Question 9 of 16intermediate🔧 ApplyNumerical3 marks
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?
(i)
Answer
The focal length of the lens required for correcting distant vision is -18.2 cm (or -0.182 m).
(ii)
Answer
The focal length of the lens required for correcting near vision is 66.7 cm (or 0.667 m).
Explanation
The power of a lens (P) is related to its focal length (f) by the formula P = 1/f, where f is measured in metres. A negative power indicates a concave lens used for correcting myopia (near-sightedness), while a positive power indicates a convex lens used for correcting hypermetropia (far-sightedness).