Question

The near point and far point of a person are at 60cm and 300 cm respectively. Find the power of lens he should use while reading 30 cm. With this lens on the eye, what maximum distance is clearly visible?

Answer 1

Persons near point,v = -60 cm

Least distance of distinct vision,u = -30 cm

By lens makers formula

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

Putting all the values

$\frac{-1}{60}+\frac{1}{30}=\frac{1}{f}$

After solving

f = 60 cm

We know,

Power = $\frac{1}{f}=\frac{1}{0.6}$

= 1.666 D

Hence the lens will be Convex lens

To read a book kept at the least distance of 30 cm he should use a convex lens of power 1.666 D.

Now,

focal length,f = far point = -x m

Power of lens = $\frac{1}{f}$

Or,

1.66 D = $\frac{1}{-x}$

=- 0.6 m

Hence, the maximum distance up to which clearly visible = 600 cm