Ricocheting Lasers

This experiment will help students understand the importance of the critical angle and the index of refraction.

Materials

• Clear Jell-O
• A protractor
• Square, open-topped box made of clear plastic
• Petri dish
• laser

Directions

1. Make Jell-O according to directions.
2. Fill the petri dish and the plastic box half and inch full with Jell-O and cool in the fridge.
3. Hold the laser parallel to the table and shine it through one side of the plastic box. Start with the beam perpendicular to the edge of the dish and notice how the laser passes straight through the gelatin.
4. Rotate the laser so the beam hits the flat edge of the dish at an angle.
5. Use the protractor to measure the angle the laser is to the dish.
6. Measure the angle of the light in the box with respect to the plat edge.
7. How do these two angles relate?
8. Now shine the laser through the middle of the petri dish.
9. Move the laser to the right and left, noticing how the angle of the light changes.
10. What’s going on?

Explanation

Light enjoys moving freely through substances, but when it leaves the easy to move through air and enters a substance that slows it down, it will refract. Refraction is the bending of light. In this case, the light is bending towards the middle of the gelatin because Jell-O is harder to move through than air. If the light were coming from the Jell-O and going into the air, it would do the opposite.

The angle of the laser with respect to the box is called the incident angle. On the other hand, the angle which the light exits is called the critical angle. They are related through Snell’s law which states that  The “n” is the index of refraction, or the difficulty of movement through the substance.