• A flavored gelatin dessert (preferably a light color such as orange or lemon), made with water

• Square or rectangular clear glass baking dish

• A flashlight (preferably a focusable flashlight, which can be adjusted so that all the light is focused into one bright spot or spread evenly throughout the field of illumination)

1. Make the gelatin according to the instructions on the package, and pour into the glass baking dish.
2. The gelatin should be at least two inches deep.
3. Chill until firm.
4. Turn off the lights.
5. Place the dish on a level surface and shine the flashlight (tightly focused if possible) through one side of the dish.
6. Look from the adjacent side at the light through the gelatin.
7. You should see the beam passing through the gelatin and out the other side of the dish. You should also be able to see the flashlight’s bulb.
8. Move the flashlight so that the light enters one side of the dish and hits the adjacent side.
9. Looking from the top, you should now see the beam of light bounce off the glass and reflect back into the gelatin.
10. Look through the gelatin and see if you can still see the flashlight’s bulb.

Gelatin contains very large protein molecules-large enough to scatter light. The molecules scatter the light in all directions, and you can see the beam in the gelatin as much as you can see headlight beams in a fog. You can also see the angle the beam makes when it reaches the glass-air boundary and the critical angle when it is totally internally reflected. When the light is not at the critical angle, it passes through both the gelatin and the glass dish. If the light from the bulb of the flashlight is reflected off the side of the dish before it reaches your eyes, you will not see the bulb.

Want to Learn More? Read an article related to this activity:

• The Reflection of Light

• The Refraction of Light

To find this experiment and many more please read Light Action! Amazing Experiments with Optics by Vicki Cobb and Josh Cobb, illustrated by Theo Cobb. To purchase a copy click here.