To Do and Notice
- Shine the laser through the red gelatin first. Notice that a beautifully visible beam travels through it. Then shine the laser through the blueberry gelatin.
- Using the red gelatin and the curved side of the dish, shine the laser parallel to the central axis. Move the laser from the center to the outside edge. Notice the change and measure the angle of refraction.
- Turn the dish around. Shine the laser perpendicular to the straight edge. Move the laser to the outside edge so that the beam reflects off the surface of the gelatin and is re-emitted parallel to the incoming beam. Repeat the activity, bouncing the beam off the straight side, to measure the critical angle.
- If you have two lasers, align them parallel and shine them through the curved side of the dish. Use a white piece of paper or waxed paper as a screen to find the focal point. Invert the gelatin lens onto a piece of waxed paper--it may need a little prying to get it to come out. Use the empty dish as a cookie cutter to make a concave lens out of the original convex gelatin lens. Shine parallel lasers through the new lens.
Use gelatin as a smoked lens, to view total internal reflection, and as a color filter.
D-shaped refraction or Petri dishes
One red-colored and one blueberry package or Jello™
A laser (preferably HeNe)
20 - 25 minutes
Make the gelatin according to the Jigglers™ recipe. Then, shine the laser through it.
Exploratorium - Science Activities
What's Going On?
- Red objects appear red because they don't resonate at red frequencies. The red wavelength energy is either reflected or transmitted. Gelatin is colloidal and scatters enough of the beam to make it visible. The blueberry Jello™ is actually cyan, a mixture of blue and green (check it on the overhead with a spectrum created by a diffraction grating). Cyan is the complementary color of red. The blueberry color transmits green and blue, but changes red wavelengths into heat.
- As light enters the gelatin, the change in medium causes a change in speed. A ray of light will refract similar to the way a car will change direction as it moves at an angle from asphalt to soft dirt or loose gravel.
- As light tries to travel from a slower (or more optically dense) substance to a faster medium, it may reflect similar to the skimming of a stone off the surface of water. If the beam hits at an angle that is small relative to the surface, then the light will reflect; if the angle is closer to perpendicular, then the beam will refract.
- Light traveling through the convex lens will converge. Light traveling through the concave lens will diverge. This might be a good time to introduce the reversibility of light. Shine the light through the Jello™ lens. Mark its path into and then out of the Jello™. Then shine a laser backwards along this path. That is, shine it into the path that the original beam exited. The light path followed by the reversed beam will be exactly the same.