Question

a short sighted person can see most clearly at a distance of 15 cm,acquires spectacles enabling him to see most clearly objects at a distance of 60 cm. calculate the total length of the lens and power of the lens

Answer 1

focal length is given as

1/f = 1/v - 1/u

here

u = -60 cm

v = -15 cm

so,

1/f = (1/-15) - (1/-60)

1/f = (-4 + 1)/60

or focal length

f = -60/3 cm = -20 cm

and power of lens

[here f = -20cm = -0.2 m]

P = 1/f = 1/-0.2 = -5D