Good question, Esme!
You are exactly right that both cameras and light houses use
lenses. But they both use convex lenses. The difference between convex
lenses (also called converging lenses) and concave lenses (also called
diverging lenses) is their shape. This difference in shape causes a
light to be refracted (or bent) differently when it passes through
In a camera, a converging lens is used so that a real, inverted, smaller image is formed on the light-sensitive film.
To be fully accurate, nearly all modern camera lenses fancier than
the disposable one-use cardboard box ones use combinations of
converging and diverging lenses. There is a variety of reasons for
doing this. The main reason is to free a camera lens from annoying
distortions that people who take pictures like to get rid of. One of
the worst is called "chromatic abberation", in which a simple lens made
out of glass will focus light of different colors in different places.
Slice a lens near the edge, and you have what looks and acts just like
a prism! The effect on pictures is that bright objects in your picture
will have little colored fringes around them (yuck!). This is
compensated for by combining a convex lens made of one kind of glass
and a concave lens made of another kind of glass such that the net
effect is to focus the light, but that the color dependence is canceled
between the two lenses. Most high-quality lenses have their elements in
pairs like this.
Other effects are geometrical distortions of the image and can be
cured by adding more complicated lens combinations involving both
convex and concave lenses.
Some diagrams of compound lenses used in cameras can be found at
Many modern cameras get much much fancier.
In a lighthouse, a converging lens is used to make a very
concentrated beam of light able to reach many miles to ships at sea.
Converging lenses are often used to focus parallel rays of light down
to a point. In a lighthouse, however, this is done in reverse (yes,
lenses work backwards, too!) -- because the light source is a point and
emits its light in all directions, a converging lens is used to bend
the light so that as many of the rays from the original source are
parallel. That way, they all go in the same direction, and someone far
out at sea is more likely to see it because more of the light is sent
Large converging lenses with short focal lengths are very thick
and large and difficult to build and ship. See this great web site on
lighthouse optics for the solution by Fresnel to the practicality
problem of making converging lenses for lighthouses: http://www.terrypepper.com/lights/fresnel/fresnel.htm
Jessica (and Tom)
(republished on 07/29/06)