Question

Find the minimum focal length of a normal human eye. If the distance of eyelens from retina is $2\text{cm}$

• A. $2\text{cm}$
• B. $1.85\text{cm}$
• C. $1.92\text{cm}$
• D. $1.76\text{cm}$

Answer 1

The correct option is B $1.85\text{cm}$

Image of the object forms on retina for a normal human eye.
$\therefore$ Image distance, $v=2\text{cm}$
Focal length of normal human eye is minimum, when object is placed at near point $(D=25\text{cm})$
So,$u=-25\text{cm}$
Using the formula:
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\Rightarrow \frac{1}{2}-\frac{1}{-25}=\frac{1}{f}$
$\frac{1}{f}=\frac{27}{50}$
$\therefore f=\frac{50}{27}=1.85\text{cm}$

$$\begin{array}{c}\text{Why this Question ?}\\ \text{Concept : When the eye is focused on a closer object, the ciliary muscles are strained and the focal length of eye lens decreases. Ciliary muscles adjust the focal length in such a way that image is formed on retina. and we see the object clearly.}\end{array}$$