3 Tutor System
Starting just at 265/hour

# 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.