Question

A concave lens forms the image of an object kept at a distance 20cm in front of it at a dostance 10cm on the side of the object whats is the nature of the image find the focal length of the lens

Answer 1

Answer: a) virtual,erect,diminished image will be form.

b) f = -20cm

Explanation:as it is concave lens. So,the v and u will be (-ve).

Answer 2

Answer:

The focal length of the concave lens is 20cm.

Explanation:

Given object is placed in front of the concave lens, therefore the object distance is negative and hence $u = -20cm$

The image is formed on the side of the object, therefore the image distance is also negative, $v = - 10cm$

Given the image is formed on the same side of the object, also the lens is a concave lens, hence the image formed will be a virtual, erect and diminished image.

The len's formula for focal length is given by

$\dfrac{1}{f} =\dfrac{1}{v} -\dfrac{1}{u}$

Substituting the given values,

$\dfrac{1}{f} =\dfrac{1}{(-10)} -\dfrac{1}{(-20)}$

$\implies \dfrac{1}{f} =\dfrac{1}{-10} +\dfrac{1}{20}$

$\implies \dfrac{1}{f} =\dfrac{-2+1}{20}=-\dfrac{1}{20}$

Thus, the focal length of the concave lens is 20cm.