What is a magnifying glass?

A magnifying glass is a convex lens, its makes objets look larger than they really are. The glass is thicker in the middle than it is at its edges. Its magnifying power is measured by the number of times larger an objet seen through it appears. For example, if the letter A on a poster is one inch tall but through a magnifying glass appears to be three inches tall, the magnifying power is three.

Convex lenses may magnify in another way, too, producing enlarged images on a movie screen. In this case the image is a real image which is always inverted. For this reason, the film is put into the projector upside down.