# Refraction through a Prism

Concave spherical surface : Let S be spherical refracting surface, which separates media ‘1’ and ‘2’. Medium ‘1’ is rarer and medium ‘2’ is denser. The refractive indices of media ‘1’ and ‘2’ are respectively (n1 < n2). Let P be the pole and C the center of curvature and PC the principal axis of spherical refracting surface.

O is a point-object on the principal axis. An incident ray OA, after refraction at A on the spherical surface bends towards the normal CAN and moves along AB. Another incident ray OP falls on the surface normally and hence passes un deviated after refraction. These two rays, when produced backward meet at point I on principal axis. Thus I is the virtual image of O.

#### Optical Center :

If the lens sufficiently thin ,the rays going through center 0 of the lens , suffer negligible displacement .

#### Principal Axis :

The line joining the center of curvature of the two faces of the lens is called its principal axis.

#### Principal Focus and Focal lens :

If a parallel beam is incident on a convex lens in a direction parallel to its principal axis ,after reflection through the lens , all rays coverage at the point of the principal axis .This point F is called Principal Focus .