Question

Define power of a lens. Write its units Deduce the realtion
$\frac{1}{f1 } + \frac{1}{f2} = \frac{1}{f}$
for two thin lenses kept in contact coaxially

Answer 1

power of lens = 1/ f
it's SI unit is dioptre

Answer 2

Answer:

Explanation:Power of lens: It is the reciprocal of focal length of a lens. P = 1/f (f is in metre) Unit of power of lens: Diopter. An object is placed at point O. The lens A produces an image at I1 which serves as a virtual object for lens B which produces final image at I. Given, the lenses are thin. The optical centres (P) of the lenses A and B is co-incident. For lens A, we have If two lenses are considered as equivalent to a single lens of focal length f, then 1/v - 1/u = 1/f ....(iv) From equation (iii) and equation (iv), we can write 1/f = 1/f1 + 1/f2