Geometrical Optics

Try it: Focusing of parallel rays.

You can verify that a converging lens, such as a magnifying glass, will focus parallel rays. Using the sun as a source, hold a comb perpendicular to a piece of paper so that the shadows of its teeth form a set of long parallel lines on the paper.

Now hold the lens so that the light passes through the comb, then through the lens, and then hits the paper. Notice that the previously parallel shadows become converging lines that meet at eh focal point of the lens. Use ruler to measure the focal length of the lens.

You can also try this with a cylindrical lens (one shaped liked a cylinder rather than a sphere). A glass of water makes a nice, converging, cylindrical lens.