Adam Harrington, age 14, of Haggerstown, Md., for his question:
JUST WHAT IS A LENS?
A lens is a transparent object that is used for refracting, or changing the angle of, the light rays that pass through it. The most common type of lens found in nature is part of the eye. Included among man made lenses are eyeglasses, cameras and telescopes.
There are two groups of lenses: convex and concave.
And there are two types of images: virtual and real.
Virtual images are those formed by diverging rays rays that spread apart. They are always right side up. A coin seen through a magnifying glass is an example of a virtual image.
Real images are made by converging rays rays that come to a focus. Real images are always upside down, or inverted.
The convex lenses are thicker in the middle than at the edges. They are a bit like two prisms, set base to base. They cause light rays to converge and they can be used to produce either virtual or real images.
The concave lenses are thinner in the middle than at the edges. They are more like two prisms set edge to edge. They cause the light rays to diverge and they produce virtual images only.
A lens is usually made of glass and is almost always shaped like a disk. The angle of light changes as it passes through a lens because its speed is changed temporarily by the different density of the lens material. The angle at which the light is refracted depends on the lens curvature and on the type of material from which the lens is made.
The human eye is the most remarkable lens. This lens is somewhat elastic and can focus on small print a few inches away or on stars light years away. This is done by using tiny muscles that are attached to the rim of the lens. They constantly adjust its focus as the eye roves from one object to another at different distances. When these muscles become weak, the lens can no longer be focused properly. Eyeglasses then can help correct the images.
A piano convex lens has one plane surface and one convex surface. It is used in some types of slide projectors.
A double convex lens has two convex surfaces. It is used in many types of magnifying glasses.
A concavo convex lens has one convex surface of greater curvature than the concave surface. It is used to help correct farsightedness.
A piano concave lens has one plane surface and one concave surface. It is combined with other lenses for cameras.
A double concave lens has two concave surfaces. It is used in reducing glasses.
A convexo concave lens has one concave surface of greater curvature than the convex surface. It is used to help correct nearsightedness.