Refraction: How Fast Does Light Travel in Air or Water

The Idea

Light travels at its top speed in a vacuum and at (nearly) its top speed in air. When light moves through other transparent materials, it slows down. If it goes from one material to another at an angle, the light will bend. The more it slows down, the more it bends. In this project you will compare how much light bends in various materials. This bending is called refraction and it gives us a way to determine how fast light travels in a transparent material.

What You Need

• Square or rectangular piece of glass about ¼ inch thick and a few inches in length and width (at least two opposing sides must be clear)
• Laser pointer
• Semicircular plastic container filled with water
• Protractor

Method

Laser

1. Place the piece of glass on the paper.
2. Trace the shape of the glass.
3. Darken the room.
4. Put a dot on one side of the glass to provide a target for the laser.
5. Draw a line perpendicular to the edge of the glass at that point. Extend the line, so it extends under the glass, as well as going into it.
6. Place the laser, so its beam forms an angle with respect to the perpendicular line you just drew. Mark the position of the laser.
7. Darken the room.
8. Shine the laser and direct its beam toward the target dot. Angle the beam vertically, so you can see its path both before entering and after exiting the glass. (It is OK if you don't see the entry and exit beams at the same time.)
9. Place a dot where you see the laser beam exit from the glass and one or two dots along its path.
10. Connect the dot where the light strikes the glass with the point where the light ray emerges from the glass back into the air.
11. Measure the angles that:
    • the incoming ray made with the perpendicular line (θⁱ).
    • the ray going through the glass made with the edge where the light entered the glass (θ_r).
12. Try this with other transparent materials such as water (in a plastic case).

Expected Results

Going from air into water, the light path is bent to give a smaller angle with respect to the perpendicular line. Specific results are given in the following chart:

Why It Works

The relationship between the incident and refracted angles is given by Snell's law, which states:

n_i sin(θ_i) = n_r sin(θ_r)

where n_i is the index of refraction where the ray is incident and n_r is the index of refraction where the ray is refracted. Both are measures of the speed of light in the various materials, The index of refraction for any material is given by n = c/v. The physical arrangement for this is shown in Figure 85-1.

The index of refraction for air, which is 1.0, indicates that light is traveling at its maximum speed.

Because light slows down in glass, n for glass is 1.45.

Other Things to Try

Find the index of refraction for the glass or water using the equation:

n_isin(θ_i) = n_rsin(θ_r)

using ni for air = 1.0 and = θ_I and θ_r as defined in the previous step to find the index of refraction in the material of interest. The index of refraction is given by:

n = c/v

The index of refraction is a measure of the speed of light in a particular material, v, compared to the speed of light in a vacuum which is given by c = 3.0 × 10⁸ m/s.

The speed of light in a particular medium is given by v = c/nr.

The Point

Light travels at a slower velocity when it goes through materials other than vacuum or air. When light hits a boundary going from a material where the light is faster to one where it is slower, the light bends toward the perpendicular line.

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125 Physics Projects for the Evil Genius
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