## Wednesday, 25 March 2015

### Lens

👀👓 CARDINAL DATA OF A LENS::
➡ CENTRE OF CURVATURE (C) of the spherical lens is the centre of the sphere of which the refracting lens surface is a part.
➡ RADIUS OF CURVATURE of the spherical lens is the radius of the sphere of which the refracting surface is a part.
➡ The PRINCIPAL AXIS (AB) of the lens is the line joining the centres of curvatures of its surfaces.
➡ OPTICAL CENTRE (O) of the lens corresponds to the nodal point of a thick lens. It is a point on the principal axis in the lens, the rays passing from where do not undergo deviation. In meniscus lenses the optical centre lies outside the lens.
➡ The PRINCIPAL FOCUS (F) of a lens is that point on the principal axis where parallel rays of light, after passing through the lens, converge (in convex lens) or appear to diverge (in concave lens).
➡ The FOCAL LENGTH (f) of a lens is the distance between the optical centre (O) and the principal focus (F).
➡ POWER OF A LENS (P) is defined as the ability of the lens to converge a beam of light falling on the lens. For a converging (convex) lens the power is taken as positive and for a diverging (concave) lens power is taken as negative. It is measured as reciprocal of the focal length in metres i.e. P = 1/f. The unit of power is dioptre (D). One dioptre is the power of a lens of focal length one metre.