Q. 19

A normal eye has retina 2 cm behind the eye-lens when the eye is (A) fully relaxed, (B) most strained?

Answer :

(a)When the eye lens is fully relaxed , u = ∞


The retina is 2 cm behind the eye-lens,hence v = 2 cm = 0.02 m


Using lens formula –



Substituting the values-




Hence, the power of the eye lens is



So, when the eyes are fully relaxed condition, eye lens has a power of 50 D


(b)When the eye lens is strained-


Distance of object u = – 25 cm = – 0.25 m


Using lens formula –



Substituting the values-




Hence, power of the eye lens becomes-



So, when the eyes are most strained condition, eye lens has a power of 54 D.


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