A doctor prescribes a corrective lens of power -0.5 D to a person. Find the focal length of the lens. Is this lens diverging or converging? Give reason. How does the property of this lens can be used to correct eye defects ?

Explanation

(i) He is suffering from myopia because the focal length required for the lens is negative.
(ii) Given P = -1.2 D.
`f = (1)/(p_(1))` [where, P = power of lens and f = focal length]
`= (1)/(-1.2) = -0.83 m`.
(iii) The lens is diverging because power is negative.

Or
Given, Power (P) = = -0.5 D, Focal length (f) = ?
`p = (1)/(f) rArr f = (1)/(P) rArr f = -(1)/(0.5) = 2m`.
* This lens is diverging.
* Because power of a lens is negative means the given lens is concave.
* The diverging property of the lens is used to correct myopia. In myopic eye the image of a distant object is formed in front of the retina and not on the retina.
* A concave lens of suitable power diverges the light rays and brings the image back on the retina.