Question

A needle placed at 30 cm from the lens forms an image on a screen placed 60cm on the other side of the lens. Identify the type of lens and determine the focallength.

Answer 1

Answer:

20 be the focal length of the given lens

Answer 2

Answer:

Focal length f = 20 cm

Type of lens is a concave lens

Explanation:

We have been given that

A needle placed at v = 30 cm

Image on screen is u = 60 cm

Now

We know that  focal length

$\dfrac{1}{f}\ =\ \dfrac{1}{v}+\dfrac{1}{u}\\\\\dfrac{1}{f} = \dfrac{1}{30} + \dfrac{1}{60}\\\\\dfrac{1}{f}\ =\ \dfrac{3}{60}$

Focal length f = 20 cm

Type of lens is a concave lens = This is a concave lens = concave lens are thinner at the middle .rays of light pas through the lens and spread they diverge