Demos: 7A-21 Refraction
A transparent, semicircular cell is mounted at the center of a circular board (concentrically). A light source with a slit is mounted so that it can move along the perimeter of the board, thus insuring that the beam will be normal to the curved surface of the cell. Both ordinary refraction and total internal reflection can be demonstrated.
Directions: For optimal effect, dim the room lights. Turn on the lamp and make any adjustments necessary to achieve a thin, visible beam of light. Make sure the light beam is aimed directly at the center of curvature of the cell, otherwise the beam will not strike the cell perimeter at a 90 degree angle. Start the beam at the bottom so that there will be no refraction. Slowly move the lamp around the circle. At some point, you will reach the critical angle and the beam will be entirely reflected back into the cell. Move on past 90 degrees and the light will strike the cell on its flat surface. This is the case for light moving from air into water.
Suggestions for Presentation: After you have shown that the beam refracts at the flat surface, point out (if a student doesnt first) that there was no refraction at the perimeter of the cell. Discuss the fact that the light beam is traveling along a radius of the circle which always meets the perimeter at right angles. Thus, this set up (light from below) simulates light coming entirely from within the cell. We suggest laying a straight edge or otherwise produce a vertical line passing through the center of curvature of the cell (see diagram). This will allow the students to see more clearly the relationship between the incident and refracted angles. If you are exploring total internal reflection, you can also point out that this effect occurs only when the refraction takes place under the conditions that the beam is passing from water into air, not the other way.