Question

Calculate the focal length of a eye lens when object placed at infinity and image at least distance of distinct vision​

Answer 1

Given :

Object is placed at infinity

Image is formed at least distance of distinct vision.

To find :

Focal length of eye lens.

Solution :

Least distance of distinct vision is the object's minimum distance from the eye so that it can have clear vision.

In case of normal eye, least distance of distinct vision is 25 cm from the eye.

Object distance = infinity.

Image distance = 25 cm.

The lens formula :

$\\ \bf \: \frac{1}{f} = \frac{1}{v} + \frac{1}{u}$

where,

f = Focal length of lens.

v = Image distance = 25 cm.

u = Object distance = Infinity.

Putting all the values in lens formula, we get :

$\\ \bf \: \frac{1}{f} = \frac{1}{ - 25} + \frac{1}{ \infty} \\ \\ \bf \: \frac{1}{f} = \frac{1}{ - 25} + 0 \:$

So,

Focal length of eye piece f = -25 cm.