Science Standards by State
your teacher wants to project a transparency, she or he will probably use
an overhead projector. The projector probably uses a Fresnel (pronounced
lens. Most lenses have a curved surface, but a Fresnel lens appears
flat, with many faint concentric circles around its center. In an
ordinary overhead projector, the Fresnel lens is the flat, glass-like surface
on which a page-size transparency is placed in order to project it.
Look at one closely, and you will see many concentric circles, all having
the same center. The Fresnel lens in an overhead projector is larger
in area than a sheet of paper. Some other Fresnel lenses are no larger
than a 3x5 card, and can fit into a shirt pocket. They are helpful
in reading menus or road maps, especially in dim light. Still others
are larger for use in reading books with small print. Fresnel lenses
are a special form of a plano-convex lens - a lens that is flat
on one side, but curved on the other. How can a Fresnel lens be flat
on both sides, yet still be a lens?
Imagine a large-diameter plano-convex lens made of a material that is transparent, but can be sliced. Picture this lens run throught a machine that cuts thin slices from the flat side of the lens to the rounded top. Imagine that the first (bottom) slice is thin, with the edge having the same slope as the lens from which it was cut. Like a slice of transparent baloney! The edge of the slice slopes just the way the lens did before it was cut.
Now imagine that the whole center part of the slice is removed and discarded, leaving only a ring whose outer edge slopes just the way the lens did. The same thing is true of the next slice, and the next, and the next .... Most of the center of each slice is removed, leaving only a ring whose outer edge slopes the way the original lens did. Imagine all these rings set, each inside the rest, so that finally there is just a flat sheet of rings, each ring having the same slope as the lens from which it was cut. That is what a Fresnel lens is like.
Think of how heavy an overhead projector would be if it had a glass lens an inch or more thick in the center! But a Fresnel lens, made of clear plastic, is light-weight and flat so can put a paper or plastic sheet on it and draw. Yet it is a pretty good magnifier. A glass lens of that diamtere would be heavy, might smash if dropped, is expensive, and needs a strong frame to hold it in place. A Fresnel lens of the same diameter is light-weight, doesn't break if dropped, is less expensive than a glass lens, and doesn't need a heavy frame to hold it.
Fresnel lenses are also used in modern lighthouses because they are not nearly so heavy or costly as older, thicker glass lenses. The lenses are used to focus a rotating light so ships at sea can tell where harbors are, as well as where dangerous rocks and shallow places are so they can be avoided.
Fresnel lenses are interesting
and useful. The next time you see a Fresnel lens, take a close look
at it. Try to imagine what a glass lens of the same diameter and
magnifying power would be like! Would you want to carry such a glass
lens in your pocket? But you can carry an equally strong Fresnel
lens - becuase it's flat and usually made of plastic, not glass.