Convex or Convergent Lens
Back to Top
Convex lens are thin at the edges and thick at the centre.
Concave or Divergent Lens
Back to Top
Concave lens are thick at the edge and thin at the centre.
Important Terms
Back to Top
Principal Axis
Back to Top
It is defined as a straight line passing through the centre of curvatures of two surfaces of a lens.
Optical Centre (C )
Back to Top
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)
Back to Top
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
Back to Top
In case of a concave lens, the rays after refraction appear to come from F and hence F is a virtual focus.
Convcave Lens
Back to Top
According to sign conventions F is positive for convex lens and is negative for concave lens.
