Question

A person needs a lens of power -5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?

Answer 1

The power P of a lens of focal length f is given by the relationP= 1/f

 

(i) Power of the lens used for correcting distant vision = - 5.5 D
Focal length of the required lens, f= 1/Pf= 1/-5.5 = -0.181 m
The focal length of the lens for correcting distant vision is - 0.181 m.
 

(ii) Power of the lens used for correcting near vision = +1.5 D
Focal length of the required lens, f= 1/P

f= 1/1.5 = +0.667 m
The focal length of the lens for correcting near vision is 0.667 m.