Lens (Physics): Definition, Types & How They Work

You encounter lenses every day. Whether it's the lens on your cell phone camera, the lenses on the eyeglasses or contact lenses you use to see clearly, magnifying glasses, microscopes, telescopes or something else entirely, the physics of lenses explains how a simple piece of glass can be used to magnify, minimize or bring images into focus for any purpose.

Essentially, lenses work by bending light rays that pass through them through refraction, but this basic point can be implemented in different ways that varies according to the lens type. Luckily, the basics of such lenses are easy to understand when you learn a little more about how they work.

A lens is a piece of transparent material that is shaped so as to cause light rays to bend in a specific way as they pass through it, whether that means making the rays converge to a specific point or to diverge as if from a specific point. The material used could be a piece of glass or plastic, and the shape of the lens determines whether it causes light rays to converge or diverge. The word "lens" comes from the Latin word for "lentil," due to the similarity in shape between a converging lens and the legume.

The actual bending of light rays produced by a lens occurs because the lens material has a different index of refraction than the surrounding air. This behavior is described by Snell's law for refraction, which relates the different in angle between the incident and refracted light ray to the indices of refraction for the two materials.

In short, the law says that if you're going from a lower refractive index substance to a higher one (e.g., from air to glass), the light ray is deflected towards the "normal" to the surface (i.e., towards the direction perpendicular to the surface at that point) and that the opposite is true for light rays going from a higher refractive index material to a lower one.

There are quite a few unique terms used in optics, and understanding these is crucial if you're studying the physics of lenses.

• The
focal point is the point where
parallel rays converge when after passing through a lens.
• The focal length of a lens is the
distance from its center to the focal point, essentially defining the "bending
power" of a lens.
• The
optical axis is the line of symmetry
for the lens.
• A
light ray is an approximation of the
path of light, where straight lines are used to represent the motion of light
waves (or photons). Every point on an object produces light rays in every possible
direction, but usually a few specific rays are chosen to determine the location
of the resulting image.
• An
optical lens is a single piece of
material designed to cause light rays to converge (convex lens) or diverge
(concave lens).
• A biconvex lens is a simple optical
lens with two convex sides (producing the lentil-like shape that gave lenses their
name), sometimes called a convex-convex lens and having a positive focal length
by definition. They're used in magnifying glasses, telescopes, microscopes and
even the human eye.
• The
depth of field describes the range
of distances at which objects are in focus when viewed through a lens, and is
common terminology in photography in particular. Since the light sensors in
cameras are of a fixed size, if the image is slightly unfocused but the amount of error is sufficiently small,
it won't actually register as out of focus. This range of focus is the depth of
field.
• The
prime lens is a lens used in
photography with a fixed focal length, as opposed to zoom lenses where the
focal length can be changed. In other contexts, though, prime lens can be used
to mean the primary lens in a system composed of multiple lenses.  

Ray diagrams are an extremely useful tool in optics, and they are used to find the location where an image will be formed based on the location of the object and the lens. By drawing some key light emerging from an object and marking their path as they pass through the lens, the point where they meet is where the image will be formed.

This process can be done using Snell's law of refraction, but a few tricks can simplify the process too. For example, a ray that passes through the center of the lens is barely deflected at all, and one that strikes the lens perpendicular to the optical axis is refracted to pass through the focal point for the lens.

The image produced by the lens can be real or virtual. For a real image, the light rays converge to form an image at a specific location, and you could see that image if you placed a screen at that location. In the human eye and the region behind a camera lens, photosensitive cells or materials are used to pick up this image.

A virtual image is different: when the rays diverge from a lens, their orientation makes it look like they came from the location of the virtual image. In other words, if you follow the refracted rays backwards but only following straight lines, they'll all converge at the location of the virtual image. However, light rays don't physically converge at this location, and if you placed a screen there, you wouldn't see an image.

A camera lens is one of the most familiar types of lenses you encounter on a daily basis, and these come in many different types, although they all share the same basic principles of operation outlined previously.

A prime lens is a basic lens with a fixed focal length, and a zoom lens has a variable focal length, so you don't have to physically change your location to get something in focus. A wide-angle lens is a type of lens with a very small focal length that dramatically increases the field of view, and a fisheye lens is essentially an extreme version of a wide angle lens.

Other examples are telephoto lenses, which have very long focal lengths and are intended to capture subjects that are far away, and macro lenses that are intended to focus at very close ranges and either produce life-size or magnified versions of objects.

Other common types of lenses are eyeglasses lenses or the contact lens, and both of these work to correct the problems with your vision. If you're "nearsighted," this means your eye lenses create images in front of the light-sensitive retina in your eye, and so you need diverging (concave) lenses to move the image further back.

If you're "farsighted," the lenses in your eyes would produce an image further back than your retinas, so you need converging lenses to correct this issue.

Both contact lenses and eyeglasses correct this in the same way – by adding an additional corrective lens to make the effective focal length of your eye match the distance to your retina – but there are differences because contact lenses sit directly on your eyes. In a contact lens, the lens doesn't need to cover as much space (it only needs to be big enough for your pupil at its maximum dilation) and can achieve this with less material. For eyeglasses lenses, the lens needs to cover a much larger area and is thicker as a result.