Lenses

Lens is a portion of transparent refracting medium bound by two spherical surfaces or one spherical surface and the other plane surface.

Lenses are divided into two classes:

Convex or Convergent Lens

Convex lens are thin at the edges and thick at the centre.

Concave or Divergent Lens

Concave lens are thick at the edge and thin at the centre.

Important Terms

Principal Axis

It is defined as a straight line passing through the centre of curvatures of two surfaces of a lens.

Optical Centre (C )

It is a point lying on the principal axis of the lens so that a ray of light whose refracted path passes through this point will have its emergent path parallel to the direction of the incident ray.

Principal Focus (F)

When a beam of light is incident on a lens in a direction parallel to the principal axis of the lens, the rays after refraction through lens converge to or appear to diverge from a point on the principal axis.

In case of a convex lens, the rays after refraction actually come to a focus and hence a real focus is obtained at F.

Convex Lens

In case of a concave lens, the rays after refraction appear to come from F and hence F is a virtual focus.

Convcave Lens

According to sign conventions F is positive for convex lens and is negative for concave lens.