Question

A nearsighted person cannot see beyond $25cm$. Assuming that the separation of the glass from the eye is $1cm$, find the power of lens needed to see distant objects.

Answer 1

For a near sighted person.

$u=\infty$

$v=$ distance of image from glass

$=$distance of image from eye $-$ separation between glass and eye

$=25$cm $-1$cm $=24$cm

Using the lens formula:

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

$\frac{1}{f}=\frac{1}{-24}-\frac{1}{\infty }$

$\frac{1}{f}=-\frac{1}{24}$

$f=-24cm=-0.24m$

So, Power $=\frac{1}{f}=\frac{100}{-24}=-4.16D$.