Question

a far sighted person cannot focus distinctly objects closer than 120 cm. The lens that will permit him to read from a distance of 40 cm will have focal length
1)+30cm 2)-30cm 3)+60cm 4)-60cm

Answer 1

Answer : option (3)+60 cm

explanation : A far sighted person cannot focus distinctly objects closer than 120cm. i.e., v = -120cm

focal length of lens ‘f’ is used to read from a distance of 40cm.

so, u = -40cm

using lens maker formula,

1/v - 1/u = 1/f

or, 1/-120 - 1/-40 = 1/f

or, 1/-120 - 3/-120 = 1/f

or, 3/120 - 1/120 = 1/f

or, 1/60 = 1/f => f = +60cm

hence, focal length of lens is +60cm