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:

P = -5.5D


P = 1/f


f = 1/P = 1/(-5.5) = -0.1818 m = -18.18cm


(ii) For Near Vision:


P = 1.5D


P = 1/f


f = 1/P = 1/1.5 = 0.6666 = 66.66 cm


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