University of Virginia
Physics Department

Total Internal Reflection

A Physical Science Activity

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?