Question

a concave mirror gives a real image magnified 4 times if the object moves 3cm the magnification of the real image is 3 times. find focal length

Answer 1

We can directly find the focal length by using

$\frac{f}{f - u} = - 3$

f/f-u+3=-3

f=-3f+3u+9

4f=3u+9

f=3u+9/4__1)

Initial U we need to find

-v/u=-4

v=4u

1/f=1/4u+1/u

1/f=5/4u

f=4u/5___2)

4u/5=3u+9/4

u=45

u=(-45) Applying sign convention

put this value in 1)

f=-36cm