Question

(a) Where is the near point of an eye for which a contact lens with a power of +2.95 diopters is prescribed?
cm (in front of the eye)

(b) Where is the far point of an eye for which a contact lens with a power of -1.35 diopters is prescribed for distant vision?
cm (in front of the eye)

Answer 1

The purpose of the corrective lens is to take an object 25 cm from the eye and form a virtual image at the eye’s near point. Given P = 2.95 D => 1/f = 2.95 m

Therefore, 1/f = 1/s + 1/s'

=> 1/s' = 1/f - 1/s = 2.95 - 1/0.25 = -1.05

=> s' = 0.9523 m = -95.23 cm

Therefore, the eye's nearpoint is 95 cm from the eye.

b) The purpose of the corrective lens is to take an object at infinity and form a virtualimage of it at the eyes far point.

P = -1.35 D => 1/f = -1.35 m

=> 1/s' = -1.35 - 0 = -1.35 m = -0.7407 = -74.07 cm

Therefore, the far point is 74 cm.