Spherical Mirrors — Definition

Definition

Imagine taking a hollow sphere made of glass and cutting out a small section from its surface. If you then silver one side of this curved section, you create what is known as a spherical mirror. These mirrors are not flat like the ones you might have at home; instead, they have a distinct curvature, which allows them to form images in unique ways. There are two main types of spherical mirrors: concave and convex.

A concave mirror is like the inside surface of a spoon. Its reflecting surface is curved inwards, towards the center of the sphere from which it was cut. When parallel rays of light strike a concave mirror, they converge (meet) at a single point after reflection.

This point is called the principal focus. Because they converge light, concave mirrors are also known as converging mirrors. They can form both real and virtual images, depending on the object's position, and are commonly used in applications like shaving mirrors, dental mirrors, and in headlights, where a concentrated beam of light is needed.

A convex mirror, on the other hand, is like the outer, bulging surface of a spoon. Its reflecting surface is curved outwards, away from the center of the sphere. When parallel rays of light strike a convex mirror, they diverge (spread out) after reflection, appearing to come from a single point behind the mirror.

This point is also called the principal focus, but it's a virtual focus. Because they spread out light, convex mirrors are also known as diverging mirrors. They always form virtual, erect, and diminished images, regardless of the object's position.

Their primary use is in applications where a wider field of view is required, such as rearview mirrors in vehicles and security mirrors in shops.

Key terms associated with spherical mirrors include the pole (P), which is the geometric center of the mirror's reflecting surface; the center of curvature (C), which is the center of the sphere from which the mirror is a part; the radius of curvature (R), the distance between the pole and the center of curvature; the principal axis, an imaginary line passing through the pole and the center of curvature; the principal focus (F), the point on the principal axis where parallel rays converge (concave) or appear to diverge from (convex) after reflection; and the focal length (f), the distance between the pole and the principal focus.

Understanding these terms and the basic behavior of light rays with these mirrors is fundamental to mastering the topic.