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. ( CBSE 2007)

(i) For distant vision:

Power of lens, P = -5.5 D

Thus, the focal length of the lens required for correcting distant vision is, -18.2 cm.
Note: The negative sign of the length tells us that it is a concave lens.

(ii) For near vision:

Power of lens, P = + 1.5 D

So, Focal length, f = + 66.66 cm (or + 66.7 cm)

Thus, the focal length of the lens required for correcting near vision is + 66.7 cm.

Note: The positive sign of focal length tells us that it is a convex lens.