# Refraction of Light Part-2

**Sign Convention for Spherical Lenses**

For lenses, we follow sign convention, similar to the one used for spherical mirrors. We apply the rules for signs of distances, except that all measurements are taken from the optical centre of the lens. According to the convention, the focal length of a convex lens is positive and that of a concave lens is negative.

**Lens Formula and Magnification:**

** **In optics, the relationship between the distance of the image (v), the distance of the object (u), and the focal length (f) of the lens is given by the formula known as the Lens formula. The Lens formula is applicable for convex as well as concave lenses. These lenses have negligible thickness. The formula is as follows:

** ****Use of Concave Lens:**

** ****Use of Convex Lens:**

** **

** ****Power of Lens:**

The power of a lens is defined as the reciprocal of its focal length. It is represented by the letter P.

The power P of a lens of focal length f is given by P = 1/f

The SI unit of power of a lens is ‘dioptre’. It is denoted by the letter D.

If f is expressed in metres, then, power is expressed in dioptres. Thus,.1 dioptre is the power of a lens whose focal length is 1 metre. 1D = 1 $m^{-1}$`m`^{−1}

The power of a convex lens is positive and that of a concave lens is negative.