Question

find the position of an object placed in front of a concave mirror of focal length 20 cm so as to have an image 4 time inlarge.

Answer 1

Heya user,

We consider : m = 4 or -4

For m = 4,
---> - v / u = 4
==> v = -4u

Applying the formula,
=>$\frac {1}{f} = \frac{1}{u} + \frac{1}{v}$;
=>$\frac{1}{-20cm} = \frac{1}{u} + \frac{1}{-4u} = \frac{-3}{u}$
=> u = 20 * 3 = 60cm

Similarly, for m = -4;
----> v = 4u 
==> u = 5 * -f = -100cm

Hence, the object can be placed at -100cm or 60cm to get the image 4 times large

Answer 2

Answer:

25 cm for real image and inverted image and 15 cm for the virtual and erect image