University of Virginia
Physics Department

## Total Internal Reflection

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.9

Objectives

Students will

• refresh their understanding of reflection and refraction;
• learn how total internal reflections occurs;
• observe total internal reflection;
• discuss the important applications of total internal reflection.

Motivation for Learning

Laser light in a Water Tank

Material List:

• aquarium (fish tank, 5 gallon size is ideal)
• water
• laser pointer (He-Ne, red/orange in color)
• powdered milk (coffee creamer)
• white poster board, at least 12" on a side to stretch across the top of the fish tank

Procedure

This is a very nice set up to demonstrate both reflection and refraction.

Look here for laser pointer safety guidelines.

1. Fill the aquarium (clear glass) almost full of water; leave about 5 cm unfilled.
2. Mix in less than a teaspoon of powdered coffee creamer and stir. You may have to do this in stages to obtain the optimum amount, but if you put in too much, you will have to start over.
3. Shine the laser pointer from the outside of one end of the aquarium from near the bottom pointing so the light will come out through the surface of the water (see photo below). You should be able to see the red path of the laser as it passes through the water. You will not be able to easily see laser light pass through clear water, but the powder adds larger masses that will scatter the laser light so you can see it. You should be able to see the light reflect off the top water surface. If you shine the laser pointer towards the side of the aquarium, you may be able to see light reflected off the glass side.
4. You will not be able to see light coming out of the aquarium into air, because air molecules are too small to effectively scatter visible light. However, you can place the white poster board over the top and sides of the aquarium and see the laser light shine on the white board.
5. If the laser light is quickly attenuated inside the aquarium, you have put too much coffee creamer in the water. Start over. If you can't see the light or if you can barely see it, then you need to put more creamer in and stir. It may take some practice to get this just right. Measure what you put in so you can reproduce the amount later. This photo was produced using a more powerful laser than a laser pointer, so the light could easily be seen in the 5-gallon fish tank for the photo. We used less than one teaspoon of coffee creamer. The laser can be seen in the lower right of the photo (red spot). The path of the laser light cannot be seen until after it enters the water. The light moves to the left and is totally internally reflected at the water surface. Note the straight lines that the laser light makes inside the aquarium. The fish tank can be seen as outlined by the cloudy area in the photo.
6. Shine the laser light so that it reflects off the water surface (from inside the water) at different angles. Use the poster board to see if light is being transmitted through the water/air interface (that is, is light being refracted?). You should easily be able to see the laser light reflect off the glass surface where it enters the aquarium (use poster board).
7. You should be able to find an angle (called the critical angle) where, as the angle with the water surface gets smaller, all the light will be reflected from the water surface back into the aquarium and none will be refracted out into the air. This angle should be about 40 degrees from the water surface for water and air. Smaller angles should have the light totally reflected back into the water. You will have to move the poster around on top of the aquarium to see the red spot, because the direction of the refracted moves dramatically with entrance angle.

Background Information

The many website references given in this activity serve as excellent background sources.

There are some fundamental rules to remember:

1. Light travels in a straight line.

2. Light intensity falls off as the square of the distance from the light source.

3. In light reflection, the angle of incidence always equals the angle of reflection.

4. In light refraction, light bends at the interface between two media. If we measure the angle from the perpendicular to the interface, the angle in the media with the higher index of refraction is the smallest.

5. In an interface between two media, light may be both reflected and refracted.

It is important for students to already understand reflection and refraction before doing this activity. Even though they should have already studied this in previous grades, they may have not retained much of their earlier understanding. If you feel they are unprepared, then it would be best to do a reflection and/or refraction activity before attempting the present activity. If you think a brief refresher would be satisfactory, then you can do that before beginning this activity. You may use your own resources for a refresher, which may consist of demonstrations or computer simulations, for example. (Remember that a website in place today may be gone tomorrow!) Some excellent websites have suitable material such as:

Reflection:

Refraction:

Light travels in a straight line. We can see this by observing shadows from the sun or from bright lights. We can draw a straight line from the light source to the edge of the obstruction and note that the edge of the shadow is along this straight line. We can think of light traveling in rays as if little bullets were moving along straight lines. This is the origin of the particle theory of light. We can explain both reflection and refraction using the particle theory of light. (Do activity with flashlight and holes in cards).

Light traveling in a straight line:

Total internal reflection occurs when light is passing through a medium (call it 1) like glass or plastic with an index of refraction n1. Outside this material is a medium like air (call it 2) with an index of refraction n2. For certain angles of incidence for light striking the interface and going from medium 1 to medium 2, light will not pass through the interface, but will be totally reflected inside medium 2. This occurs primarily when the light strikes a glancing blow on the interface. For a thin material of medium 2, like a fiber, these angles will always be small, and the light will stay inside the fiber. This process, called total internal reflection, leads to one of the most important technological applications of physical science, because most of our telecommunications now passes through fiber optic cables. These fibers can be made very small, and thousands of fibers can be attached together in a cable.

Total Internal Reflection:

Aswers to Data Sheet:

1. Yes, the holes are lined up in a straight line.
2. The light does not pass through when one of the cards is moved out of line.
3. Yes, light travels in a straight line.
4. You should not see light come all the way through the straw after you bend it.

1. The path of the light inside the test tube is a straight line.
2. Water molecules are too small to effectively scatter light. The powder provides larger mass that increases the probability of light scattering so that we can see the path of the light.
3. If you do not have too much powder in the milk, you may be able to see the laser light. Eventually the laser light will be scattered out of the test tube due to the powder.
4. The laser light can travel for many kilometers through clear plastic. We could not see the path of the light traveling through the plastic, because it would not be scattered. It would only be imperfections in the material that would scatter the light.
5. This should be able to be done. The laser light will be at an greater angle than when it was totally reflected.

1. The stream of water has light inside it. The light breaks up with the water as it hits the water collector.
2. Light reflects off the inside of the can and comes out the hole. The light reflects off the inside of the water stream. The angles are so large that it is improbable that the light refracts out of the water. This is the effect of total internal reflection.
3. This is what happens with optical fibers where light passes down long thin plastic fibers.

### Student Activity

I) Does Light Travel in a Straight Line?

Materials

• flashlight
• 4 - 3" x 5" index cards
• means of supporting the index cards
• flexible soda straw

Procedure

1. Punch a small hole in each of the index cards at precisely the same position.
2. Stick each card into a white support to hold the card upright, or fold the card at one end and stand it up leaning on that fold.
3. Place the cards about 15 cm apart with holes in a straight line.
4. Shine the flashlight so that the light travels through the hole in each card.
5. Move one of the cards a little and observe if the light passes through.

6. Now try looking through the flexible straw while it's straight at a light source at eye level.
7. Bend the straw and look at the same source.

Questions:

1. Look down the cards when the light is passing through all the cards. Are the holes lined up in a straight line?

2. What happened when you moved one of the cards a little?

3. Can we conclude that light travels in a straight line?

4. Did you see light come all the way through the straw after you bent it?

II) Light in a Test Tube

Materials

• long test tube (the longer the better)
• laser pointer
• powdered milk or a few drops of liquid milk
• water in container so water can be poured into test tube

Procedure

1. Take a clean test tube and put a small amount of powdered milk in it (only a pinch). Fill the test tube with water and shake to mix up the powder and water.
2. Make sure the outside of the test tube is clean and dry.
3. You will be using the laser pointer to shine into the test tube. MAKE SURE THAT YOU NEVER SHINE THE LASER INTO YOUR OWN OR ANYONE ELSE'S EYES! Click here for laser pointer safety precautions.
4. Hold the laser pointer at the bottom of the test tube and shine it up through the test tube. The powder in the water has large enough particles that the laser will scatter light into your eyes so that you can see the path of the laser light through the test tube. If you have time try shining light through a test tube of clear water. What do you think you will see?
5. Now rotate the laser so its light reflects off the side of the test tube. What do you see? Have one of your group members put a finger near the test tube where the light hits the test tube. Do you see laser light on the finger? If you do, try to shine the laser at a smaller glancing angle onto the side of the test tube. Eventually all (or almost all) the light will be reflected. THE INTENSITY OF LASER POINTERS IS SMALL ENOUGH THAT IT WILL DO NO DAMAGE ON YOUR SKIN, BUT DO NOT SHINE THEM IN ANYONE'S EYES.
6. Try to shine the light on the sides so that it reflects several times before coming out of the top of the test tube. You have demonstrated total internal reflection.
7. If the light disappears inside the test tube, you have most likely added too much powdered milk. Pour this out and start over. If you don't see the laser light inside the test tube, you need to add a little more powdered milk.

Questions:

1. Can you conclude that light travels in a straight line inside the test tube?

2. What is the effect of the powdered milk? Why is it needed?

3. What do you think would happen if the test tube were ten times longer? Would you still be able to see the zigzag path of the light? Why or why not?

4. Why would a thin, clear plastic fiber be better than the test tube with water and powdered milk? Would we be able to see the path of the laser light in the plastic?

5. Were you able to shine the laser inside the test tube at an angle such that a lot of the light came outside the test tube? If so, draw a diagram showing the angle of the laser light path through the test tube.

III) Water Stream of Light

Materials:

• flashlight
• soda can
• tape
• nail
• water
• pan to collect water
• can opener

Procedure:

1. Use a can opener to take off the lid of the soda can that has the pull-tab. Clean out the inside of the can.
2. Use the nail to make a hole in the side of the soda can near the bottom of the can. It is important that this hole have very smoot edges so that the stream of water is not broken up.
3. Put a piece of tape over the nail hole. Fill the can almost full of water.
4. Use the nail to make another hole in the can toward the top as an air hole to allow the water to flow more freely.
5. Place a flashlight on top of the open can with the light pointing towards the bottom. It might be helpful to tape the flashlight in place using black electrical tape or duct tape.
6. Place the can over a dish or bucket to catch the water.
7. Turn out the room lights and turn on the flashlight
8. Hold the can above the collector and take off the tape letting the water pour out slowly into the dish. If you have difficulty seeing the stream of light lower the can so the stream is short and gradually lengthen it. Or put your hand in the path of the water and move it along the stream to watch the light spot on your hand.
9. Observe carefully the water coming out of the soda can.

Questions:

1. What do you observe happens in the stream of water?

2. Why does the light appear in the water stream?

3. Can you think of any application of this technique?

Extensions

A very nice and useful extension to do is to allow students to handle and use fiber optics. You can obtain a sampler kit of fiber optics from Edmund Industrial Optics (Optical Grade Fiber Sampler, Catalog #K53-883 (year 2000) for \$31; 101 East Gloucester Pike, Barrington, NJ 08007-1380, 800-363-1992). The kit contains a range of sizes and lengths of fibers that have outer covering (called jacketed) and nonjacketed. The students can shine flashlights into one end and see light out the other end even when the fibers are turned at angles. They can place them on top of overhead projectors and see light shining through. They can also shine a laser pointer into the end, and it should come out the other end. They must be careful when using laser pointers to not let either the direct light from the laser pointer or the light coming out of the fiber optic to shine in anyone's eyes. It is fun to place various color filters over the flashlight and see that the colors will pass through the optical fibers. Color plastic can be obtained from art supply stores.

Students with Special Needs

Some students may need help in handling the fish tank, putting water and creamer in, and in handling the laser pointer. Safety is very important in using the laser pointer. Teachers may want to do the laser pointer activity as a demonstration.

Click here for further information on laboratories with students with special needs.

Assessment

The students should each write down what they observe in these activities on a Data Sheet and discuss their answers to the questions. It is okay for the students to discuss the questions and answers, but the students should write out the answers in their own words.