Lenses and Image Formation

A lens is a transparent piece of glass or plastic with curved surfaces that bends light through refraction to form an image. Lenses fall into two broad types. A converging, or convex, lens is thicker at its centre and brings parallel rays of light together at a point called the focal point. A diverging, or concave, lens is thinner at its centre and spreads parallel rays apart so they appear to come from a focal point on the same side as the incoming light.

The focal length is the distance from the lens to its focal point and measures how strongly the lens bends light; a shorter focal length produces stronger refraction. The relationship between the object distance, the image distance and the focal length is expressed by the thin-lens equation, which allows the position and size of an image to be calculated. Depending on where the object is placed, a converging lens can form an image that is real or virtual, enlarged or reduced, and upright or inverted.

These principles underlie many optical instruments. A magnifying glass uses a single converging lens, while cameras, telescopes, microscopes and the human eye combine lenses to focus light onto a sensor, film or the retina. Because the amount of refraction varies slightly with wavelength, simple lenses can introduce chromatic aberration, a color fringing that quality instruments correct by combining lenses of different glasses.

Frequently asked questions

What is the difference between a convex and a concave lens?

A convex (converging) lens bends parallel rays toward a single focal point, while a concave (diverging) lens spreads them apart so they seem to come from a focal point.

What is focal length?

Focal length is the distance from a lens to the point where it focuses parallel light. A shorter focal length bends light more strongly than a longer one.

What is a real image?

A real image is formed where light rays actually converge and can be projected onto a screen, unlike a virtual image, which only appears to be located behind the lens.