LOOKING AT A MIRROR from one side, you see, not yourself reflected, but an object off to the other side. You can demonstrate what this angle of reflection is by standing crayons equally spaced in a semicircle and tracing lines from them to the center of a flat mirror. Sight across any crayon and take notice of the color of the one you see reflected; then find its position in the semicircle, and you will see it is at the same angle from the plane of the mirror as the one across which you sighted. For example, as you look at the photo at right you see the mirror from across the second crayon from the left Note that the one reflected is the second from the other end.

reflected is the secondHOW MUCH LIGHT IS BENT

HOW MUCH LIGHT IS BENT as it passes through a medium depends on the index of refraction-the ratio of its velocity in a vacuum to that in the given medium. To compare liquid refractive indexes, cover a square bottle with black paper, cut a disk on one side, cross the hole with narrow paper strips for reference, and make a 1/16" slit on an adjacent side. Fill the bottle to the horizontal strip and direct a light through the slit to hit the cross at the center. Mark the amount of refraction and try other liquids. A drop of milk may make reading easier.

WHEN REFRACTION bends light at such an angle that it can't escape from a medium, we have a phenomenon called total internal reflection. A startling demonstration of this can be made with a glass of water and a test tube in which a pencil has been placed. Lower the test tube into the water and look at it from the side, and the tube will be as transparent as ever. But look into the glass from the top while you hold the test tube at an angle and, as if by magic, that part of the tube under the water will appear as opaque as if it were filled with quicksilver. The pencil in it will be completely invisible, as shown in the photo at right above, for the water will have bent the light at such an angle that it can't enter the air in the tube. Put water in the tube, and it becomes transparent again.

WHEN REFRACTION