Question

What is meant by the power of a lens?  What is its S.I. unit ? Name the type of lens whose power is positive. The image of an object formed by a lens is real, inverted and of the same size as the object. If the image is at a distance of 40 cm from the lens, what is the nature and power of the lens? Draw ray diagram to justify your answer.

Answer 1

(i) Power of lens - Ability of a lens to converge or diverge light rays / Degree of convergence or divergence of light ray achieved by a lens / Reciprocal of focal length of the lens

(ii) S.I.  unit is dioptre 

(iii) Convex lens has positive power

(iv) v= +40 cm; h' = h

The lens is real, inverted and same sized

So, the object is at 2F