How to Measure the Speed of Light in JELL-O With a Laser Pointer

Refraction doesn't just occur with high-tech equipment.

The speed of light changes depending on what material it is traveling through. It travels fastest in a vacuum where there is no air and slows down in transparent substances such as water, glass or JELL-O. The speed of light in JELL-O can be measured at home using simple techniques, and you can still eat the JELL-O afterwards.

Things You'll Need

• Laser pointer
• Red JELL-O ingredients
• Pizza cutter
• Protractor
• Graph paper
• Scientific calculator

Instructions

Make the JELL-O

1. Mix your JELL-O ingredients according to the packet instructions.

2. Pour 1 inch of the JELL-O mixture in a plastic bowl or tub and leave it to set in the refrigerator.

3. Remove the JELL-O from the mold and cut into a square with a pizza cutter.

Take Measurements

1. Draw a 3-inch straight line on the graph paper using the faint lines as a guide.

2. Put the JELL-O square onto the graph paper so that it half covers the line.The line must be at 90 degrees to the surface of the Jell-O's side. This line is called the "normal."

3. Angle the laser beam so that it hits the JELL-O at 45 degrees at the point of the line. Use the protractor to find the angle needed.

4. Observe how the laser beam bends at the surface of the JELL-O and again when it comes out the other side. Mark the point where the beam exits the JELL-O.

5. Switch off the laser and remove the JELL-O from the paper. Connect the point where the laser entered the JELL-O and its exit with a straight line.

6. Measure the angle between the beam path and the "normal" line. This angle is called the angle of refraction (r). The beam entered the JELL-O at 45 degrees. This angle is called the angle of incidence (i).

7. Divide the sine of the angle of incidence by the sine of the angle of refraction to determine the refractive index.

   Refractive index = sin(i)/sin(r)

8. Divide the speed of light in a vacuum, 299,792,458 m/s, by the refractive index to find the speed of the light that passes through your JELL-O.

Tips & Warnings

• Use red JELL-O for a red laser pointer.

• The square of JELL-O should be smaller than the width of the graph paper, preferably measuring no more than 4 inches on each side.

• Be careful not to point the laser into anyone's eyes as laser beams can damage the retina.

• JELL-O will eventually set at room temperature. However, if you choose to eat the JELL-O after the experiment, it should first be refrigerated to set.

References

• Physics.org: Refraction
• Encyclopedia Brittanica: Refractive Index