Question Detail
Question
A myopic eye has its far point at 40 cm. What power of lens is required to see clearly an object at infinity?
Correct Answer
Option B is the correct answer.
Detailed Explanation
For a myopic eye, the corrective lens needs to form a virtual image at the eye's far point (40 cm) when the object is at infinity. Using the lens formula
1/f = 1/u + 1/v:u = infinity (object at infinity)v = -40 cm (virtual image formed at the eye's far point, in front of the lens)1/f = 1/infinity + 1/(-40)1/f = 0 - 1/40 = -1/40f = -40 cm = -0.40 mPower = 1/f = 1/(-0.40) = -2.5 Diopters.
Hint
For myopia, the corrective lens must create a virtual image of distant objects at the wearer's far point.