Let's first talk about why people need glasses sometimes. In order to
see things clearly, the light must hit our eyes at a certain point
which is called the retina. In the front of our eye, we have a lens
that bends the light so that it hits the retina in just the right way
so that we see a clear image. Many people have lenses that aren't quite
the right shape to do the job perfectly. Without the right lens, the
image we see becomes very very fuzzy.
Now on to lenses. There are 2 main types of lenses, convex and
concave. The convex lens is wider in the middle and narrow on the
edges. And the concave lens is wider at the edge than it is in the
middle. Each of these lenses will bend light. Concave bends light out
and convace bends it in. Here's the neat part. When you put too lenses
next to each other, their effects add up. So if you have a convex lens
and a concave lens that are both the same power, they totally cancel
each other out.
So we put those 2 things together and get glasses. The optometrist
will check your eyes and find out which type of lens (concave or
convex) you need. Then he figures out what power lens, when added to
the strength of the one in your eye, gives the clearest vision.
So when you look through glasses, you are looking through 2
different lenses on each eye, but it acts just like you had the right
lens in your eye to begin with.
(republished on 07/28/06)