Question

Show that when the light is incident at polarising angle, the reflected and refracted rays are mutually perpendicular to each other.

Answer 1

i. In the figure,

AB = incident ray

BD = refracted ray

BC = reflected ray

We have to show BD ⊥ BC

i.e. ∠DBC = 90°

iv. In the figure, i_p = r [from laws of reflection]

But i_p + r_p + ∠DBC = 180°

 \(\therefore\) 90° + ∠DBC = 180° [From equation (3)]

\(\therefore\) ∠DBC = 90°

\(\therefore\) BD ⊥ BC

Hence reflected and refracted rays are perpendicular at polarising angle.

Note:

1. When rays of light are allowed to fall on a transparent or translucent object, then some part of light gets reflected and remaining part gets refracted. This phenomenon is called partial reflection.

2. In 1808, Malus discovered the phenomenon of polarisation by reflection.

3. When a beam of unpolarised monochromatic light is incident on a plane glass plate, some part of light is reflected while the rest is transmitted. The reflected light is partially polarised. At certain angle of incidence, reflected ray is completely polarised.