Q: If the focal length of a magnifier is 5 cm calculate

(a) the power of the lens

(b) the magnifying power of the lens for relaxed and strained eye.

Sol: (a) As power of a lens is reciprocal of focal length .

$\large P = \frac{1}{5\times 10^{-2}} = \frac{1}{0.05}$

P =20 D

(b) for relaxed eye, MP is minimum and will be

$\large MP = \frac{D}{f} = \frac{25}{5} $

MP = 5

While for strained eye, MP is maximum and will be

$\large MP = 1+ \frac{D}{f} = 1 + \frac{25}{5} $

MP = 1 + 5 = 6