Question

A boy uses spectacles of focal length -60cm. Name the defect of vision he is suffering from. Which lens is used for the correction of this defect? Compute the power this lens.

Answer 1

Focal length = f 

f = -60 cm (Given) = -0.60m 

The negative sign of focal length indicates that the lens used is concave hence the boy is suffering from myopia (short sightedness).

Power of the lens = \(\frac{1}{f(in\,m)}\)

= \(\frac{1}{(-0.60)}\)

= -1.667 D