Magnifying Glass

Magnifying glass is a lens which makes close objects appear larger. Both sides of the lens are usually curved to form a double convex lens. The magnifying glass can give two kinds of images. A glass held close to a page in a book forms a virtual image. The light rays which produce this image diverge (spread out) as they pass through the lens and appear to originate on the same side of the lens as the page. The virtual image appears upright and larger than the object.

A real image is formed when light rays from an object pass through the lens and are focused on the other side. The real image appears inverted, or upside down. Its size depends on the distance of the object from the lens. The distance from the center of the lens to the point where parallel light rays are focused is called the focal length. If the object is more than twice the focal length away from the lens, the image will be smaller. If the object is less than twice the focal length away, the image will be larger.

The magnifying power of a lens depends on its focal length. The greater the curve of a lens, the shorter its focal length and the greater its power. It bends the rays more, and they meet at a smaller distance from the lens. A lens with a focal length of 5 inches (13 centimeters) magnifies an image about two times.

A magnifying glass held between a piece of paper and the sun can be used to start a fire. Heat from the many rays focusing at a common point (focus) on the paper will make the paper burn.

Contributor: Joseph A. Muscari, Ph.D., Program Manager, Martin Marietta.

Magnifying Glass

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