Let's start with the human eye. Light enters the eye and gets focused
onto the retina in the back. The retina changes the light into a signal
the the brain reads and interprets into the image we "see". In front if
the eye is a lens. Like the lenses you see in people's glasses, this
helps bend the light.
What makes the lens in our eye even more special is that it is
flexible. There are some muscles in the eye that stretch and compress
the lens to change its strength. Much like how a camera moves 1 of its
2 main lenses to focus an image, our lens changes shape to focus an
The lens doesn't always work perfectly. It can get tougher and
won't change shape exactly like it should. That makes it hard to see
certain distances clearly. Depending on the way your lens is stuck, you
might see far things better than near (far-sighted) or you may see near
by things better (near-sighted). Then, if you look at some things, you
lens can't focus the light properly and the image looks fuzzy.
An optometrist fixes this by putting another lens in front of your
eye. This bends the light the rest of the way that your own lens no
longer can. Your lens will still adjust itself to keep things in focus,
but now you can see everything correctly.
(republished on 07/29/06)