Teaching Notes

Suggest that the students use a file card to follow the beam after it hits the glass rod. If the students say that they have difficulty seeing the refracted beam as they approach the critical angle, point out that less and less of the beam is refracted and more and more is reflected (as they approach the critical angle). You can introduce Steps 7 and 8 by pointing out that Steps 1 through 6 produced relatively small angles of incidence inside the gelatin block (as the light approached the gelatin-air surface). This occurs because the light is bent toward the normal upon entering the block, and the angle of refraction on the front surface is equal to the angle of incidence on the rear surface. However, in Step 8, the angle of incidence inside the block is the complement of the angle of refraction in the block, so a small angle of refraction entering the block makes a large angle of incidence leaving.

If the students use lasers, be sure they understand the importance of keeping their laser beam within their group’s work area (by building some sort of backstop).

If your class makes observations with laser beams, have each group build backstops of cardboard to absorb the beam. Also, explain to the students that they must not eat the gelatin. Keep the gelatin blocks covered and refrigerated before use. If your light source is a candle, have the students remove any loose papers from the area around the candle.

The concept that reflected light fans out from each point on an object is extremely difficult for students. Consequently, when asked to explain examples of refraction, they typically make a broad statement like “The water bent the light,” rather than considering the path of light reflecting from the object in the water. For this reason it is particularly effective to have them trace the path of the laser beam. Asking a qualitative question, like “How does the laser beam bend when it goes from air into the gelatin,” can help students organize their observations and contrast what happens in going from air to block with what happens going the other way.