MCAT Content / Geometrical Optics / Spherical Mirrors

Spherical mirrors

Topic: Geometrical Optics

A mirror is a reflective surface that light does not pass through, made by a layer of silver nitrate or aluminium behind a piece of glass.

When you place an object in front of a mirror, you see the same object in the mirror (the image). The object is the source of the incident rays, and the image is formed by the reflected rays. An image formed by reflection may be real or virtual. A real image occurs when light rays actually intersect at the image and is inverted, or upside down. A virtual image happens when light rays do not meet at the image. Instead, you “see” the image because your eye projects light rays backwards. A virtual image is right side up (upright).

Spherical mirrors can be either concave or convex. The center of curvature is the point at the center of the sphere and describes how big the sphere is. The principal axis is a line that is perpendicular to the center of the mirror.

The focal point is halfway between the mirror and the center of curvature on the principal axis. The distance to the focal point from the mirror is called the focal length. We can see from the figure that this focal length is also equal to half of the radius of the curvature.

Concave mirrors are characterised by converging rays that create a real image which inverted in front of the mirror.

Convex mirrors are characterised by diverging light and creating an upright virtual image behind the mirror.


Key Points

• Images in mirrors can be either real or virtual.

• A summary of the properties of the concave mirrors are shown below: converging real image inverted image in front of the mirror.

• A summary of the properties of the convex mirrors are shown below: diverging virtual image upright image behind the mirror.


Key Terms

concave: curved like the inner surface of a sphere or bowl

convex: curved or bowed outward like the outside of a bowl or sphere or circle

center of curvature: the point at the center of the sphere and describes how big the sphere is

principal axis: a line that is perpendicular to the center of the mirror

focal point: halfway between the mirror and the center of curvature on the principal axis

focal length: the distance to the focal point from the mirror



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