A magnifying glass is a lens used to make objects seen through it appear larger. Magnifying glasses are used for examining small objects or the fine details of large objects. Watchmakers, jewelers, and eye doctors use them for much of their work.
To examine an object in detail without a lens, the object is held at the distance of most distinct vision. For the average person this distance is about 10 inches (25 cm) from the eye. When using a magnifying glass, a person holds the glass close to his eye with the object a short distance away on the other side of it. By moving the glass in and out, it is easy to form an enlarged image of the object.
A magnifying glass is simply a convex, or converging, lens. When a parallel beam of light passes through a converging lens it is brought to a focus at a point. The distance from this point to the lens is called the focal length of the lens. An object being examined through a magnifying glass is always kept at a distance from the lens that is less than the focal length. If the object is at a distance greater than the focal length of the lens, an inverted image is formed.
The magnifying power of a lens depends only on the focal length of the lens and increases as the focal length becomes shorter. The most powerful magnifying glass is a sphere, since it has the shortest possible focal length. However, the field of view through such lenses is small.