## Geometrical Optics

• OP2.1 Brewster's Law

Here the vu graph is used as the source of light which is unpolarized. Using a piece of glass to reflect the light before it hits the mirror in the vu graph, any transmitted light will be polarized slightly. The angle A at which this is greatest obeys Brewster's law: tan A = n'/n, where n' is the refractive index of the second medium and n is that of the first medium, in this case air.

• OP2.2 Fresnel Lens

This is a method of making lenses thinner. Instead of having one thick lens, the lens is separated into a series of cuts. Each cut has the same radius of curvature as the original lens, but is located at the surface of the lens. This makes the lens thinner. The accuracy of the focal length has a greater error, but if accuracy is not very important, this is a good way to reduce the thickness of a lens.

• OP2.3 Lenses: Plane, Convex, Concave

This is a good demonstration of lenses. A box is screwed to the blackboard that has a light source. This source puts out five rays of white light via an arrangement of slits and mirrors. The direction of the rays can be changed by turning the knobs on the box. Using different lenses, the class can see that convex lenses are converging and concave ones are diverging. Arrangements of these can be made with the vast assortment of lenses (C53-4). Turning off the board lights enhances the audience's view. Another version is a laser used with lenses held in a circular apparatus. As the laser scrapes the edge of the lens, the different paths are easily seen. In order to understand the reason convex lenses converge and concave diverge, use a laser (locked cabinet), a water bath, and the hallow lenses in C64. The two sides of the lenses are separated by air and not glass, thus creating a transition from a higher index of refraction to a lower and then back to a higher, which is opposite of glass lenses in air. The convergent properties of these lenses are opposite to that of ordinary lenses.

• OP2.4 Light Pipe

This shows the bare basics of fiber optics. It consists of a base with a light source and a piece of plexiglass that is inserted into the base. The base is designed so that the light travels only into the pipe. The pipe bends into a spiral. The pipe ends at the center of the spiral as it bends and faces the class. The light shines through only at the end of the pipe.

• OP2.5 Mirrors

Two big mirrors are attached via hinges. By placing the light source in between the two, multiple reflections can be seen, their number varying with angle. We also have the convex and concave mirrors to be used with the board lights (see lenses above).

• OP2.6 Prisms

Passing a white light through a prism will shine the spectrum of the light onto a screen pulled down for this purpose. Dimming room lights might help. Also, filling the plastic prisms with carbon disulfide will produce the a spectrum of much greater dispersion.

• OP2.7 Total Internal Reflection

A prism is placed on the optical disk, a vertical rotatable platform. A laser with light spread out in the horizontal direction sends a beam through the lens. By slanting the platform the incoming and outgoing rays are revealed. When the prism is rotated beyond the critical angle, total internal reflection is observed.