Question

The near point of a person is 50cm and the far point is 1.5m. The spectacles required for reading purpose and for seeing distant objects are respectively:

• A. $+2D$, $-\frac{2}{3}$D
• B. +$\frac{2}{3}$D, $-2D$
• C. $-2D$, +$\frac{2}{3}$D
• D. $-\frac{2}{3}$D, $2D$

Answer 1

The correct option is A $+2D$, $-\frac{2}{3}$D

For reading purpose,

$u=50cm,v=25cm$
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\frac{1}{25}-\frac{1}{50}=\frac{1}{f}$
$f=50cm$

$P=\frac{1}{f}$

$f$ should be in metre

$P=\frac{100}{50}$
$P=+2D$
For seeing distance objects,
$u=1.5m,v=\infty$

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$f=-\frac{3}{2}m$

$P=\frac{1}{f}$
$P=\frac{-2}{3}D$