an equation relating the radii of curvature of the lens, the refractive index of the given material of the lens and the required focal length of the lens. The lens maker’s formula is one such equation. It is given as f = ( µ − ) R − R . .
. . .( . ) where µ is the refractive index of the material of the lens; R and R are the radii of curvature of the two faces of the lens; f is the focal length of the lens.
. POWER OF A LENS When a ray of light falls on a lens, the ability to converge or diverge these light rays depends on the focal length of the lens. This ability of a lens to converge (convex lens) or diverge (concave lens) is called as its power. Hence, the power of a lens can be defined as the degree of convergence or divergence of light rays.
Power of a lens is numerically defined as the reciprocal of its focal length. P = f . . .
) The SI unit of power of a lens is dioptre. It is represented by the symbol D. If focal length is expressed in ‘m’, then the power of lens is expressed in ‘D’. Thus 1D is the power of a lens, whose focal length is 1metre.
1D = 1m - . By convention, the power of a convex lens is taken as positive whereas the power of a concave lens is taken, as negative. More to Know The lens formula and lens maker’s formula are applicable to only thin lenses. In the case of thick lenses, these formulae with little modifications are used.
Table . Differences between a Convex Lens and a Concave Lens S. No Convex Lens Concave Lens A convex lens is thicker in the middle than at edges. A concave lens is thinner in the middle than at edges.
It is a converging lens.