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Q & A: Lighthouse Lenses

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Most recent answer: 10/22/2007
Q:
Are concave and convex lenses used in cameras and light houses?
- Esme (age 13)
London, Great Britain
A:
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 them.
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 his way.

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: .

Jessica (and Tom)

(published on 10/22/2007)

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