Lens Basics

Lens Basics

by Donald E. Simanek

This document was previously frivolously titled Physics for Dummies. After a polite, but threatening, letter from IDG Books Worldwide, Inc., I was persuaded to change it. This publisher has cornered the market in books for Dummies and have even registered "... For Dummies" as a trademark. I find it immensely amusing that they have taken Dummies (R) to be their "intellectual property." Think about the implications of that!
[Watch this document for the addition of more diagrams and expanded discussion. June 1998.]

Lens surfaces are usually spherical or near-spherical. They may be concave, convex, or flat (infinite radius). A lens has two surfaces through which light passes. These surfaces may be mixed in type: concave, convex, or flat.

If both surfaces are convex (curved outward from the body of the lens), the lens is thicker at its center than its edges. A lens with one surface convex and one concave is called meniscus. A lens with one flat surface is called plano-concave or plano-convex, depending on the nature of the other surface.

Whatever the mix of surfaces, if the lens is thicker at its center than its edges it is called a converging lens (having positive focal length). If it is thinner at its center than its edges it is called diverging (having negative focal length). Sometimes they are just called `positive' and `negative'.

Rays from a point source diverge from that point. Rays from a common point are called a bundle. When such a bundle enters a lens, each ray is refracted on passing through a surface. Refraction changes the direction of the ray. Because of this, the rays of the bundle may emerge from the lens either more divergent or less divergent, depending on the nature of the lens.

Some lenses change the direction of the rays enough to cause the rays in a bundle to emerge convergent, that is, converging toward a common point. This is the most well-known situation. If the incident rays come from a point source of light located at least one focal length from a converging lens, they emerge from that lens convergent to a point at least a focal length distant from the lens.

We call a point source of light a real object, and the point of convergence of the bundle of rays emergent from the lens is a real image of that object.

An important case of wide application is an array of point sources spread over a surface, usually a flat surface. An example is a painted picture drawn on the surface of a flat frosted glass and illuminated from behind. Another is a photographic color transparency illuminated from behind so that light from it passes through a lens to cast a much enlarged image on a flat screen. In these cases we speak of an object plane and an image plane, rather than object point and image point. The points in the image plane have a 1:1 correspondence with points in the object plane. Geometric patterns in the image plane are similar (in the geometric sense) to the patterns of points in the object plane, though the image may be inverted up/down or left/right with respect to the object.

Whenever emergent rays converge to a point, that point is called a real image. Whenever they emerge divergent from a common point, that point is called a virtual image. When an image can be found sharply detailed on a screen, it is called real. When an image is seen only by looking through a lens back toward the source of light, that image is called virtual. The image of yourself which you see in a mirror is virtual. The image you see when looking through a telescope is virtual. The image a camera lens casts on film is real.

The focal point of a lens is found by letting a bundle of parallel rays enter it. The point where they converge after passing through the lens is defined to be the focal point of that lens. The distance from the focal point to the lens is defined to be the focal length of the lens. Parallel rays can be made to enter from the other side of the lens, too, so we can find a focal point on either side of the lens. Each lens has two focal points and two focal lengths. If the lens is thin compared to its focal lengths, the two focal lengths are approximately equal in size. This is the most familiar case.

Lenses are usually symmetric about an axis, called the lens axis. For a single-lens system, this axis is also called the optical axis. Usually multiple lens systems have all lenses coaxial, their lens axes all lying along the same line, called the optical axis of the system.

A converging lens is said to have positive focal length. A converging lens causes exiting rays to be more convergent coming out than they were entering the lens.

A diverging lens is said to have negative focal length. A diverging lens causes exiting rays to be more divergent coming out than thay were enetering the lens.

A converging lens can form a real image or a virtual image of a real object. Only when the object is a distance from the lens greater than the focal length will a real image be formed.

A diverging lens always forms virtual images of real objects. Only when incident rays are very convergent entering a negative lens (convergent toward a point somewhere between the lens and the focal point on the far side of the lens), can the emergent rays still be convergent, forming a real image.

One needs to be careful to distinguish convergence/divergence of rays from convergence/divergence of a lens. A set of rays associated with an object or image point are said to be divergent if they spread out, and convergent if they `come together'. In any coaxial optical system, the optic axis represents a legitimate ray path. A ray along this axis passes through the lenses without any change of direction due to refraction. This is, in fact, a good definition of optic axis.

A ray which gets farther from the optical axis the farther it goes is called a divergent ray. One which gets nearer to the optical axis the farther it goes is a convergent ray. One which is parallel to the optic axis has zero convergence/divergence. So, when we speak of the divergence/convergence of a single ray it is with reference to the optic axis.

A lens which deviates the path of a ray so that it is deflected more toward the optic axis is a converging lens. Such action makes converging rays more convergent. It makes diverging rays less divergent. It may, if strong enough even make diverging rays non-divergent (parallel) or even convergent. Likewise a diverging lens can make diverging rays more divergent, converging rays non- convergent, or even divergent.

A lens with two convex surfaces, fatter at the center than at the edges, can be used as a simple magnifier, as a hand lens (Sherlock Holmes lens). When used this way you are looking through it at a virtual, enlarged image. A camera lens, however, forms a real image on film, an image usually reduced in size compared to the object. The power of a lens to change the convergence of light is called its power. The power is expressed as a diopter rating. The diopter rating is D = 1/f, where f is the focal length measured in meters. A 5 diopter lens has a focal length of 20 cm. Your eye doctor writes your eyeglass prescription in diopters. Say he writes 5.2 diopters. The lens shop then takes a lens off the shelf already ground to 5 diopters at the factory, and grinds one surface a bit to add 0.2 diopters. The principle here is that thin lenses, or two surfaces of a thin lens close together, obey the law that its diopter rating is approximately the sum of the two diopter ratings: D = D1 + D2.

In Galileo's time (early 1600s), spectacle lenses were widely available in Europe, usually made in Holland, and were sold by street vendors. Galileo heard that someone in Holland used two of them together in a tube to make distant objects appear larger. Galileo used a long focal length converging lens in one end of the tube (the objective lens) and a short focal length diverging lens at the other end (the lens nearest the eye, or eyelens). If the focal length of the objective is Fo and the focal length of the eyelens is -Fe, the distance between them must be Fo - Fe, and the power (angular magnification) is Fo/Fe. This is called the Galilean telescope, or opera-glass.

Three kinds of telescopes. (A) Keplerian (astronomical), (B) Galilean, (C) Newtonian. Rays are shown from an on-axis, infinitely distance source. The image is virtual, and located at infinity. These few rays are not sufficient to locate the intermediate image, nor show the size of that image. To do that, one must also consider the rays from off-axis points.

Galileo's telescope had a power of about 5 or 6, comparable to hand-held binoculars today. This power is quite adequate for some fascinating astronomical observations: craters on the moon, four moons of Jupiter, rings of Saturn, phases of Venus, nebulae and star clusters, and faint stars in the Milky Way.

Kepler heard about all of this (he and Galileo corresponded) and made another form of telescope with two converging lenses. The larger focal length one was the objective, focal length Fo, and the shorter focal length positive eyelens of focal length Fe was at the other end of the tube. The lenses were separated by distance Fo + Fe, and the angular magnification is Fo/Fe. This Keplerian (or astronomical) telescope inverted the image, but who cares if stars or the moon are seen upside-down? It had a more uniform field illumination than Galileo's telescope, and was more comfortable to use, for one could keep one's eye in a fixed location and see the entire field of view from edge to edge (indeed, one had to keep the eye there). It could also be made in higher powers than Galileo's, without seriously degrading image quality.

Both telescopes suffered from spherical aberration (causing incompletely focussed images) and chromatic aberration (causing colored fringes). Kepler (and Newton) thought that these defects could never be overcome. (They did not anticipate achromatic lenses, which didn't come along until the 19th century.)

Gregory suggested that mirrors be used for telescope objectives since mirrors have no color fringing. Newton took the idea and made the Newtonian form of the telescope, using a concave silvered mirror and a positive eyelens. He gave one of these to the Royal Society, and I think it is still there, on display.

Newton's Telescope

A one-lensed telescope can cast an image on a screen or photographic film. This requires a long focal length positive lens for adequate magnification, say 1/2 meter, 1 meter, or many meters. This arrangement is often used for astronomical photography. It may seem paradoxical to one unfamiliar with optics that in this application, a weaker, lower power lens (long focal length) gives the greatest magnification.

Lately we've heard speculation that ancient cultures might have had telescopes, because they made small glass spheres (like clear marbles). The problem with this is that we don't know what they used these for, and they certainly couldn't form the basis of a very good telescope. They can be used for magnification of small objects, but the image quality is very poor.

The focal length of a perfect sphere of glass is very short and forms a real image very near the sphere. Furthermore the image aberrations (geometric distortions) are severe. Try it with a glass or plastic sphere, crystal ball, or a clear marble (if you can find one). Actually the problem here is the distance of separation between the two surfaces.

However, if a deep equatorial groove is ground in the spherical glass lens, to block rays which cause image imperfections, it is transformed from a very mediocre magnifier into an excellent one. This innovation is attributed to Coddington, and the Coddington magnifier may be purchased today in small hand magnifiers for examination of very small objects. There's no evidence anyone did this before the 19th century, however.

The student may easily confirm much of the above. A simple magnifier is a positive, converging lens. Negative lenses are not so easy to come by, but the eyeglasses of someone who is nearsighted are negative. (Best if the lenses don't have astigmatism correction, however.) The bowl of a spoon is a converging mirror. (Soup spoons work best.) The back of the spoon is a diverging mirror. Shaving mirrors are flat on one side and concave (converging) on the other side. A silvered garden ornament sphere has convex mirror surfaces (diverging).

So have fun with optics, but never look at the sun directly, through a lens, or through a telescope. And never assume that a pair of crossed polarizers can be used to darken the sun enough to view sunspots or a solar eclipse. Crossed polarizers do not cut out the infrared and ultraviolet which do the most damage to your retina. A telescope, binoculars, or just a simple lens, can be used to safely cast a real image of the sun onto a sheet of paper, or other flat surface, as Galileo did. Even then, the sun's image can be pretty bright to view, so mask off the lens to smaller area, or use a small diameter lens.

As a supplement to the discussion above, I've written an instructive program to demonstrate thin lens ray tracing. It is free and may be downloaded.

Input and suggestions are welcome at the address shown to the right. When commenting on a specific document, please reference it by name or content.

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