The image of an object placed at a point A before a plane mirror LM is seen at the point B by an observer at D as shown in Figure. Prove that the image is as far behind the mirror as the object is in front of the mirror.
Given : ABC is an triangle and let O is a point on AB.
To prove : OA = OB
Proof : Since i and r is angle of incidence and reflection respectively.
CN is normal then
LCA = 90 – i, MCD = 90 – r
ACN = DCN = i = r (Given)
LCA = MCD = 90 – i = 90 – r (Since, i = r )
BCL = 90 – r (vertically opposite angles)
BCL = LCA
L bisects BA
Hence, OB = OA,
Hence, proved.