Question

The furthermost point of distinct vision of a short-sighted person is 200 cm. What spectacles would she require to enable her to see distant objects clearly?

• A. -0.5 m^-1
• B. -1 m^-1
• C. 1 m^-1
• D. 0.5 m^-1

Answer 1

The correct option is A

-0.5 m^-1

The virtual image of distant objects must be formed 200 cm from the eye. Therefore v = -200 cm and u is infinite.
$\frac{1}{u}+\frac{1}{v}=\frac{1}{f}$
$\frac{1}{\infty }+\frac{1}{-200}=\frac{1}{f}$
$0-\frac{1}{200}=\frac{1}{f}$
f = -200 Therefore diverging lenses of focal length 200 cm are required.
The power of the lens = $\frac{1}{f}$ where f is expressed in meters. Power = $\frac{1}{-2}=-0.5m^{-1}$. The power of the lenses is $-0.5m^{-1}$.