Lenses

Last updated: August 31, 2009.

Microscopes let us peer inside invisible worlds our eyes could never see, telescopes take us far beyond the Earth to the stars and planets of the night sky, movie projectors throw enormous images onto screens, and lighthouses cast reassuring beams of light far across the ocean. Amazing curves of glass or plastic called lenses make all these things possible. Let's take a closer look at what they are and how they work!

Photo: Lenses in the headlamps of this car focus beams of light down onto the road so you can see where you're going. Some car headlights use Fresnel lenses to make powerful beams, just like lighthouses!

What are lenses?

A lens is a transparent piece of glass or plastic with at least one curved surface. It gets its name from the Latin word for "lentil" (a type of pulse used in cooking), but don't let that confuse you. There's no real reason for this other than that the most common kind of lens (called a convex lens) looks very much like a lentil!

Photo: Lentils gave lenses their name. Convex lenses bulge out in the middle like lentils, while concave lenses "cave in" in the middle and bulge out at the edges.

How do lenses work?

A lens works by refraction: it bends light rays as they pass through it so they change direction. (You can read a full explanation of why this happens in our article on light.) That means the rays seem to come from a point that's closer or further away from where they actually originate—and that's what makes objects seen through a lens seem either bigger or smaller than they really are.

Types of lenses

There are two main types of lenses, known as convex (or converging) and concave (or diverging).

Convex lenses

In a convex lens (sometimes called a positive lens), the glass (or plastic) surfaces bulge outwards in the center giving the classic lentil-like shape. A convex lens is also called a converging lens because it makes parallel light rays passing through it bend inward and meet (converge) at a spot just beyond the lens known as the focal point.

Photo: A convex lens makes light rays converge (come together) at the focal point or focus. The distance from the center of the lens to the focal point is the focal length of the lens.

Convex lenses are used in things like telescopes and binoculars to bring distant light rays to a focus in your eyes.

Concave lenses

A concave lens is exactly the opposite with the outer surfaces curving inward, so it makes parallel light rays curve outward or diverge. That's why concave lenses are sometimes called diverging lenses. (One easy way to remember the difference between concave and convex lenses is to think of concave lenses caving inwards.)

Photo: A concave lens makes light rays diverge (spread out).

Concave lenses are used in things like TV projectors to make light rays spread out into the distance. In a flashlight, it's easier to do this job with a mirror, which usually weighs much less than a lens.

Compound lenses

It's possible to make lenses that behave in more complex ways by combining convex and concave lenses. A lens that uses two or more simpler lenses in this way is called a compound lens.

How do you measure the power of a lens?

If you've ever looked through binoculars, a telescope, or a magnifying glass, you'll know that some lenses magnify (or reduce) the apparent size of an object much more than others. There's a simple measurement that tells you how powerful a lens is and it's known as the focal length. The focal length of a lens is the distance from the center of the lens to the point at which it focuses light rays. The bigger the focal length, the more powerful the lens.

You'll find focal lengths written either in ordinary units of length (such as centimeters, millimeters, or inches) or in special optical units called diopters. The diopter measurement of a lens is the reciprocal of the focal length in meters (one divided by the focal length), so 1 diopter = 1 m, 2 diopters = 0.5 m, 3 diopters = 0.33 meters, and so on. Eyeglass prescriptions from opticians typically show the strength of the corrective lenses you need in diopters.

The focal length isn't the only important feature of a lens. Bigger lenses gather more light than smaller ones, so they make a brighter image. This is particularly important if you're choosing a lens for a camera, because the amount of light the lens gathers will determine what the image looks like. Camera lenses are usually rated with a measurement called the f-number, which is the focal length divided by the diameter. Generally speaking, lenses with a small f-number make brighter images. Lenses with a higher f-number have a bigger depth of focus: essentially, more of the object you're photographing and its surroundings are in focus at the same time. (If you want to know more, take a look at Louis Bloomfield's explanation of lens size.)

How are lenses made?

Until plastics became common in the 20th century, virtually all lenses were made by grinding solid pieces of glass into different shapes. Convex lenses were made by using a concave-shaped grinding tool (and vice-versa), and then the roughly shaped lens was polished to make its final shape. The ordinary glass we use in windows and crockery isn't good enough to use for lenses, because it contains air bubbles and other imperfections. These cause light rays to divert from their correct path, making a fuzzy image or one that makes different colors of light behave in different ways (problems that optical scientists refer to as aberrations). Instead, lenses are made using a more refined material known as optical glass. For eyeglasses, many people now prefer plastic lenses because they're much lighter and safer than optical glass. Plastic lenses can be molded to shape, instead of being ground, so they can be made in huge quantities far more cheaply than glass lenses. Although ordinary plastic scratches easily, it can be coated with a thin layer of a protective material such as diamond-like carbon (DLC) to reduce the risk of damage.

Photo: This magnifying glass uses a single convex lens made from plastic.

What are lenses used for?

Lenses are everywhere in the world around us—in everything from car headlamps and flashlights to the LED lights used in electronic instrument panels.

Photo: You might not have noticed, but the tiny LEDs (light-emitting diodes) used in instrument panels have tiny plastic lenses built into them to magnify the light they produce. The lens is the curved plastic on the left (the top of the LED that shines toward you.)

Our eyes contain probably the most amazing lenses of all. Think what happens when you look at the world around you. One minute you're staring at the ground in front of your feet. Seconds later, you hear an airplane screaming past, turn your head, and watch it fly by. Do this trick with a pair of binoculars and you'll find it takes you quite a while to adjust the focus from near-sight (looking at the ground) to far (watching the plane). Try it with the naked eye and you won't even notice what you're doing. That's because your eyes have flexible lenses, controlled by tiny muscles, that can bulge in and out, changing shape instantly to focus on anything from the prints on your finger to the surface of the Moon. How amazing is that?

We all have lenses in our eyes, but many of us balance extra ones on the end of our noses to correct long and short sight: more glass and plastic lenses are used for eyeglasses and contact lenses than for any other purpose. There are all kinds of eyeglass lenses, including light-sensitive photochromic ones that darken in sunlight and double-up as sunglasses.

You'll also find lenses in binoculars (which use two or three lenses in each of the cylinders serving your eyes) and telescopes, though not all microscopes use them. Ordinary (optical) microscopes use a series of glass lenses to magnify tiny objects, while super-powerful electron microscopes use electromagnets to bend electron beams that help us see in even more detail. Movie projectors and projection televisions use lenses to convert small movie pictures into giant images that lots of people can view at once. Cameras work the opposite way, catching light rays from a distance and bringing them to focus on chemically treated plastic film or light-sensitive electronic chips called CCDs. You can even find lenses built into magazine and book covers to make images change as you shift your head from side to side; this cunning trick is called lenticular printing—but it really just means "printing with built-in lenses."

Photos: Lighthouses don't use huge and heavy lenses: instead, they rely on Fresnel lenses (ones with a stepped surface pattern of concentric rings) and prisms, like the one in this exhibit at Think Tank, the science museum in Birmingham, England. Read how they work in our article on Fresnel lenses.