Question

. A 2cm high object is placed at a distance of 20cm from a concave mirror. A real image is formed at 4
0cm
from the mirror. Calculate the focal length of the mirror.

Answer 1

Given :
object distance=-20cm  [ sign conventions]
Image distance=v=?
focal length=f=-15cm

Formula to be used:
1/f=1/u+1/v
1/v=1/f-1/u
=-1/15+1/20
= -4+3/60
=-1/60
v=-60 cm
hence object is placed between F and c and image formed is beyond C .

Nature of image:

-->Real  AND INVERTED
--->MAGNIFIED
--->AT A DISTANCE OF 60CM FROM THE MIRROR.
Object height=ho=3cm
Image height=?
magnification=m=-v/u
=60/-20
=-3

m=hi/ho
hi=mxho
=-3x3
=-9cm