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