A lens is an optical device used to direct light rays. Anton Van Leeuwenhoek is perhaps the most famous
lens grinder – his work still amazes lens technicians and his techniques remain a mystery. Galileo might be
one of the most famous users of lenses. As the rays enter and exit a lens, refraction changes their path. The
lens’ shape and index of refraction determine where the light rays go as they exit the lens. There are two
main shapes of lenses and each has a few variations. A converging lens focuses rays to a point as they exit
it; light leaving diverging lenses spreads out (diverges.) Look at the shapes below.
A lens of major importance to us is the one in our eye. The following diagram summarizes the major parts
of our eye.
1. Sclera: the hard, white outer layer of the eye. It helps maintain the eye’s circular shape and protect
its inner layers.
2. Choroid layer: this pigmented layer blocks stray rays that would blur the image on the retina.
3. Retina: this layer covers the rear portion of the eye and contains light sensitive cells that change light
into electrical signals that travel to the visual cortex in the brain. There, the pattern of signals is
interpreted as an image.
4. Lens: our lens is a flexible double convex lens that projects a real, inverted image back onto the
5. Iris: this is the colored circle surrounding the pupil. It is a diaphragm that opens or closes to regulate
the amount of light entering the eye.
6. Pupil: this is the dark opening through the center of the iris. It admits light to the lens.
7. Cornea: this is the transparent, convex front of the sclera. The cornea is at least as important as the
lens in refracting light rays onto the retina.
8. Ciliary muscles: these are a miniature belt around the edge of the lens. For close focusing, the lens
must become more convex by budging out more; this happens when the ciliary muscles tighten.
The lens in an average eye can become convex enough to allow one to