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 ?

The power P of a lens of focal length f is given by the relationP= \(\frac{1}{f} \)

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

Focal length of the required lens,
f= \(\frac{1}{Pf} =\frac{1}{-5.5} = -0.181 m \)

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= \(\frac{1}{P} \)

f= \(\frac{1}{1.5} \) = +0.667 m

The focal length of the lens for correcting near vision is 0.667 m.