Reading Glass Lens Information 

© 1998-2005 by Glenn Elert
All Rights Reserved, Fair Use Encouraged

Introduction

If you have mammalian eyes, you have lenses. Common optical devices that use lenses include reading glasses, eyeglasses, contact lenses, magnifying glasses, cameras, projectors, telescopes, binoculars, and microscopes. All sorts of things will act like lenses, even things that aren't normally thought of as lenses. As long as they're transparent and curved, they satisfy the definition and will behave in a manner similar to the devices commonly thought of as lenses. Car headlights and taillights are covered with lenses. (They're even called lenses in auto parts catalogs.) A drop of water is a lens. So are fishbowls, light bulbs, and drinking glasses. In general, a lens is any piece of transparent material with at least one curved surface.

The origins of the word lens can be traced back to the Seventeenth Century. Scientists at the time wanted a word to describe the shape of the glass pieces used in telescopes, magnifiers, and reading glasses. For some unknown reason they decided against the logically descriptive term "biconvex" (curved outward on two sides) and instead chose to name them for the small, flat beans they resemble, lentils. Lens lentis is the Latin word for lentil. It's good word for us now because hardly anyone makes the connection and because not all lenses are biconvex.

What makes a lens deferent from any other transparent object is its ability to focus light. A focus is a meeting point. The origins of this word are also Latin. Focus is the Latin word for fireplace. Like the word lens, the word focus first started to appear in scientific writing in the Seventeenth Century. Many people are familiar with the ability of magnifying glasses to concentrate sunlight to the point where it can burn paper. It is uncertain whether the scientist of this time were referring to the fire produced by this method or the fact that the fireplace was the meeting point (or focus) of the home at the time.

In any case, it's only half true that a lens can concentrate sunlight to a point that might then be used to a start a fire. Only one kind of lens will do that. A lens that will converge parallel rays of light down to a point. Some lenses do just the opposite, that is, they diverge parallel rays of light from a point. Thus lenses are divided into two major categories called converging lenses and diverging lenses, respectively. The point to which these rays converge or from which they appear to diverge is called the focus or focal point.

The way to distinguish among the two types of lenses is to look at the relative thickness of two parts, the center and the edges. Converging lenses are thicker in the middle than they are at the edges, while diverging lenses are thicker at the edges than they are in the middle. Converging lenses include those that are biconvex (curved outward on both sides), plano-convex (flat on one side and curved outward on the other side), and convex meniscus (curved inward on one side and outward on the other side more strongly. Diverging lenses include those that are biconcave (curved inward on both sides), class=keyother>plano-concave (flat on one side and curved inward on the other side), and concave meniscus (curved inward on one side and curved outward on the other side less strongly.

Nearly every lens, including reading glass lenses, has a line of symmetry down its center (which may or may not be its geometric center, but usually is). Given a standard, simple lens like eyeglasses, contact lenses, or the lens in a movie projector, there is an obvious axis of symmetry about which the lens could be rotated and not have any effect on the image produced. Such an axis is called the principal axis. To keep life simple, when parallel rays are shown entering a lens, they are often all drawn parallel to the lens' principal axis. Such rays are said to be paraxial; that is, parallel to the principal axis. As I have just said,this is merely done for convenience. Actually, for an ideal lens, parallel rays always converge or diverge from a point. The aberrations of a real lens from an ideal lens are dealt with in another section of this book. For a lens with spherical symmetry, the collection of focal points for any group of parallel rays from a focal plane.

It is often more convenient to work with this equation if we give special names to the inverse quantities. The inverse of focal length is the refracting power (usually just called of the lens and the inverse of the object and image distances are called vergences. Using these new terms, the lens equation can be stated more compactly in words and symbols. "The power of a lens is the sum of the object and image vergences." Since it is a linear relationship, it is also much easier to handle mathematically.

Summary

A lens is any piece of transparent material with at least one curved surface.

There are two types of lenses.

A converging lens is any piece of transparent material that will converge parallel rays of light down to a point.

Converging lenses are thicker in the middle than at the edges.

A diverging lens is any piece of transparent material that will cause parallel rays of light to appear to diverge from a point.

Diverging lenses are thicker at the edges than in the middle.

The focal point or focus is the point which rays of light initially parallel converge towards after emerging from a converging lens or diverge from after emerging from a diverging lens.

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