Question

State the condition for each of the following :

(a) a lens has both its focal lengths equal.

(b) a ray passes undeviated through the lens.

Answer 1

i) If the medium on both sides of the lens is the same and the curvature of both sides are equal, the first and second focal lengths are equal.

ii) When the ray is incident towards the optical centre of the lens, it passes undeviated.