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# The Index of Refraction and Snell’s Law

Media Type:
Video

Running Time: 4m 04s
Size: 12.1 MB

or

Source: The National Center for Information and Communications Technologies

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In this video from the ICT Center, learn about the index of refraction and Snell’s law. Review a simple example of refraction, the speed of light in materials, and the formula for calculating the index of refraction. Explore a table of the refractive indices of common media and materials used in fiber optics and semiconductors. Finally, examine Snell's law to understand the relationship between incident and refracted angles.

Discussion Questions

• What does light do as it enters a denser material?
• How would you express the relationship between n1 and n2 when light moves from a less dense to a more dense material?
• If light travels from a material with a larger n into a material with a smaller n, how does the light bend?
• How does Snell's law apply to fiber optics?
• What is the important angle to achieve in order to have light pass through an optical fiber?

Teaching Tips

Here are suggested ways to engage students with this video and with activities related to this topic.

• Beginning a lesson: Fill a small, clear plastic or glass tank with water. Shine a laser pointer through the water to demonstrate that the beam of light is difficult to see without an additive. Add a few drops of milk (or grains of milk powder) and stir so that the water is slightly cloudy. Direct the laser pointer into the water again and slowly change the angle at which the beam strikes the water. Which way does the beam bend? Ask students to observe the incident and refracted angles. Note: A slight water mist, baby powder, or chalk dust in the air over the tank makes the beam visible above the water's surface.
• Viewing the video: Use the following suggestions to guide students' viewing of the video:
• Before: Review with students how angles are measured in optics. Draw a diagram showing a surface and the normal (perpendicular) line, and indicate how the incident and refracted angles are measured.
• During: Ask students to pay particular attention to how the angles of incidence and refraction are measured. What does it mean to measure angles from the normal? How does the direction of the final beam segment compare to the direction of the initial beam segment?
• After: Ask students how they would react if someone handed them a piece of glass and told them that the index of refraction was less than one. What would that mean? If a student calculated index of refraction values of less than one using Snell’s law, what would the most likely error be?
• Doing a classroom activity: Provide students with cross-sectional drawings of biconvex and biconcave lenses. Indicate on the drawings the optical axis through the center of each lens and two rays of light entering the lens parallel to the axis (draw one ray above the axis and one ray an equal distance below the axis). Ask students to predict what will happen to the rays in each case. Do they bend toward or away from the normal as they encounter the surfaces of the lenses? After students have made their predictions on the drawings, show the Refraction of Light Demonstration.

Standards

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