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Gelatin can be used for much more than a sweet treat. It can also act as a smoked lens—which allows you to view total internal reflection—or as a color filter.
Hold the red laser flat against the table so the light beam is parallel to the table. Shine the laser through the middle of the round dish of red gelatin—a beautifully visible beam travels through it. Then shine the laser through the blue gelatin. Notice that the beam gets dimmer almost as soon as it hits the gelatin. Shine the laser through the dish of clear gelatin. Notice that you can see the beam very clearly
Shine the green laser through the round dish of blue jello and observe the beam as it travels through the jello. Shine the green laser through the red jello and notice that the beam gets dimmer as soon as it enters the jello.
Hold the laser parallel to the table and shine it through one side of a square dish of red or clear gelatin. (Use the red laser for the red gelatin; use either the green or red laser for the clear gelatin.) Start with the beam perpendicular to the edge; notice that it passes through the gelatin in a straight line. Now rotate the laser so that the beam hits the flat edge of the dish at an angle. As you do so, notice how the beam bends towards the center of the dish. Use the protractor to measure the angle of incidence between the beam and a line perpendicular to the flat edge, and the angle of refraction after it enters the gelatin.
Next, take the round Petri dish of blue gelatin. Holding the green laser parallel against the tabletop, shine the laser through the middle of the curved edge of the dish (it should look as though the laser is bisecting the circle). Now, starting from the laser's original position, slide the laser in a straight line to the right and then the left, so that the beam moves toward the outer edges of the dish. Notice how, as you do so, the beam bends towards the center of the dish. (Note: Hold your two lasers parallel to one another and shine both beams through the curved edge of the dish, and then use a piece of white paper or waxed paper as a screen to find the focal point where the two beams cross—see the What's Going On? section below for more information.)
Finally, take a square dish of red or clear gelatin. Holding either laser parallel to the table (use the red laser for the red gelatin; use either the red or the green for the clear gelatin), shine the beam through one edge of the box and notice the beam coming out the far side. Now, shine the beam through one side of the box so that it hits the adjacent side at a glancing angle—you'll notice that no light exits the box through that side! (You may need to play around a bit to find the right glancing angle.) The largest angle at which no light escapes is called the critical angle.
Gelatin is colloidal—its large molecules are suspended in solution in such a way that they don't settle out—and so it scatters enough of the laser beam to make it visible. Red dye in the red gelatin doesn't absorb red light, so you can see the red beam when it shines through it. The blue gelatin (which is actually cyan) absorbs red light (but not blue or green), so the red beam isn't visible.
As light enters the gelatin, the change in medium causes a change in the speed of the light and a change in the index of refraction. This change in speed causes the direction of the beam to refract, or bend. When going from a high-speed material such as air to a lower-speed material such as gelatin, the beam will bend into, or towards, the gelatin.
Light traveling through a convex lens will converge. If you shined two parallel beams through your gelatin, you saw that parallel beams of light will come together at a point on the far side of a curved lens (in this case, the curved side of the dish)—this is called the focal point. (Teachers! This might be a good time to introduce the reversibility of light. Shine a laser through your gelatin "lens"—mark its path into and then out of the gelatin. 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.)
As light travels from a slower (or more optically dense) substance to a faster medium, it may reflect in a similar way 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 completely reflect—this is called total internal reflection. If the angle is closer to perpendicular, then the beam will exit out the side of the dish.
As it moves from a higher-speed medium to a lower-speed one, a ray of light will behave similarly to the way a car behaves as it moves from paved asphalt to soft dirt or loose gravel. The car will change direction by angling into the gravel, the same way a beam of light will refract by turning in a similar direction towards the lower-speed material.
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Attribution: Exploratorium Teacher Institute