A converging lens of focal length 5 cm is placed at a distance of 20 cm from a screen. How far from the lens should an object be placed so as to form its real image on the screen?

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#### Solution

Given:

Focal length, *f *= 5 cm

Image distance, *v *= Distance of the lens from the screen = +20 cm (for real image)

Object distance, *u *= ?

Applying lens formula, we get:

1/*v-*1/*u *= 1/*f*

1/20-1/*u *= 1/5

Or,

1/*u *= 1/20-1/5 =-3/20

Or,

*u *=-20/3 =-6.6 cm

Thus, to form a real image, the object should be placed at a distance of 6.6 cm from the lens.

Concept: Concave Lens

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