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Q & A: Lenses Have Many Uses

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Q:
uses for lenses?
- Bradley Hacker (age 12)
Hengrove, Bristol
A:
Lenses are used to focus light. Because focusing light is so important, you can find lenses in many places.

Perhaps the most common lens that we see are the ones in peoples glasses. There is a small lens in each of our eyes. When that lens isn't shaped quite right, light doesn't focus clearly and it's hard to see things. Glasses put another lens in front of the eye to help focus the light better for us. Then we can see clearly.

Another place to find a lens in in a CD player. A CD player works by shining a laser (which is a type of light) onto a disc. The lens in the CD player helps focus the laser onto the disc. Without that lens, the CD player can't read anything.

Lenses are also used in telescopes. When we look up at the sky, we see lots of stars. But we can use a telescope to look at a specific star. The lenses in the telescope focus the light from a single star so that the star we're looking at appears larger so it is easier to see.

Adam

(republished on 07/29/06)

Follow-Up #1: Concave and Convex Lenses

Q:
What are the uses of concave and convex lens in day to day life?
- mayank (age 19)
India
A:
Hi, Mayank. It should come as no surprise to you that convex lenses are much more widely used than concave in day-to-day life. This is because convex lenses magnify images, or make them appear larger. For this reason, a single convex lens fitted to a frame or handle is called a magnifying glass or magnifying lens.

Convex lenses cause incident light rays to converge, creating a point of greater light intensity (as seen below). For this reason, large convex lenses have been used since antiquity as burning-glasses. Nowadays, this same technology is applied to the concentration of solar energy on solar ("photovoltaic") cells. Smaller cells can harvest more solar energy through the use of a convex lens, eliminating the need for larger, more expensive cells.


Concave lenses, on the other hand, make any given image appear smaller. This may not sound like the most useful attribute, but they have the added bonus of providing a sharper, clearer image and are therefore immensely handy in compensating for flaws (called "aberrations") when used in conjunction with convex lenses. Specifically, concave lenses correct chromatic aberrations (i.e. the result of non-uniform refraction of different colors of light), which would otherwise prevent the image produced by a convex lens from converging properly -- this is evident in the diagrams below. The final image illustrates the result of a such an aberration. It is worth noting that aberrations may be also be spherical (i.e. the result of less-than-perfect lens curvature), though correcting these is much more complex and not necessarily a job for a mere concave lens.



Most high quality cameras, telescopes and binoculars use concave lenses to improve the quality of the images they provide. Microscopes and reflecting telescopes make use of a concave mirror, a plane mirror, and a convex lens, while refracting telescopes use two convex lenses. Monoculars are just refracting telescopes modified through the use of prisms. Cameras make use of numerous lens elements (some convex, some concave) in series.

In summary, concave and convex lenses have many uses in day-to-day life, but most of these involve some combination of the two, as well as -- perhaps -- prisms and curved or planar mirrors. For further reading on lenses, try this answer to an earlier question on how lenses work: http://van.physics.illinois.edu/qa/listing.php?id=1961.

- Becca

(published on 03/02/11)

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