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Question

A convex lens of power 5D is placed at a distance of 30 cm from a screen. At what distance from the lens should the object be placed so that its image is formed on the screen? [3 MARKS]

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Solution

Lens formula: 1 Mark
Formula for power: 1 Mark
Answer: 1 Mark

Given:
Power of the lens, P=+5D

Focal length, f = 15=0.2m=20cm

Here, the screen is placed 30 cm from the lens.

v=+30cm,f=+20cm,u=?

Using the lens formula, 1v1u=1f

1301u=120

130120=1u

2030600=1u

10600=1u

we get u=60cm

Therefore, the object should be placed at 60 cm from the lens.


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