Question

The near point of short sighted person is 10 cm and he desires to read the book at 30 cm away from him. Calculate the power of lens

Answer 1

Given:

The near point of short sighted person is 10 cm and he desires to read the book at 30 cm away from him.

To find:

Power of lens

Calculation:

The object placed at 30 cm from the eye after refraction from the correction lens will produce a virtual image at 10 cm (near point) from the eye. This will help in proper visualisation of the object.

Applying Lens Law :

$\therefore \: \dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u}$

$= > \: \dfrac{1}{f} = \dfrac{1}{( - 10)} - \dfrac{1}{( - 30)}$

$= > \: \dfrac{1}{f} = - \dfrac{1}{10} + \dfrac{1}{30}$

$= > \: \dfrac{1}{f} = \dfrac{ - 3 + 1}{30}$

$= > \: \dfrac{1}{f} = \dfrac{ - 2}{30}$

So , power will be :

$= > P = \: \dfrac{1}{f} = \dfrac{ - 2 \times 100}{30}$

$= > P = \: - 6.67 \: D$

So , final answer is :

Power is lens is -6.67 Dioptre.