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 relation between the power and focal length is given by:

Power (P) = 1/f

(i) Power of the lens (used for correcting distant vision) as given is – 5.5 D

Focal length of the lens (f) will be 1/P, we get,

f = 1/-5.5

f = -0.181 m

As a result, the focal length of the lens (for correcting distant vision) is calculated as – 0.181 m.

(ii) Power of the lens (used for correcting near vision) as given is +1.5 D

Focal length of the required lens (f) will be 1/P, we get,

f = 1/1.5 = +0.667 m

As a result, the focal length of the lens (for correcting near vision) is calculated as 0.667 m.