Question

Far point of a person is $5m$. In order that he has normal vision what kind of spectacles should be use

• A. Concave lens with focal length $5m$
• B. Concave lens with focal length $10m$
• C. Convex lens with focal length $5m$
• D. Convex lens with focal length $-2.5m$

Answer 1

The correct option is A Concave lens with focal length $5m$

$x-$ distance of far point $=5meters$

$f=$focal length of correcting lens

For lens, $v=$image distance$=-x$ ,$u=$object distance $=\alpha$

$\frac{1}{f}=\frac{1}{v}+\frac{1}{u}$

$=\frac{1}{-x}+\frac{1}{\alpha }$

So $f=-5m$

Since it is negative ,it is concave lens.