Question

A convex lens in a human eye adjusts its focal length based on the object position. If it can form a clear image on retina for objects as close as 25 cm and as far as infinite distance, find the range offocal length of lens in the human eye?

Answer 1

• Distance between the lens and the retina is 2.5 cm

• So v = +2.5 cm

• Let us take the first case where the object is kept at 25 cm from the eye.

• So u = -25 cm

• The Lens formula is 1/f = 1/v - 1/u.

• 1/f = 1/2.5 - 1/-25

• 1/f = 11/25

• fat 25 cm = ≈ 2.3 cm

• Now let us take the second case where the object is kept at close to infinity from the eye.

• v = +2.5 cm

• u ≈ -∞

• The Lens formula is 1/f = 1/v - 1/u.

• 1/f = 1/2.5 - 1/-∞

• u approaches infinity so 1/u approaches 0

• 1/f = 1/2.5

f at ∞= 2.5 cm