(a) Define the term power of a lens. Write its SI unit.
(b) Derive the expression for the power of two thin lenses placed coaxially in contact with each other.
a) Power of a lens :
Power of a lens is a measure of the convergence or divergence which a lens introduces in the light falling on it.
It is defined as the tangent of the angle by which it converges or diverges a beam of light falling at unit distance from the optical centre.
SI unit of power of a lens is dioptre (D).
b) The two lens - the system is regarded as equivalent to a single lens of focal length f,