Question

If a spherical lens has a power of, −0.25 D, the focal length of this lens will be :
(a) −4 cm
(b) −400 mm
(c) −4 m
(d) −40 cm

Answer 1

(c) $-$4 m

Focal length = $\frac{1}{\mathrm{Power}}=\frac{1}{-0.25}=\frac{-100}{25}=-4\mathrm{m}$