# Reflections from a Curved Mirror

The axis of symmetry of a curved mirror is known as the Optic Axis. A mirror that reflects on its "inside" is a concave mirror while one that reflects on its "outside" is a convex mirror. Consider incoming, incident rays of light that are parallel to the Optic Axis. You may think of these as rays of light from an object infinitely far away, an infinitly far distant object.

When these rays strike a concave mirror, they are reflected so they go through a common point and then go on their way; this point is called the focal point. The distance from the mirror to the focal point F is called the focal length and is labeled f. The focal length f is positive for a concave mirror.

When these rays strike a convex mirror, they are reflected so they diverge from a common point as they go on their way; this point is called the focal point. The distance from the mirror to the focal point F is called the focal length and is labeled f. The focal length f is negative for a convex mirror. This focal length is one-half the radius of curvature for the lens,
f = (1/2) R

R and f are positive for a concave mirror. R and f are negative for a convex mirror.   Return to Ch 24, Mirrors and Lenses