# How to use a laser to measure the speed of light

Written by john brennan
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The speed of light is a constant. In a vacuum, it is always the same. In translucent materials like water or glass, however, light travels at slower speeds. Different frequencies of light can also travel at different speeds in the same material. To measure the speed of light accurately, you need sophisticated tools. You can, however, get a rough estimate of its speed using some simple materials, a laser and a mathematical formula.

Skill level:
Moderate

### Things you need

• Laser pointer(any colour)
• Ruler
• Block of glass
• Block of ice
• Gelatin powder
• Water
• Plastic container
• Protractor
• Calculator

## Instructions

1. 1

Mix the gelatin powder in water to make clear jello. Follow the instructions on the back of the package (different brands may have different water-to-gelatin ratios). Add the jello mix to the plastic container. Put it in the refrigerator and allow to solidify.

2. 2

Place the laser pointer at an angle to the plastic container and use the ruler to mark the direction of the laser pointer. The laser beam will not be visible in air but inside the jello it will be visible.

3. 3

Use the protractor to determine the angle between the normal (an imaginary line running perpendicular to or straight out of a surface) from the plastic container and the direction of the laser beam and record this value. Now measure the angle between the normal and the laser beam inside the jello. The two angles should be different.

4. 4

Determine the index of refraction in jello using the Snell's Law equation, n1 ( sin θ1 ) = n2 ( sin θ2 ). n is the index of refraction, the speed of light in vacuum divided by the speed of light in a particular medium. The index of refraction for air is 1.000293, meaning that the speed of light in air is very close to the speed in vacuum. Rearrange the Snell's Law equation so that n1 = ( n2 sin θ2 ) / sin θ1, where θ1 is the angle between the normal and the beam in the jello, θ2 is the angle between the normal and the beam where it enters the plastic container, and n2 is the index of refraction for air. Use the values you recorded above to calculate n1.

5. 5

Divide the speed of light in a vacuum (roughly 186,282 miles per second) by the index of refraction for jello to determine the speed of light in jello.

6. 6

Repeat the procedure to determine the speed of light in other materials like glass, ice or water. Use Snell's Law to figure out the speed of light in the medium, if you can see the path of the beam of laser light in the material and measure the angles.

#### Tips and warnings

• Never aim a laser pointer directly at your or anyone else's eyes.

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