Question

Power of a lens is +4 D. What is the nature and focal length of this lens?

(a) Convex, +25 cm

(b) Biconcave, ± 25 cm

(c) Concave, -25 cm

(d) Plano-convex, ± 25 cm​

Answer 1

Answer:

Since focal length is positive , the nature of the lens is convex and the correct answer is 1st option

Answer 2

Answer:

$power = \frac{1}{f} \\ 4 = \frac{1}{f} \: \: \: \: \: \: \: \: \: \: \: \\ f = \frac{1}{4} \: \: \: \: \: \: \: \: \: \: \: \: \\ f = 0.25 \: m \\ \\ \\ \\ \\ convex = 0.25m \\ = 25cm$