refraction experiment to deduce the speed of light in glass

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Light Refraction Experiment

March 30, 2020 By Emma Vanstone Leave a Comment

This light refraction experiment might be one of the most simple to set up science experiments we’ve ever tried. It is a bit tricky to explain, but impressive even if you can’t quite get your head around it!

If you like this activity don’t forget to check out out our other easy science experiments for kids .

Materials for Light Refraction Experiment

Paper or card

Instructions

Fill the glass almost to the top.

Draw arrows on one piece of of card or paper. Place the paper behind the glass and watch as the arrow points the other way.

Now try to think of a word that still makes sense if you put it behind the glass.

We tried bud , the green ( badly drawn ) plant is on the opposite side when the paper is not behind the glass.

NOW works well too 🙂

How does this work?

Refraction ( bending of light ) happens when light travels between two mediums. In the refraction experiment above light travels from the arrow through the air, through the glass, the water, the glass again and air again before reaching your eyes.

The light reaching your eye (or in this case our camera) coming from the arrow is refracted through the glass of water. In fact the glass of water acts like a convex lens (like you might have in a magnifying glass). Convex lenses bend light to a focal point . This is the point at which the light from an object crosses.

The light that was at the tip of the arrow is now on the right side and the light on the right side is now on the left as far as your eye is concerned (assuming you are further away from the glass than the focal point.

If you move the arrow image closer to the glass than the focal point it will be the way around you expect it to be!

More Refraction experiments

Create an Alice in Wonderland themed version of this too!

Find out how to make your own magnifying glass .

We’ve also got a fun disappearing coin trick .

Or try our light maze to learn about reflection .

Last Updated on February 22, 2021 by Emma Vanstone

Safety Notice

Science Sparks ( Wild Sparks Enterprises Ltd ) are not liable for the actions of activity of any person who uses the information in this resource or in any of the suggested further resources. Science Sparks assume no liability with regard to injuries or damage to property that may occur as a result of using the information and carrying out the practical activities contained in this resource or in any of the suggested further resources.

These activities are designed to be carried out by children working with a parent, guardian or other appropriate adult. The adult involved is fully responsible for ensuring that the activities are carried out safely.

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NOTIFICATIONS

Refraction of light.

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Refraction is the bending of light (it also happens with sound, water and other waves) as it passes from one transparent substance into another.

This bending by refraction makes it possible for us to have lenses, magnifying glasses, prisms and rainbows. Even our eyes depend upon this bending of light. Without refraction, we wouldn’t be able to focus light onto our retina.

Change of speed causes change of direction

Light refracts whenever it travels at an angle into a substance with a different refractive index (optical density).

This change of direction is caused by a change in speed. For example, when light travels from air into water, it slows down, causing it to continue to travel at a different angle or direction.

How much does light bend?

The amount of bending depends on two things:

• Change in speed – if a substance causes the light to speed up or slow down more, it will refract (bend) more.
• Angle of the incident ray – if the light is entering the substance at a greater angle, the amount of refraction will also be more noticeable. On the other hand, if the light is entering the new substance from straight on (at 90° to the surface), the light will still slow down, but it won’t change direction at all.

Refractive index of some transparent substances

All angles are measured from an imaginary line drawn at 90° to the surface of the two substances This line is drawn as a dotted line and is called the normal.

If light enters any substance with a higher refractive index (such as from air into glass) it slows down. The light bends towards the normal line.

If light travels enters into a substance with a lower refractive index (such as from water into air) it speeds up. The light bends away from the normal line.

A higher refractive index shows that light will slow down and change direction more as it enters the substance.

A lens is simply a curved block of glass or plastic. There are two kinds of lens.

A biconvex lens is thicker at the middle than it is at the edges. This is the kind of lens used for a magnifying glass. Parallel rays of light can be focused in to a focal point. A biconvex lens is called a converging lens.

A biconcave lens curves is thinner at the middle than it is at the edges. Light rays refract outwards (spread apart) as they enter the lens and again as they leave.

Refraction can create a spectrum

Isaac Newton performed a famous experiment using a triangular block of glass called a prism. He used sunlight shining in through his window to create a spectrum of colours on the opposite side of his room.

This experiment showed that white light is actually made of all the colours of the rainbow. These seven colours are remembered by the acronym ROY G BIV – red, orange, yellow, green, blue, indigo and violet.

Newton showed that each of these colours cannot be turned into other colours. He also showed that they can be recombined to make white light again.

The explanation for the colours separating out is that the light is made of waves. Red light has a longer wavelength than violet light. The refractive index for red light in glass is slightly different than for violet light. Violet light slows down even more than red light, so it is refracted at a slightly greater angle.

The refractive index of red light in glass is 1.513. The refractive index of violet light is 1.532. This slight difference is enough for the shorter wavelengths of light to be refracted more.

A rainbow is caused because each colour refracts at slightly different angles as it enters, reflects off the inside and then leaves each tiny drop of rain.

A rainbow is easy to create using a spray bottle and the sunshine. The centre of the circle of the rainbow will always be the shadow of your head on the ground.

The secondary rainbow that can sometimes be seen is caused by each ray of light reflecting twice on the inside of each droplet before it leaves. This second reflection causes the colours on the secondary rainbow to be reversed. Red is at the top for the primary rainbow, but in the secondary rainbow, red is at the bottom.

Activity ideas

Use these activities with your students to explore refration further:

• Investigating refraction and spearfishing – students aim spears at a model of a fish in a container of water. When they move their spears towards the fish, they miss!
• Angle of refraction calculator challenge – students choose two types of transparent substance. They then enter the angle of the incident ray in the spreadsheet calculator, and the angle of the refracted ray is calculated for them.
• Light and sight: true or false? – students participate in an interactive ‘true or false’ activity that highlights common alternative conceptions about light and sight. This activity can be done individually, in pairs or as a whole class .

Useful links

Learn more about different types of rainbows, how they are made and other atmospheric optical phenomena with this MetService blog and Science Kids post .

Learn more about human lenses, optics, photoreceptors and neural pathways that enable vision through this tutorial from Biology Online .

See our newsletters here .

Cool Light Refraction Science Experiment – Arrow Changes Direction!

Magic trick? No, but the results of this experiment are pretty surprising. Kids (and adults) will stare in amazement and scratch their heads wondering what causes the arrow in this experiment to change direction right before their eyes! The secret is light refraction.

Exploring light refraction couldn’t be easier or more fun, simply preview the experiment with our demonstration video below and find an easy to understand explanation of how it works below.

JUMP TO SECTION:   Instructions  |  Video Tutorial  |  How it Works

Supplies Needed

• Piece of Paper

Light Refraction Science Lab Kit – Only $5

Use our easy Light Refraction Science Lab Kit to grab your students’ attention without the stress of planning!

It’s everything you need to  make science easy for teachers and fun for students  — using inexpensive materials you probably already have in your storage closet!

Light Refraction Science Experiment Instructions

Step 1 – Get a sheet of paper and draw two arrows on it. One arrow near the top and one arrow near the bottom. Make the arrows point in the same direction.

Step 2 – Fill a glass with water.

Step 3 – Slowly lower the piece of paper behind the glass of water.

Step 4 –  Look through the glass of water and watch what happens. Do you know why the arrow appears to change directions? Find out the answer in the how does this experiment work section below.

Video Tutorial

How Does the Science Experiment Work

The scientific concept that is at work in this experiment is called refraction. Refraction is the bending of light. Refraction occurs when light travels from one medium to another (ie. air to water, water to air).

During the experiment, the light traveled from the image through the air, then through the glass cup into the water, and finally out of the glass cup and into the air once more before it reached our eyes. Light refracts as it passes from one medium to the next because it travels at different speeds through those mediums. Light travels fastest through air, a little slower through water, and even slower through glass.

This means that the light bends once when it travels through the glass cup into the water, and then it bends again when it travels out of the glass cup and into the air. As a result, the light paths cross and the image appears to be flipped horizontally (left/right).

Light Refraction Examples

The following are examples of refraction that occur all around us.

• Glasses or Contacts – The lenses of glasses and contacts are designed to bend light in ways that help a persons improve vision.
• Rainbow – Rainbows are formed when the rays of sunlight bend (refract) when they travel through rain drops.
• Cameras – A camera works because the lens causes the light rays to refract. 

More Experiments that Show Light Refraction

Refraction of Light Science Experiment – Watch as the straw appears to bend in this experiment that shows refraction in action.

Ruler Changes Size Science Experiment  – Observe how the size of an object changed when viewed through different liquids. 

I hope you enjoyed the experiment. Here are some printable instructions:

Light Refraction Science Experiment

Instructions.

• Get a sheet of paper, and draw two arrows on it. One arrow near the top and one arrow near the bottom. Make the arrows point in the same direction.
• Fill a glass with water.
• Slowly lower the piece of paper behind the glass of water.
• Look through the glass of water and watch what happens.

Reader Interactions

February 5, 2017 at 9:25 am

THIS IS COOL. MY DAUGHTER WON THE SCHOOL WIDE SCIENCE PROJECT. THANK YOU SO MUCH FOR DOING THIS EXPERIMENT!

April 20, 2018 at 3:07 pm

Cause of the reflection of the water.

September 10, 2019 at 11:45 am

*refraction

January 7, 2021 at 3:53 pm

I can’t get this to work. I have used a round glass and a square plastic container. I’ve moved the piece of paper close to the container of water and father back. I have lowered the paper quickly and very slowly. Clearly it works, so what am I missing? The size of the arrows? The size of the paper?

Help! I teach a science class to elementary school children and would love to do this. Please answer [email protected]

May 23, 2018 at 7:33 am

This is because of refraction

January 22, 2019 at 3:42 am

Wonderful. Thanks for sharing

May 29, 2019 at 8:03 am

It was very useful and unique. It impressed my teacher a lot.

January 7, 2021 at 4:11 pm

I was finally able to get the arrow to change direction, but it appears that the mechanism is not the water, but the shape of the glass. It did not work with a square or wide straight sided glass. It did work in a straight sided narrow glass, but the arrow was distorted and could be manipulated back and forth by moving the paper.

March 2, 2022 at 2:52 am

Wow, this helped me for my school project i won second place thank you so much

August 4, 2022 at 7:27 pm

I tried this in a square glass container and the arrow does not change direction.

Does the concave/convex shape of the glass have something to do with the result?

May 22, 2023 at 10:07 am

That’s a great question. Do you have multiple glass containers to try the experiment with? That way you can test to see if the shape of the contain changes the results of the experiment. If you try it, come back to let us know what you find.

July 31, 2023 at 6:30 pm

It was refraction that caused the change of direction

It is caused by the refraction or the shape of the glass.

September 28, 2023 at 6:22 am

Thnx, I got 3rd position in my competition! 🤤

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Refraction of Light ( CIE IGCSE Physics )

Revision note.

Ray Diagrams for Refraction

• The angle of the wave approaching the boundary is called the angle of incidence (i)
• The angle of the wave leaving the boundary is called the angle of refraction (r)
• The line at right angles (90°) to the boundary is known as the normal
• An incident ray has an arrow pointing towards the boundary
• A refracted ray has an arrow pointing away from the boundary
• The angles of incidence and refraction are usually labelled i and r respectively

A ray diagram for light refracting at a boundary, showing the normal, angle of incidence and angle of refraction

Refraction of Light

• Refraction occurs when light passes a boundary between two different transparent media
• At the boundary, the rays of light undergo a change in direction
• This line is perpendicular to the surface of the boundaries and is usually represented by a straight dashed or dotted line
• From less dense to more dense (e.g air to glass), light bends towards the normal
• From more dense to less dense (e.g. glass to air), light bends away from the normal
• When passing along the normal (perpendicular) the light does not bend at all

How to construct a ray diagram showing the refraction of light as it passes through a rectangular block

• When light passes into a denser substance the rays will slow down , hence they bend towards the normal
• Different frequencies account for different colours of light (red has a low frequency, whilst blue has a high frequency)
• When light refracts, it does not change colour (think of a pencil in a glass of water), therefore, the frequency does not change

Worked example

Draw a third parallel ray entering and passing through prism B .

Step 1: Draw a parallel ray on the left

Step 2: Draw the refracted ray at the first surface

• As the ray enters the block it bends towards the normal since it is going into a denser material
• In this case, the angle of refraction is smaller than the angle of incidence

Step 3: Draw the refracted ray at the second surface

• As the ray leaves the block it bends away from the normal
• In this case, the angle of refraction is larger than the angle of incidence

Practice drawing refraction diagrams as much as you can! It's very important to remember which way the light bends when it crosses a boundary:

As the light enters the block it bends towards the normal line

Remember: Enters Towards

When it leaves the block it bends away from the normal line

Remember: Leaves Away

Investigating Refraction

Aim of the experiment.

• To investigate the refraction of light using rectangular blocks, semi-circular blocks and triangular prisms
• Independent variable = shape of the block
• Dependent variable = direction of refraction
• Width of the light beam
• Same frequency / wavelength of the light

Equipment List

• Protractor = 1°
• Ruler = 1 mm

Diagram showing a ray box alongside three different shaped glass blocks

Apparatus to investigate refraction

• Place the glass block on a sheet of paper, and carefully draw around the rectangular perspex block using a pencil
• Switch on the ray box and direct a beam of light at the side face of the block
• A point on the ray close to the ray box
• The point where the ray enters the block
• The point where the ray exits the block
• A point on the exit light ray which is a distance of about 5 cm away from the block
• Draw a dashed line normal (at right angles) to the outline of the block where the points are
• Remove the block and join the points marked with three straight lines
• Replace the block within its outline and repeat the above process for a ray striking the block at a different angle
• Repeat the procedure for each shape of perspex block (prism and semi-circular)

Analysis of Results

• Consider the light paths through the different-shaped blocks

Refraction of light through different shapes of perspex blocks

• The final diagram for each shape will include multiple light ray paths for the different angles of incidences ( i ) at which the light strikes the blocks
• Label these paths clearly with (1) (2) (3) or A , B , C to make these clearer 
• Angles i and  r are always measured from the  normal
• For light rays entering the perspex block, the light ray refracts towards the central line:
• For light rays exiting the perspex block, the light ray refracts away from the central line:
• When the angle of incidence is 90° to the perspex block, the light ray does not refract, it passes straight through the block:

Evaluating the Experiment

Systematic Errors:

• Use a set square to draw perpendicular lines

Random Errors:

• Use a sharpened pencil and mark in the middle of the beam
• Use a protractor with a higher resolution

Safety Considerations

• Run burns under cold running water for at least five minute
• Avoid looking directly at the light
• Stand behind the ray box during the experiment
• Keep all liquids away from the electrical equipment and paper

In your examination, you might be asked to write a method explaining how you might investigate the refraction of light through different shaped blocks

As part of this method you should describe:

• What equipment you need
• How you will use the equipment
• How you will trace the rays of light before, while and after they pass through the block

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Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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Refraction of Light Experiment: Easy Science for Kids

By: Author Jacquie Fisher

Posted on Published: February 1, 2019

Categories Kids Activities & Crafts , Science Experiments

Learn about refraction of light with this easy science experiment that can be done at home or in the classroom.

Our easy science experiments are back and this week, we’re adding a touch of ‘magic’!

In my experience, you can AMAZE kids with two types of science activities — the first type are those that show some behind-the-scenes phenomenon like this How Does a Leaf Breathe? experiment!

And the second type are those that seem to work like magic.

Today we’re going to play with that sense of wonder and highlight the science behind light refraction as we bend a pencil without breaking it !

Refraction & Light Experiments: Easy Science for Kids

In the 28 Days of STEM series, this week’s topic is STEM Challenges so we thought we would introduce a science challenge that ‘tricks’ your eyes.

Similar to our Optical Illusion science experiment , light experiments also rely on what our eyes see. 

This quick experiment (actually 2) is so fun to do with kids, takes only 5 minutes and is SUPER EASY — all you’ll need is a pencil and a glass of water. We’re also including affiliate links to some great books and items we used for this experiment.

Since we always pair our experiments with books (it’s a great way to explain & extend the science 😉 here are a few of our favorites on light:

Light is All Around Us – a great introduction to sunlight, shadows and the speed of light.  This is from a great set of early science books which always include a ‘Learn More’ section and easy experiments kids can do after reading the book.  You can see all the books in this series here:  Let’s Read & Find Out Science series for kids !

Explore Light & Optics with 25 Great Projects – includes a number of other cool activities and experiments kids can do with light!  I love the Explore Your World project books — so many awesome themes & topics!

The Bending Pencil Experiment: Light Refraction

Step 1:  Fill a drinking glass or glass jar with water.

We used a mason jar since all of our drinking glasses have an etched design in them.  You’ll get the best results with a clear, glass container for this experiment.

Step 2:  Hold the tip of the pencil while inserting it into the water

You can already see that the pencil appears larger once in the water (as water is also a magnifier — learn more about that in this pine cone experiment ).

Step 3:  Look at the pencil through the side of the glass or jar — did it bend?

You may have to lean the pencil closer to the front of the glass but you should see a distinct ‘break’ in the pencil when you peep through the side of the glass.  As you can see above, the part of the pencil in the water appears to be shifted to the left of where you would expect to see it. 

You can tell kids that this is an optical illusion — your eyes are being ‘tricked’ because the pencil still looks the same when you pull  it out of the water.

Pretty darn cool, huh!?!

Here’s the science behind this cool experiment …

Why Does a Pencil Look Bent in Water?

Refraction is a physics concept that refers to how a wave travels through a medium. 

In this case, we’re looking at how light waves travel through the air, through water and through glass.

Light waves travel faster thought air (which is a less dense material) than they do through water or glass (a more dense material).  So as the light waves enter the glass and water, they slow down and bend into the water. 

Since the light waves are no longer traveling at the same angle, this makes the pencil appear to ‘break’ when you look at it through the glass of water.

How Light Waves Travel through Water

We’re going to try a second cool science experiment:  The Glowing Jar!

We’ll use a jar, some water and a flashlight to get an close-up look at refraction.

Let’s look at light waves without the pencil. 

Since both the glass and the water are more dense materials than air, when you shine a flashlight into the glass (on the left), the light waves will slow down slightly as they enter and then return to normal as they exist the other side so you see one round spot of light.

However, once you add water to the glass (on the right), you can see that the light shining through the glass is more dispursed — now there is BOTH a spot of light and a ‘glowing’ outline of the jar showing on the wall due to how the light waves are refracting (or bending) as they travel through the water.

If you’ve ever been on a boat and looked over the side into the water, this explains why you can see so much below the water — sunlight is refracting (bending) to illuminate a wider area.

One way to explain this to kids is to think about running at the beach and into the water.  When you run on the sand, you can move quickly because you are only running through air. 

However, when you continue to run into the water, you will not be able to move as quickly because water is more dense and slows you down. 

The same applies to beams of light — as they enter water, they slow down and can’t move as quickly.

If you want to see another refraction experiment, visit my friend Erica at What do We do all Day to see how they made an arrow change directions with a glass of water !

If you’d like to explore more with light science and optical illusions, a few fun items along with affiiliates links include:

More Easy Science Experiments!

The Science of Sound: Create Your own Gong

How do Leaves Breathe?

Be sure to join us for the 28 Days of Hands-on STEM Activities hosted by Left Brain, Craft Brain this month! 

Every day, you can find new activities that cover science, technology, engineering and math perfect for kids from preschool thru middle school!

16.2 Refraction

Section learning objectives.

By the end of this section, you will be able to do the following:

• Explain refraction at media boundaries, predict the path of light after passing through a boundary (Snell’s law), describe the index of refraction of materials, explain total internal reflection, and describe applications of refraction and total internal reflection
• Perform calculations based on the law of refraction, Snell’s law, and the conditions for total internal reflection

Teacher Support

The learning objectives in this section help your students master the following standards:

• (D) investigate behaviors of waves, including reflection, refraction, diffraction, interference, resonance, and the Doppler effect; and
• (F) describe the role of wave characteristics and behaviors in medical and industrial applications.

In addition, the High School Physics Laboratory Manual addresses content in this section in the lab titled: Mirrors and Lenses, as well as the following standards:

• (D) investigate behaviors of waves, including reflection, refraction, diffraction, interference, resonance, and the Doppler effect.

Section Key Terms

The Law of Refraction

[BL] [OL] Remind students that the maximum speed of light is its speed in a vacuum. This is a fundamental constant of physics. The maximum speed of light is equal to 3.00 × × 10 8 m/s. Have your students memorize this value.

You may have noticed some odd optical phenomena when looking into a fish tank. For example, you may see the same fish appear to be in two different places ( Figure 16.16 ). This is because light coming to you from the fish changes direction when it leaves the tank and, in this case, light rays traveling along two different paths both reach our eyes. The changing of a light ray’s direction (loosely called bending ) when it passes a boundary between materials of different composition, or between layers in single material where there are changes in temperature and density, is called refraction . Refraction is responsible for a tremendous range of optical phenomena, from the action of lenses to voice transmission through optical fibers.

[BL] An angle is the measure of the separation of two lines or rays originating from a single point. The length of the lines is not relevant.

[OL] [AL] The trigonometric function sine (sin) for a given angle is the ratio of the side of a right triangle opposite that angle to the hypotenuse of that triangle.

Why does light change direction when passing from one material (medium) to another? It is because light changes speed when going from one material to another. This behavior is typical of all waves and is especially easy to apply to light because light waves have very small wavelengths, and so they can be treated as rays. Before we study the law of refraction, it is useful to discuss the speed of light and how it varies between different media.

The speed of light is now known to great precision. In fact, the speed of light in a vacuum, c , is so important, and is so precisely known, that it is accepted as one of the basic physical quantities, and has the fixed value

where the approximate value of 3.00 × × 10 8 m/s is used whenever three-digit precision is sufficient. The speed of light through matter is less than it is in a vacuum, because light interacts with atoms in a material. The speed of light depends strongly on the type of material, given that its interaction with different atoms, crystal lattices, and other substructures varies. We define the index of refraction , n , of a material to be

where v is the observed speed of light in the material. Because the speed of light is always less than c in matter and equals c only in a vacuum, the index of refraction (plural: indices of refraction) is always greater than or equal to one.

Table 16.2 lists the indices of refraction in various common materials.

Figure 16.17 provides an analogy for and a description of how a ray of light changes direction when it passes from one medium to another. As in the previous section, the angles are measured relative to a perpendicular to the surface at the point where the light ray crosses it. The change in direction of the light ray depends on how the speed of light changes. The change in the speed of light is related to the indices of refraction of the media involved. In the situations shown in Figure 16.17 , medium 2 has a greater index of refraction than medium 1. This difference in index of refraction means that the speed of light is less in medium 2 than in medium 1. Note that, in Figure 16.17 (a), the path of the ray moves closer to the perpendicular when the ray slows down. Conversely, in Figure 16.17 (b), the path of the ray moves away from the perpendicular when the ray speeds up. The path is exactly reversible. In both cases, you can imagine what happens by thinking about pushing a lawn mower from a footpath onto grass, and vice versa. Going from the footpath to grass, the right front wheel is slowed and pulled to the side as shown. This is the same change in direction for light when it goes from a fast medium to a slow one. When going from the grass to the footpath, the left front wheel moves faster than the others, and the mower changes direction as shown. This, too, is the same change in direction as light going from slow to fast.

Bent Pencil

A classic observation of refraction occurs when a pencil is placed in a glass filled halfway with water. Do this and observe the shape of the pencil when you look at it sideways through air, glass, and water.

• A full-length pencil
• A glass half full of water

Instructions

• Place the pencil in the glass of water.
• Observe the pencil from the side.
• Explain your observations.

Look up the refractive indices of air, glass, and water in Table 16.2 . Think about how a ray of light changes direction for these transitions: air to glass and glass to water.

Virtual Physics

Bending light.

The Bending Light simulation allows you to show light refracting as it crosses the boundaries between various media (download animation first to view). It also shows the reflected ray. You can move the protractor to the point where the light meets the boundary and measure the angle of incidence, the angle of refraction , and the angle of reflection. You can also insert a prism into the beam to view the spreading, or dispersion , of white light into colors, as discussed later in this section. Use the ray option at the upper left.

• The medium below the boundary must have a greater index of refraction than the medium above.
• The medium below the boundary must have a lower index of refraction than the medium above.
• The medium below the boundary must have an index of refraction of zero.
• The medium above the boundary must have an infinite index of refraction.

Have students try all the different tabs at the top of the simulation. Point out to students that, although the tools work in both Ray and Wave mode, some may be easier to use in Wave mode because the region where the tool is able to read is larger.

[BL] Be sure students understand that if c is always greater than v , n must always be greater than one. Demonstrating division using numbers that can be divided easily can reinforce student understanding.

[OL] Explain that, unlike the law of reflection, the law of refraction is most easily expressed as an equation, rather than in words. Walk students through the lawnmower analogy in Figure 16.17 . Suggest other wheeled vehicles with which they may be more familiar, and other surfaces, such as sand.

[AL] Ask students to try to explain why a prism separates white light into a rainbow of colors, but a window pane does not. If they cannot explain it, show them a ray diagram of light transmitted through a flat sheet of glass.

The amount that a light ray changes direction depends both on the incident angle and the amount that the speed changes. For a ray at a given incident angle, a large change in speed causes a large change in direction, and thus a large change in the angle of refraction. The exact mathematical relationship is the law of refraction, or Snell’s law , which is stated in equation form as

In terms of speeds, Snell’s law becomes

Here, n 1 and n 2 are the indices of refraction for media 1 and 2, respectively, and θ 1 and θ 2 are the angles between the rays and the perpendicular in the respective media 1 and 2, as shown in Figure 16.17 . The incoming ray is called the incident ray and the outgoing ray is called the refracted ray . The associated angles are called the angle of incidence and the angle of refraction . Later, we apply Snell’s law to some practical situations.

Dispersion is defined as the spreading of white light into the wavelengths of which it is composed. This happens because the index of refraction varies slightly with wavelength. Figure 16.18 shows how a prism disperses white light into the colors of the rainbow.

Rainbows are produced by a combination of refraction and reflection. You may have noticed that you see a rainbow only when you turn your back to the Sun. Light enters a drop of water and is reflected from the back of the drop, as shown in Figure 16.19 . The light is refracted both as it enters and as it leaves the drop. Because the index of refraction of water varies with wavelength, the light is dispersed and a rainbow is observed.

Watch Physics

This video explains how refraction disperses white light into its composite colors.

• Colors with a longer wavelength and higher frequency bend most when refracted.
• Colors with a shorter wavelength and higher frequency bend most when refracted.
• Colors with a shorter wavelength and lower frequency bend most when refracted.
• Colors with a longer wavelength and a lower frequency bend most when refracted.

Have students note that the dependence of the index of refraction on the speed of light implies a dependence on wavelength, as discussed in the video. This is because v = f λ v = f λ for light in a medium. Different wavelengths of light travel at different speeds, and so refract differently at media boundaries. Also, have students note that the frequency of light is not affected by refraction; it remains constant.

A good-quality mirror reflects more than 90 percent of the light that falls on it; the mirror absorbs the rest. But, it would be useful to have a mirror that reflects all the light that falls on it. Interestingly, we can produce total reflection using an aspect of refraction. Consider what happens when a ray of light strikes the surface between two materials, such as is shown in Figure 16.20 (a). Part of the light crosses the boundary and is refracted; the rest is reflected. If, as shown in the figure, the index of refraction for the second medium is less than the first, the ray bends away from the perpendicular. Because n 1 > n 2 , the angle of refraction is greater than the angle of incidence—that is, θ 2 θ 2 > θ 1 θ 1 . Now, imagine what happens as the incident angle is increased. This causes θ 2 θ 2 to increase as well. The largest the angle of refraction, θ 2 θ 2 , can be is 90°, as shown in Figure 16.20 (b). The critical angle , θ c θ c , for a combination of two materials is defined to be the incident angle, θ 1 θ 1 , which produces an angle of refraction of 90°. That is, θ c θ c is the incident angle for which θ 2 θ 2 = 90°. If the incident angle, θ 1 θ 1 , is greater than the critical angle, as shown in Figure 16.20 (c), then all the light is reflected back into medium 1, a condition called total internal reflection .

Recall that Snell’s law states the relationship between angles and indices of refraction. It is given by

When the incident angle equals the critical angle ( θ 1 θ 1 = θ c θ c ), the angle of refraction is 90° ( θ 2 θ 2 = 90°). Noting that sin 90° = 1, Snell’s law in this case becomes

The critical angle, θ c θ c , for a given combination of materials is thus

for n 1 > n 2 .

[OL] The superscript in sin −1 is not a power. It indicates arcsine, which is an inverse trigonometric function. It means, “that angle whose sine equals (in this case) ( n 2 / n 1 ).”

Total internal reflection occurs for any incident angle greater than the critical angle, θ c θ c , and it can only occur when the second medium has an index of refraction less than the first. Note that the previous equation is written for a light ray that travels in medium 1 and reflects from medium 2, as shown in Figure 16.20 .

There are several important applications of total internal reflection. Total internal reflection, coupled with a large index of refraction, explains why diamonds sparkle more than other materials. The critical angle for a diamond-to-air surface is only 24.4°; so, when light enters a diamond, it has trouble getting back out ( Figure 16.21 ). Although light freely enters the diamond at different angles, it can exit only if it makes an angle less than 24.4° with the normal to a given surface. Facets on diamonds are specifically intended to make this unlikely, so that the light can exit only in certain places. Diamonds with very few impurities are very clear, so the light makes many internal reflections and is concentrated at the few places it can exit—hence the sparkle.

A light ray that strikes an object that consists of two mutually perpendicular reflecting surfaces is reflected back exactly parallel to the direction from which it came. This parallel reflection is true whenever the reflecting surfaces are perpendicular, and it is independent of the angle of incidence. Such an object is called a corner reflector because the light bounces from its inside corner. Many inexpensive reflector buttons on bicycles, cars, and warning signs have corner reflectors designed to return light in the direction from which it originates. Corner reflectors are perfectly efficient when the conditions for total internal reflection are satisfied. With common materials, it is easy to obtain a critical angle that is less than 45°. One use of these perfect mirrors is in binoculars, as shown in Figure 16.22 . Another application is for periscopes used in submarines.

Fiber optics are one common application of total internal reflection. In communications, fiber optics are used to transmit telephone, internet, and cable TV signals, and they use the transmission of light down fibers of plastic or glass. Because the fibers are thin, light entering one is likely to strike the inside surface at an angle greater than the critical angle and, thus, be totally reflected ( Figure 16.23 ). The index of refraction outside the fiber must be smaller than inside, a condition that is satisfied easily by coating the outside of the fiber with a material that has an appropriate refractive index. In fact, most fibers have a varying refractive index to allow more light to be guided along the fiber through total internal reflection. Rays are reflected around corners as shown in the figure, making the fibers into tiny light pipes .

Links To Physics

Medicine: endoscopes.

A medical device called an endoscope is shown in Figure 16.24 .

The word endoscope means looking inside . Doctors use endoscopes to look inside hollow organs in the human body and inside body cavities. These devices are used to diagnose internal physical problems. Images may be transmitted to an eyepiece or sent to a video screen. Another channel is sometimes included to allow the use of small surgical instruments. Such surgical procedures include collecting biopsies for later testing, and removing polyps and other growths.

• The process is refraction of light.
• The process is dispersion of light.
• The process is total internal reflection of light.
• The process is polarization of light.

Calculations with the Law of Refraction

[BL] If an equation has two variables and a constant, such as n = c / v , n = c / v , the value of only one variable is needed to find the other.

[OL] Explain the difference between sine and arcsine. Explain why sin 90° = 1. Note that the index of refraction is a dimensionless number.

[AL] Show why the values sin 0°, sin 30°, sin 45°, sin 60°, and sin 90° can be expressed in the form

Show that the numerical values of these expressions are 0, 0.5, 0.707, 0.866, and 1.00, respectively.

The calculation problems that follow require application of the following equations:

These are the equations for refractive index, the mathematical statement of the law of refraction (Snell’s law), and the equation for the critical angle.

Snell’s Law Example 1

This video leads you through calculations based on the application of the equation that represents Snell’s law.

• The two types of variables are density of a material and the angle made by the light ray with the normal.
• The two types of variables are density of a material and the thickness of a material.
• The two types of variables are refractive index and thickness of each material.
• The two types of variables are refractive index of a material and the angle made by a light ray with the normal.

Worked Example

Calculating index of refraction from speed.

Calculate the index of refraction for a solid medium in which the speed of light is 2.012 × × 10 8 m/s, and identify the most likely substance, based on the previous table of indicies of refraction.

We know the speed of light, c , is 3.00 × × 10 8 m/s, and we are given v . We can simply plug these values into the equation for index of refraction, n .

This value matches that of polystyrene exactly, according to the table of indices of refraction ( Table 16.2 ).

The three-digit approximation for c is used, which in this case is all that is needed. Many values in the table are only given to three significant figures. Note that the units for speed cancel to yield a dimensionless answer, which is correct.

Calculating Index of Refraction from Angles

Suppose you have an unknown, clear solid substance immersed in water and you wish to identify it by finding its index of refraction. You arrange to have a beam of light enter it at an angle of 45.00°, and you observe the angle of refraction to be 40.30°. What are the index of refraction of the substance and its likely identity?

We must use the mathematical expression for the law of refraction to solve this problem because we are given angle data, not speed data.

The subscripts 1 and 2 refer to values for water and the unknown, respectively, where 1 represents the medium from which the light is coming and 2 is the new medium it is entering. We are given the angle values, and the table of indicies of refraction gives us n for water as 1.333. All we have to do before solving the problem is rearrange the equation

The best match from Table 16.2 is fused quartz, with n = 1.458.

Note the relative sizes of the variables involved. For example, a larger angle has a larger sine value. This checks out for the two angles involved. Note that the smaller value of θ 2 θ 2 compared with θ 1 θ 1 indicates the ray has bent toward normal. This result is to be expected if the unknown substance has a greater n value than that of water. The result shows that this is the case.

Calculating Critical Angle

Verify that the critical angle for light going from water to air is 48.6°. (See Table 16.2 , the table of indices of refraction.)

First, choose the equation for critical angle

Then, look up the n values for water, n 1 , and air, n 2 . Find the value of n 2 n 1 n 2 n 1 . Last, find the angle that has a sine equal to this value and it compare with the given angle of 48.6°.

For water, n 1 = 1.333; for air, n 2 = 1.0003. So,

Remember, when we try to find a critical angle, we look for the angle at which light can no longer escape past a medium boundary by refraction. It is logical, then, to think of subscript 1 as referring to the medium the light is trying to leave, and subscript 2 as where it is trying (unsuccessfully) to go. So water is 1 and air is 2.

Practice Problems

The refractive index of ethanol is 1.36. What is the speed of light in ethanol?

• 2.25×108 m/s
• 2.21×107 m/s
• 2.25×109 m/s
• 2.21×108 m/s

Check Your Understanding

Use these questions to assess student achievement of the section’s learning objectives. If students are struggling with a specific objective, these questions help identify which one, and then you can direct students to the relevant content.

• This is Ohm’s law.
• This is Wien’s displacement law.
• This is Snell’s law.
• This is Newton’s law.
• The formula for index of refraction, n , of a material is n = speed of light in a material speed of light in a vacuum = v c , where \text{c}"> v > c , so n is always greater than one.
• The formula for index of refraction, n , of a material is n = speed of light in a vacuum speed of light in a material = c v , where \text{v}"> c > v , so n is always greater than one.
• The formula for index of refraction, n , of a material is n = speed of light in a vacuum × speed of light in a materaial = c × v , where c , 1"> v > 1 , so n is always greater than one.
• The formula for refractive index, n , of a material is n = 1 speed of light in a vacuum × speed of light in a material = 1 c × v , where "> c < v < 1 , so n is always greater than one.
• n 1 n 2 = sin ⁡ θ 1 sin ⁡ θ 2
• n 2 n 1 = ( sin ⁡ θ 2 sin ⁡ θ 1 ) 2
• n 1 n 2 = ( sin ⁡ θ 2 sin ⁡ θ 1 ) 2
• n 1 n 2 = sin ⁡ θ 2 sin ⁡ θ 1

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May 5, 2016

Now You See It... Testing Out Light Refraction

An en light ening activity from Science Buddies

By Science Buddies

Make a straw vasnish before your eyes--with the amazing duo of refracting and reflecting light. It's not magic, it's science!

George Retseck

Key Concepts Light Refraction Reflection Index of refraction

Introduction If you pour water into a clear glass, what color is it? It's clear, right? But what happens if you try to look through it to see the world on the other side of the glass? It looks a little distorted, maybe a little fuzzier and uneven. If water is clear, why can't we see through it clearly? The answer has to do with how light moves through water, glass and other transparent materials. Similar to when you try to run in a swimming pool, when light tries to move through water or glass it gets slowed down. When light is slowed down, it either bounces off the material or is bent as it passes through. We can see these changes in light, which indicates to us that something is there. In this activity you will play with light to make normal objects appear and disappear!

Background When light that is traveling through the air hits water, some of the light is reflected off the water. The rest of the light passes through the water but it bends (or refracts) as it enters the water. The same thing happens when light hits glass or any other transparent material. Some light is reflected off the object whereas the rest passes through and is refracted.

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All materials have what is known as an index of refraction, which is linked to how fast light can travel through the material. As light passes through air and into another clear material (such as glass), it changes speed, and light is both reflected and refracted by the glass. This results in us seeing the glass because it reflects and refracts light differently than the air around it does. The change in the light allows us to differentiate one object from another. If a transparent object is surrounded by another material with the same index of refraction, however, the light will not change speed as it enters the object. As a result, you will not be able to see the object.

In this activity you will observe how the index of refraction of different materials helps us to see (or not see!) the objects as light passes through them!

Two clear glass jars, tall bowls or drinking glasses that hold at least eight ounces (Tip: Pyrex glass works especially well for this activity.)

Vegetable oil, approximately 14 ounces or enough to fill one of the glasses halfway (Tip: Avoid using “light” vegetable oil for this activity.)

Glass eyedropper (A plastic eyedropper or clear plastic drinking straw also will work. If you use a drinking straw instead of a dropper, each time you immerse the straw keep your finger over the top to avoid getting liquid in the straw. The instructions will tell you when to release your finger.)

Other transparent glass objects, such as marbles, beads, a magnifying glass or glass stirrers (optional)

Preparation

Fill half of one jar with the vegetable oil.

Fill half of the other jar with water.

Make sure your eyedropper is clean before starting the activity.

Set up a flat work surface that can be cleaned if any water or oil spills on it.

Take your eyedropper (or straw) and, without squeezing it, immerse it in the jar of water. (For this step, avoid sucking up any water with the eyedropper or straw.) What do you notice about the eyedropper? Can you still see it? How clearly?

Keeping the eyedropper in the water, squeeze the top to suck up water. If you're using a straw, release your finger from the top to allow the immersed straw to fill with water. Did anything change about the eyedropper once it was filled with water? Does the eyedropper become easier or more difficult to see once it is filled with water?

Remove the eyedropper from the water and squeeze out all excess liquid.

Immerse the eyedropper in the oil, without squeezing it. Make sure to avoid sucking up any oil for this first step. What do you notice about the eyedropper? Are you still able to see it? Was it easier to see the eyedropper when it was in the water?

Squeeze the eyedropper to allow it to fill with oil. (If using a straw, remove your finger from the top to allow the immersed straw to fill with oil.) What happened? Can you still see the eyedropper? Is it easier or more difficult to see it now than it was when it was empty?

Remove the eyedropper from the oil in the jar and squeeze out the excess oil.

Slowly and gently pour the oil from the jar into the jar with the water. If you do this very carefully, the oil will sit right on top of the water! (It's ok if they mix though, they will separate once you stop pouring.)

Allow the oil and water to settle and separate (about one to two minutes). What do you notice about the oil? Are there bubbles in it? If there are bubbles, watch them closely and see if they are rising or sinking. If they sink, they are actually water bubbles trapped inside the oil!

Fill the eyedropper (or straw) with oil from the jar, and then slowly immerse it through the layer of oil, so that the dropper is visible in both the water layer at the bottom and the oil layer. Look at the dropper in the water layer, then in the oil layer. What is different about the dropper in these two layers? Is it easier to see the dropper in the oil or the water?

With the bottom tip of the dropper in the water layer, squeeze the dropper to expel the oil inside and allow it to fill with water. Again, observe the oil dropper in the water and oil layers. Is it easier to see the dropper in the oil or in the water this time?

Extra: Try repeating this activity using glass objects, such as marbles, beads, glasses or lenses. (Be sure you have permission to try out any object before using it.) Notice which things are the most difficult to see when you hold them in the oil versus the water. Why do you think that is?

Observations and results Did the eyedropper become invisible (or at least harder to see) when it was full of oil and immersed in oil? This is what is expected. It may also have been hard to see when it was in the water (and full of water) as well.

The eyedropper “disappears” because of how we see light as it encounters glass. When light hits a glass object, some of the light bounces (or reflects) off the glass. The rest of the light keeps going through the glass object, but the light is bent (or refracted) as it moves from the air to the glass.

The index of refraction for the oil is very close to the index of refraction for glass. Therefore, as light travels through the oil and into the glass eyedropper, very little of it is reflected or refracted. As a result, we see only the "ghost" of the eyedropper in the oil.

More to explore Refraction of Light Demonstration , from PBS Learning Media Using a Laser to Measure the Speed of Light in Gelatin , from Science Buddies Measuring Sugar Content of a Liquid with a Laser Pointer , from Science Buddies

This activity brought to you in partnership with Science Buddies

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An Experiment to Measure the Speed of Light in Glass.

An Experiment to Measure the Speed of Light in Glass

Apparatus –         Glass D-Block

                                 Optical pins

                                Cork board

                                Protractor printed on paper

To find the refractive index you must first find the angle of incidence and reflection. You use the method of no parallax, I will use ray tracing with the method of no parallax. This involves placing the D-Block on the protractor paper, so the normal is at 0 o , the center of the D-Block. I will place two optical pins pointing at the normal, lining up the two pins, I will place the third lined up with these two, looking through the D-Block. I will measure the angle between the normal and the line that connects the optical pins in front of the D-Block to the D-Blocks front center (i), and the angle between the normal and the line that connects the optical pin behind the D-Block to the D-Blocks front center (r) and record them. To be safe I will make sure I am careful with the apparatus.

This is a preview of the whole essay

I know that when light goes through a different medium from which it is already in (a vacuum for example) it slows down, this is caused by there being more particles in the way. Light refracts because the change in speed, hitting the medium at an angle, causes one side of the ray to slow down first, and so it turns, towards the normal. The normal is a line at right angles to the medium. If a ray went down the normal, no refraction would occur, as both sides of the ray would slow down at the same time. If light went through from a medium into a vacuum, the ray would speed up.

The value of the ratio Sini/Sinr indicates the amount of bending occurs when a ray passes from one medium to another, for two media it will be the relative refractive index, as it depends on both media, but as air is so close to a vacuum we can assume it is, giving us an absolute refractive index, from a vacuum to a medium. Dividing the speed of light in a vacuum, – the place where light travels fastest, by this ratio (from a different medium), we get the speed of light in that medium. Snell’s law states that “the refractive index of the medium light is passing into/ the refractive index of the medium light is passing out of = Sini/Sinr”. As air has a refractive index of 1, it simplifies to - the refractive index of the medium light is passing into

= Sini/Sinr *

        I think that the speed of light in glass will be less of that than in air because the speed of light in air is so close to that of a vacuum, which is where light travels fastest. It also stands to reason that a solid is denser that air, and so light will have to travel through more particles.

The independent variable is the angle of incidence; I will vary this from 10 o  till 80 o  at 10 o  intervals. At 0 o  there will be no refraction as explained, and at 90 o  the D-Block will be missed.

The dependant variable is the angle of refraction; I will repeat the results at least twice and take a wide range of readings so that the experiment will be fair, and reliable. I will read the protractor to within 0.5 o  so the readings will be precise.

Other variables will be kept constant so that any change in the dependant variable will likely be caused by a change in the independent variable. Such factors include the media and the entrance point.

Mean of Sini/Sinr         24.214     = 1.51

                                  16

Speed of light in air/vacuum = 300,000,000m/s

Speed of light in glass = 300,000,000

                                              1.51

                                =199,000,000m/s

Conclusions

• The speed of light in glass (199,000,000m/s) is less than that of air (300,000,000m/s).
• The graph shows that Sini/Sinr is constant and proportional, and can be used as the refractive index, as does the results table.
• The prediction has been fully supported though there is no proof of particles getting in the way as the cause of the light slowing down.
• This proves that light slows down from air into glass, and in doing so refracts. This refraction stays constant, as expected and the level of refraction-(which is constant, so works) can be obtained using Snell’s law. This refractive index can be used to obtain the speed of light in that particular medium, by using the speed of light in air/a vacuum as a basis.
• The results are firm enough to draw accurate conclusions, and test the prediction because the points on the graph stick very close to the line of best fit.
• The second set of results are almost all above the first set by 0.5 o  suggesting that one variable was overlooked, but didn’t change, possibly the D-Block was off the normal slightly.
• Also saying that the refractive index of air is 1; when it is 1.0003, may render the results slightly out if repeating the experiment more accurately, though here it does not make a difference.
• You could do the experiment backwards, from glass into air, to prove that Snell’s law still works. If the refractive index turned out the same then it should be proven.

* Extracts from web-site:         (Preliminary work)

Document Details

• Word Count 1081
• Page Count 4
• Subject Science

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Explain the refraction of light through a glass-slab with neat ray diagram.

On entering into the glass medium light ray bends towards the normal that is light ray gets refracted on entering the glass medium. after getting refracted this ray now travels through the glass slab and comes out of the glass slab by refraction from the other interface boundary. since ray goes from glass medium to air it again gets refracted and bends away from normal. the incident ray and the emergent ray are parallel to each other. i is the angle of incidence, r is the angle of refraction and e is the angle of emergence. angle of incidence and angle of emergence are equal as emergent ray and incident ray are parallel to each other. when a light ray is incident normally to the interface of two media then there is no bending of light ray and it goes straight through the medium..

The speed of light in air is 3 × 10 8 m s − 1 . Calculate the speed of light in glass. The refractive index of glass is 1.5.

Refractive index of a medium = s p e e d o f l i g h t i n v a c u u m s p e e d o f l i g h t i n a m e d i u m 1.5 = 3 × 10 8 s p e e d o f l i g h t i n m e d i u m speed of light in the medium = 3 × 10 8 1.5 = 2 × 10 8 m/s..

Light enters from air to glass having refractive index 1.50. What is the speed of light in the glass? The speed of light in vacuum is 3 × 10 8 m s −1 .

(1) The speed of light in a transparent medium is 2.4 × 10 8 m/s. Calculate absolute refractive index of the medium.

(2) The speed of light in water and glass is 2.2 × 10 8 m/s and 2 × 10 8 m/s, respectively. What is the refractive index of −

(a) water w.r.t. glass

(b) glass w.r.t. water.

(3) Refractive index of water is 4/3 and speed of light in air is 3 × 10 8 m/s. Find speed of light in water.

a. The speed of light in air is known to be 3.0 x 10 8 s m/s.

Outline how you would use a refraction experiment to deduce the speed of light in glass. You may draw a diagram if it helps to clarify your answer.

b. A tsunami is a giant water wave. It may be caused by an earthquake below the ocean. Waves from a certain tsunami have a wavelength of 1.9 x 10 3 m and a speed of 240 m/s.

i. Calculate the frequency of the tsunami waves.

ii. The shock wave from the earthquake travels at 2.5 x 10 3 m/s.

The centre of the earthquake is 6.0 x 10 3 m from the coast of a country.

Calculate how much warning of the arrival of the tsunami at the coast is given by the earth tremor felt at the coast. Calculate the time in s.

Step by Step Answer:

Answer a velocity 2108 ms bi frequency0126 hz bii time 24 secs ... view the full answer.

Complete Physics For Cambridge IGCSE RG

ISBN: 9780198308713

3rd Edition

Authors: Stephen Pople

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