Question

A person needs a lens of power –5.5 dioptres for correcting his near vision and he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting?

(i) distant vision

(ii) near vision

Answer 1

(i) Power of the lens used for correcting distant vision = -5.5D

Focal length of the required lens, f= $\frac{1}{P}$

f = $\frac{1}{-5.5}$ = -0.181m

The focal length of the lens for correcting distant vision is -0.181m

Negative sign implies that the lens required is concave.

(ii) Power of the lens used for correcting distant vision = +1.5D

Focal length of the required lens, f= $\frac{1}{P}$

f = $\frac{1}{+1.5}$ = +0.667m

The focal length of the lens for correcting distant vision is +0.667m

Positive sign implies that the lens required is convex.