Question

The distance of the eye-lens from the retina is x. For a normal eye, the maximum focal length of the eye-lens
(a) = x
(b) < x
(c) > x
(d) = 2 x

Answer 1

(a) = x

For a normal eye, we have:
Far point at which the object can be placed, u = $\infty$
Distance between the eye lens and the retina, v = x
Thus, we have:
$$\begin{gathered} \frac{1}{f}=\frac{1}{v}-\frac{1}{u} \\ \Rightarrow \frac{1}{f}=\frac{1}{x}-\frac{1}{\infty } \\ \Rightarrow f=x \end{gathered}$$1f=1v−1u⇒1f≅12−1−25⇒1f≅2750⇒f≅2 cm1f=1v−1u⇒1f≅12−1−25⇒1f≅2750⇒f≅2 cm