5 Polarisation
Polarisation
- The phenomenon due to which the vibrations of light are restricted in a particular plane is called the polarisation of light.
 |
 |
| Plane polarised light having vibrations in the plane of the paper | Plane polarised light having vibrations in a plane perpendicular to the plane of the paper |
- To Detect Plane Polarised Light
The naked eye or the polariser cannot make distinction between the unpolarized light and the plane polarised light. Crystal that used to analyse the nature of light is called analyser.
If the axes of the polariser and the analyser are parallel to each other, then the intensity of light is found to remain unaffected.
The intensity of light becomes minimum when the axis of the analyser and polariser are perpendicular to each other.
- Polarisation by Reflection
An ordinary beam of light on reflection from a transparent medium becomes partially polarised. The degree of polarisation increases as the angle of incidence is increased. At a particular value of angle of incidence, the reflected beam becomes completely polarised. This angle of incidence is called the polarising angle (p).
- Brewster’s Law
When light is incident at polarising angle at the interface of a refracting medium, the refractive index of the medium is equal to the tangent of the polarising angle.
Where,
μ → Refractive index of the refracting medium
p → Polarising angle
- Polarisation by Scattering
When a beam of light passes through a medium, it gets scattered from the particles constituting the medium, provided the size of the particles is of the order of the wavelength of the light. The scattered light viewed in a direction perpendicular to the direction of the beam of light is found to be plane polarised. It gives us a method to produce plane polarised light by scattering.
- Law of Malus
It states that when a completely plane polarised light beam is incident on an analyser, the intensity of the emergent light varies as the square of the cosine of the angle between the plane of transmission of the analyser and the polariser.
