Next, direct the diode laser beam through one wall of the tank, up toward the water surface. See sketch D-2. Experiment with the laser beam direction until no light emerges at the water-air surface and the beam is seen to be totally reflected back into the water. The incident angle at the water-air interface is now larger than the critical angle. This phenomenon of total internal reflection is used to trap light in fibers.
3. Focusing Parallel Light Rays with a Thin Lens.Set up a positive thin lens, several inches in diameter and of focal length around 3 inches, on an optical bench. Arrange two diode laser pointers, on stands, so that they send parallel beams onto the front surface of the lens, near its outer edge. See sketch D-3. Lower the room lights and use chalk dust as in Demonstration 1 to illuminate the beams on the imaging side of the lens. The distance from the lens to the point where the beams cross is the focal length of the lens. Repeat with a negative lens of the same diameter and focal length, sketch D-4. What do the beams do? Where is the focal point?