Question

how far should hold an object from a concave mirror of focal length 40cm.so as to get a virtual image twice the size of the object?​

Answer 1

Answer:

u=-20cm

Explanation:

the mirror equation is 1/v+1/u=1/f

as the mirror is concave the focal point will be negative ie -40cm

given that the image should be 2 times the size of object ie, v=2u

substituting this in the equation(make sure u is negative)

1/2u-1/u=1/40

-1/2u=1/40

-2u=40

u=-20cm

therefore objectt should be placed 20 cm in front of mirror

hope i helped :)