Question

A 50cm tall object is at a very large distance from a diverging lens . A virtual,erect and diminished image of the object is formed at a distance of 20cm
in front of the lens . how much is the focal length of the lens

Answer 1

When an object is at a very large distance, the image is always formed at the focus of the lens or mirror. So the focal length is 20cm. Or

object distance (u)= -∞
image distance (v)= -20
focal length = f

$\frac{1}{v} - \frac{1}{u}= \frac{1}{f} \\ \\ \frac{1}{-20} - 0= \frac{1}{f} \\ \\ f=-20cm$

So focal length is 20cm

Answer 2

Answer:

When an object is placed at a very large distance from a diverging lens, then image is formed at the focus of the lens.

Therefore, the focal length of the lens is 20 cm.

Explanation: