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The Index of Refraction

In this media-rich lesson plan, students explore the refraction of light at the boundary between materials: they learn about the refractive indices of various materials and measure the index of refraction of plastic or gelatin.

Overview

Refraction is an important behavior of light that can be used to explain the operation of lenses, prisms, and optical fiber, as well as natural phenomena such as rainbows and mirages. The index of refraction, or refractive index, is defined as the ratio of the speed of light in a vacuum to the speed of light in a material. The index of refraction is an important property of optical materials and one that can be easily measured in the laboratory.

This lesson begins with a video that introduces the concept of refraction. Students learn firsthand how when a wave of light travels from one medium to another, the change in the wave's speed leads to a change in its wavelength and the bending of the wave. Next, students investigate index of refraction through an interactive media asset, research, and a class discussion. Following a video about the index of refraction and Snell's law, students work in groups to conduct their own laboratory experiment to measure the index of refraction of gelatin (or plastic).

Note: This is the first of two optics lesson plans. You may want to follow this lesson with the Fiber Optics Lesson Plan.

Objectives

• Explain why the index of refraction is one of the most important optical properties of a material
• Describe why and how light bends when it speeds up or slows down as it enters a second medium
• Explain the relationship between angles of incidence and angles of refraction
• Demonstrate how to measure a material's index of refraction using Snell's law

Suggested Time

• Two class periods

Materials

Computers with Internet connection

For each group:

• Science notebooks
• Laser pointer
• Ruler, 12-inch
• Protractor
• Calculator
• Plastic refraction kits. If plastic kits are not available, use slabs of very stiff gelatin made from unflavored gelatin powder. For this, you will need:
• Unflavored gelatin powder
• Boiling water
• Bowl and spoon
• Cooking oil spray
• Flat-bottom pan
• Plastic knife to cut gelatin
• Curved cookie cutter or 4-cm-wide strips cut from a plastic folder (to make curved lenses from gelatin)

Before the Lesson

• If possible, arrange computer access for all students to work individually or in pairs.
• Gather all materials.
• If plastic refraction sets are not available, students can measure the index of refraction of gelatin. Unflavored gelatin powder can be purchased in grocery stores. Prepare the powder with half of the boiling water specified on the package and stir very well until all of the gelatin is dissolved. Spray a flat-bottom pan with oil and pour in the mixture. The gelatin should be at least 1.5 cm thick. Let the gelatin stand until firm; refrigerate until ready to use if the room is warm. Each group will need a rectangle about 6 cm by 8 cm.
• Reserve a large open area for the "people wave" activity. Divide the space in half using masking tape.
• Review the concepts of wavelength and frequency and how to represent wave propagation direction by a ray.

Part I: The Refraction of Light

1. Introduce the lesson by showing the Observing Refraction of Light Video. Stop the video before it explains Fermat's principle of least time. Ask students to give examples of reflection and refraction of light from their everyday experiences. Students often believe that reflection occurs only from shiny surfaces; remind them that most of the objects they see are visible because of reflected ambient light.

2. Ask students to try to explain in their own words why light waves bend when they enter a new material.

3. Have students demonstrate the behavior of light at a boundary between two materials by having them act out a "people wave." This illustrates how the changing speed and wavelength of a wave causes it to refract at the boundary.

1. You'll need a fairly large floor space for this exercise. Divide the space in half with a piece of masking tape. Explain that "waves" will travel faster on one side than on the other side.
2. Line students up on the "fast" side in several rows parallel to the tape. Explain that the rows are "wave crests" and that the distance between rows is the "wavelength." Since frequency remains constant, students must take steps at a regular rate. Choose one student to provide a frequency cue, such as hand clapping.

1. Ask students to walk forward, keeping rows straight and even. When each row reaches the tape, the wave speed slows; that is, the line of students moves more slowly after crossing the line. Because frequency (step rate) is constant, students must take shorter steps in order to move forward more slowly. Ask students to observe what happens to the wavelength (distance between rows) when the wave speed slows. This may take a few practice tries to work!
2. After students conclude that wavelength decreases when wave speed slows, ask them to predict what will happen to the wave crests if the wave crosses the tape boundary at an angle rather than straight on. Once they have agreed on a hypothesis, form the wave crest lines again, but this time so that the rows are at a 45-degree angle to the tape boundary. Again, keeping frequency (step rate) constant, ask students to move forward, decreasing speed (step size) as they cross the tape. What happens now when the wavelength shortens? Which way does the line of students bend?

Part II: Index of Refraction

4. Discuss the concept of index of refraction. Remind students that a high index of refraction means a low light speed. Light is slowed in a material due to interactions between the electric field of the light wave and the atoms that make up the material. Be sure that they understand the meaning of the index of refraction value. Have students explore The Light Stuff Interactive on their own or in pairs.

5. In small groups, have students research the index of refraction for a variety of common materials including air, plastic (acrylic), and water. Ask each group to find the values for 5 to 6 items, and use these to create a combined table for the class.

TIP: This is a good time to discuss Internet research and how students can judge the accuracy of the data they find online: finding the same values from several different sources is a good indication that the values are correct. Students should also note if the source provides the wavelength at which the measurement was made. The index of refraction will vary somewhat at different wavelengths.

6. Lead students in a discussion about the index of refraction values that they found.

1. Do they notice any trends?
2. How would the index of refraction change going from solid to liquid to gas?
3. Which substances have a high index of refraction?
4. Which substances have a low index of refraction?

Part III: Measuring the Index of Refraction with Snell's Law

7. The index of refraction of a material can be determined by measuring the refractive angle for different incident angles. Have students watch the The Index of Refraction and Snell’s Law Video. Ask students to pay particular attention to how the angles are measured. The angles of incidence and refraction are always measured from the normal line to the incident or refracted ray.

8. Have students work in small groups to conduct their own experiments to determine the index of refraction of a piece of plastic or gelatin. For each group, provide the Measuring the Index of Refraction Activity Document worksheet and the materials that will be needed.

TIP: If students are not ready for the calculation with Snell's law, they can gain important conceptual knowledge by observing the relative sizes of the incident and refracted angles.

9. Ask students to describe what happens when light travels from plastic or gelatin into air. Is the refracted angle (in air) larger or smaller than the incident angle (in plastic or gelatin)?

Check for Understanding

1. Have students discuss the following:
1. What is the meaning of the index of refraction value, or refractive index, of a material?
2. In which direction does light bend going from a medium with a low refractive index into a medium with a higher refractive index?
3. When would refraction NOT occur at a boundary between two materials?
2. Ask students to explain what happens when horizontal beams of light pass through various lens shapes. If plastic refraction shapes are not available to check answers, the shapes may be cut out of gelatin. Alternatively, students may explore the Refraction of Light Demonstration Interactive

TIP: To cut curved surfaces in gelatin, use a cookie cutter or a strip cut from a plastic folder that you bend into a curve. Push straight down firmly for the cleanest edge.

Educational Standards

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