Question

find the position of an object which when placed in front of a concave mirror of a focal length 20 cm produces virtual image which thrice the size of object​

Answer 1

Answer: The image is at distance of 20 cm from the mirror on the side opposite to that of object. ⇒ u = − 10.

Explanation:

Answer 2

Answer:

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Explanation:

Given that:

The focal length of the concave lens, f = - 20 cm

Magnification, m = 3

Two cases:-

First case when the image is virtual, m = 3

We know that magnification, m=  $\large -\frac{v}{u}$

ie $\large -\frac{v}{u}$ = -3

According to mirror formula

$\large\frac{1}{f} = \frac{1}{u} + \frac{1}{v} \\\\\large\frac{1}{-20} = \frac{1}{u} - \frac{1}{v}\\\\\large\frac{1}{-20} = \frac{3-1}{u}\\\\\large\frac{3u}{2} = -20\\\\\large 3u = -40\\\\u = -13.3\:\:cm$

∴, v = -3x - 13.3 cm = 39.9 cm

The distance of image from the mirror when the image is virtual is 39.9 cm

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Second case when the image is real, m = -3 We know that magnification, m = $\large -\frac{v}{u}$ = -3

v = 3u

According to mirror formula:

$\large\frac{1}{f} = \frac{1}{u} + \frac{1}{v} \\\\\large\frac{1}{-20} = \frac{1}{u} + \frac{1}{3u}\\\\\large\frac{1}{-20} = \frac{3+1}{3u}\\\\\large\frac{3u}{4} = -20\\\\\large 3u = -80\\\\u = -26.67\:\:cm$

∴, v = 3x -26.67 cm - 80 cm

The distance of image from the mirror when the image is real is 80 cm