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, $P = + 5 D$

Focal length, f = $\frac{1}{5} = 0.2 m = 20 c m$

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

$v = + 30 c m, f = + 20 c m, u = ?$

Using the lens formula, $\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$

$\frac{1}{30} - \frac{1}{u} = \frac{1}{20}$

$\frac{1}{30} - \frac{1}{20} = \frac{1}{u}$

$\frac{20 - 30}{600} = \frac{1}{u}$

$\frac{- 10}{600} = \frac{1}{u}$

we get $u = - 60 c m$

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

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