Question

A near sighted person cannot see distinctly beyond $50cm$ from his eye. The power in diopter of spectacle lenses which will enable him to see distant objects clearly is:

• A. +50 D
  
• B. -50 D
  
• C. +2 D
  
• D. -2 D
  

Answer 1

The correct option is D -2 D

When a person cannot see far situated object we assume object distance to infinity.

Then image distance be v = -50cm

$u=\infty v=-50cm$

then,

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\frac{1}{f}=-\frac{1}{50}f=-50cm=-0.5m$ (concave lens)

$power=\frac{1}{f}=\frac{1}{0.5}=-2D$