Question

A person is suffering from hypermetropia (or longsightedness). The near point of the person is 1.5 m (or 150 cm). Calculate the focal length and power of the convex lens used in his spectacles. Assume that the near point of the normal eye is 25 cm.
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Answer 1

hypermetropia can be corrected by using convex lens

Objects distance u = -25

Image distance v= -1. 5

Focal length = ?

using the formula

1/v-1/u = 1/f

= 1/150 - 1/25 = 1/f

1/150+1/25= 1/f

-1+6/150 = 1/f

f= 150/ 5 = 30 CM

f= 30cm

power p = 1/ f( in meter )

1/0.3

10/3 = 3.3 D

power = +3.3D