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 dioptres. What is the focal length of the lens required for correcting


(i) distant vision, and


(ii) near vision?


(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.

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