Hold the concave mirror facing some distant object like a tree. Hold a cardboard screen in front of the mirror. Adjust the position of the screen so that a sharp image of distant object is formed on the screen. The distance of screen from the mirror gives us the approximate focal length of the concave mirrror.

Suppose an object is placed u cm in front of a spherical mirror of focal length f such that the image is formed v cm from the mirror, then u, v and f are related by the equation; 1/f= 1/u + 1/v. This equation is referred to as the mirror formula. The formula holds for both concave and convex mirrors.

If the image is formed behind a concave mirror, the image distance (v) is positive but if the image is formed in front of the mirror , then the image distance will be negative.

From the experiment of measuring object distance and image distance we can observe the nature of image formed by concave mirrors and we can infer that concave mirror forms both real and virtual image.

By convention, distances are measured, along the central axis, as positive from the mirror in the direction of the object and negative away from the object. Hence the radius of curvature and the focal length are positive for concave mirrors and negative for convex mirrors.

The S I unit of power of lens is Dioptre represented by D. We know that a concave lens diverges the rays incident on it. That’s why it is also known as a diverging lens. Hence, its power is negative.

For focal length, f in lens is always taken as negative for concave and positive for convex. In case of mirror the f is taken as negative if it is in -X side i.e., is in Concave mirror and taken as positive if it is in +X side i.e., is in Convex mirror.