University of Virginia
Physics Department

## Refraction

A Physical Science Activity

2003 Virginia SOLs
• PS.1
• PS.9

Objectives

Students will

• observe that light travels in a straight line;
• observe the bending of light as it enters a new material;
• measuring the angle of refraction of light between air and water;
• use skills such as: observing, inferring, measuring, and isolating variables.

Motivation for Learning

Discrepant Events

The Bent Rod
Materials
• Wood dowel or Metal rod
• Fish tank (nearly filled with water)

Procedure

1. Place the wood dowel or metal rod in a fish tank that is nearly filled with water.
2. Let the dowel rest such that most of it is inside the water. The portion of the dowel that is in the water will appear enlarged and distorted due to the refraction of the light in the water. Explain to the students that the light reaching their eyes from under the water is bent relative to the light reaching their eyes from outside the water.

The Disappearing Penny

Materials
• Plastic cup
• Penny

Procedure

1. Place an empty cup on the table and drop a penny in it. Ask a student to come up and look down into the cup so that he can see the penny.
2. Then have him move back away from the cup slowly until the penny just disappears from his sight.
3. Slowly pour water into the cup and the penny will come back into sight.
4. Explain that this is due to the refraction of light.

Explain that in the following experiment students will be trying to measure how much light bends when it enters one substance from another.

Background Information

The fact that light bends when it moves from one material to another is a strong indication of its wave properties. Light will travel in a straight line as long as it is moving through one medium. However, when it is leaving one material and entering another at an angle, it will bend. If a ray of light travels from a less dense medium to a more dense medium (traveling from air to water), it will bend toward the normal (perpendicular line to the boundary of the two materials). If it is leaving the denser material into the less dense material, it will bend the other way, away from the normal.

Bending occurs because the speed of the light ray or wave is retarded in the new material. A portion of the wave enters the new material before the rest of the wave does and is slowed down, while the remaining portion of the wave that has not entered the new material is still traveling at the original speed. Part of the wave will be traveling at the original speed and the other part of the wave at a slower speed. This causes the wave to bend as it enters the new material.

### Student Activity

Materials

• Transparent, rectangular container at least 4 cm deep, or rectangular glass plate
• Cardboard at least 10 cm longer and wider that the plastic container
• 4 straight pins
• Ruler
• Protractor

Procedure

1. Place water in a transparent rectangular container to a depth of 3 cm.
2. Place the container on the cardboard and mark the outline of the container on the cardboard.
3. Place a pin in the cardboard behind the container. The pin should be as close to the container as possible.
4. Place a second pin 4 to 5 cm behind the first. The two pins should form a straight line which should form an angle of less than 45° with the side of the container.
5. Bend down in front of the container, so that you are looking at the two pins through the water in the container. Place a third pin in front of the container close to it. This pin should appear to line up with the two pins behind the container.
6. Place a fourth pin 4 to 5 cm in front of the third pin so that it lines up with the other pins. Readjust the positions of any of the pins so they all appear to be in a line when you look through the water in the container.
7. Carefully remove the container of water, leaving the pins in their position.
8. Use a ruler to draw a line connecting the positions of the two pins that were behind the container. Repeat this procedure for the two pins that were in front of the container.
9. Draw a third line inside the area were the water container was located connecting the other two lines. This line represents the path of the light ray through the water.
10. With a protractor, measure the angle between the two sides of the container and the lines connecting the two pins that were behind the container.
11. Repeat step 11 for the line connecting the two pins in front of the container.

Data Sheet

 Angle Measured Measurement of Angle Angle between pin line and the front side of the container Angle between pin line and the back side of the container Angle between the line inside the tray connecting the two pin lines and the side of the container

Questions:

1. Did the light bend toward or away from the side of the container as it entered the water?

2. Did the light bend toward or away from the side of the container as it left the water?

3. Were the angles measured in steps 10 and 11 equal?

4. How did the direction of the light ray entering the water compare to the direction of light leaving the water?

5. A light ray enters a piece of glass from air at an angle of 35 degrees. Use the diagram below to answer the questions.

a. At what angle did the light ray leave the glass?

b. Draw the line inside the glass representing the path of light inside the glass.

c. Are the angles between the light rays and the surface of the glass smaller inside or outside the glass?

d. When the angles between the light rays and the surface of the glass become larger, does that represent light speeding up or slowing down?

1. After the light entered the container it bent away from the container's surface.

2. As the light left the container, the ray bent toward the surface of the container making the angle outside smaller than the angle inside.

3. The angles measured should be equal.

4. The directions should be the same.

5. a. 35 degrees, c. The angles are smaller outside. d. Larger angles represent the slowing down of the ray.

Extensions

Repeat this investigation using a glass triangle instead of a rectangular container of water or glass plate. Measure the angles of the incident rays and the refracted angles as you did in this experiment. How do these results compare to this experiment? How are the angles of incidence and refraction different? Form a hypothesis to explain the differences.

Students with Special Needs

Some students may have difficulty manipulating small objects (like the pins). They may work with partners or in small groups.

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

Assessment