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. 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. Notice how the beam is bent, or refracted, as it enters the gelatin. 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 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: If you have two lasers, you can hold them 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 the laser parallel to the table, 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.