Question

A concave mirror has a focal length of 20cm. Find the position or positions of an object for which the imagesize is double of the object-size.

Answer 1

Given :

focal length = f= - 20cm

m= -v /u = 2

v= - 2u

First case : object between p and f

Virtual image

1/f= 1/v+1/u

1/2u - 1/u = 1/f

1/2u= 1/f

u= f/2 = -20/ 2 = - 10cm

second case :

1/f= - 1/2u - 1/u

= -1-2/2u

1/f= -3/2u

f= - 2u/3

u= - 3f/2 = - 3x 20/2= - 30 cm

so the positions of objects are  10 cm or 30 cm from the concave mirror

Answer 2

hope it will help you ☺️