Q21 of 22 Page 157

The focal length of a lens suggested to a person with hypemetropia is 100cm. Find the distance of near point and power of the lens.

A person suffering from hypermetropia uses a biconvex lens.

Given: focal length of lens, f = 100 cm


Power = 100/f = 100/100 = 1 dioptre


Let the distance of near point be d


using formula: f = 25d/(d-25)


100 = 25d/(d-25)


100(d-25) = 25d


100d – 2500 = 25d


100d-25d = 2500


75d = 2500


d = 2500/75 = 33.33 cm


d = 33.33 cm


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