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.

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