Question

The focal length of a normal eye-lens is about
(a) 1 mm
(b) 2 cm
(c) 25 cm
(d) 1m

Answer 1

(b) 2 cm

Given:
Near point of the human eye, u = $-$25 cm
Distance between the retina and the eye lens, v = 2 cm (approximately)
Thus, we have the focal length, f.
$$\begin{gathered} \frac{1}{f}=\frac{1}{v}-\frac{1}{u} \\ \Rightarrow \frac{1}{f}\cong \frac{1}{2}-\frac{1}{-25} \\ \Rightarrow \frac{1}{f}\cong \frac{27}{50} \\ \Rightarrow f\cong 2\mathrm{cm} \end{gathered}$$