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

Answer:

1/f =power of lens

I) 1/f=-5.5

= f= 1/5.5

= 10/-55 = - 2/11 cm

II) 1/f=1.5

f= 1/1.5

= 2/3 cm

Answer 2

Explanation:

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.