Which of the following statements is true ?
Question:

Which of the following statements is true ?

(a) A convex lens has 4 dioptre power having focal length 0.25 m.

(b) A convex lens has – 4 dioptre power having focal length 0.25 m.

(c) A concave lens has 4 dioptre power having focal length 0.25 m.

(d) A concave lens has – 4 dioptre power having focal length 0.25 m.

Solution:

(a).

Explanation: Convex lens has positive power and positive focal length.

$\mathrm{P}=\frac{1}{f} \quad \therefore f=\frac{1}{\mathrm{P}}=\frac{1}{4}=0.25 \mathrm{~m}$