Lenses redirect light by refraction. Converging lenses are thicker in the middle and bring light rays together. Diverging lenses are thinner in the middle and spread light out.

Lens terminology is very similar to that of mirrors. An important difference is that when we consider a single lens, virtual images appear on the same side of the lens as the object, and real images appear on the opposite side. Another difference is that lenses have a focal point on each side.

Ray diagrams for converging and diverging lenses are also similar to those for mirrors.

And like mirrors, lenses have equations that quantify the relative size, orientation and distance of the images they produce. In addition, the lensmaker’s equation determines the focal length of a lens with differing radii of curvature on its two sides. Sign conventions can be trickier for lenses, so pay special attention to them.

Several lenses can be used together to enhance their magnification properties, such as in a refracting telescopes.

In a microscope, a lens called the objective creates a real image, which is used as the object for a second lens called the eyepiece. The viewer looks into the eyepiece and sees a final image that is inverted, virtual, and magnified.

The human eye contains a lens that can change shape in order to create a focused image on the light-sensitive retina at the back of the eye. A person’s near point and far point are the closest and farthest distances on which she can focus.

Sometimes a lens is specified by its refractive power, which is the inverse of its focal length. Refractive power P is measured in diopters, where 1 diopter = 1 m⁻¹. This unit is commonly used with eyeglasses and contact lenses.

The angular size of an object is the angle of the field of view that is taken up by the object. It is measured in radians, which allows the use of a small-angle approximation: For small angles, the angular size of an object is approximately its height divided by its distance.

The angular magnification of a simple magnifier is the ratio of the angular size of the image to that of the object when the object is located at the near point of the human eye.

Lenses can exhibit spherical aberration just as mirrors can. They also exhibit chromatic aberration due to the differing refractive indices of different wavelengths of light. Both kinds of aberration can cause an image to look blurry.