Polarisation

The phenomenon of restricting the waves in oscillation to particular plane is called polarisation. 

                           Note

A wave having oscillation in all possible direction is said to be polarised. 

When the all oscillation get can fined along one direction the wave is called plane polarised. 

Representation of Unpolarised and polarised

Pictorial representations of Unpolarised light

Plane polarised light: (a) vibrations parallel to the plane of paper. (b) vibrations perpendicular to the plane of paper. 

Experimental demonstration of polarisation

Intensity depends upon the angle between polariser axis and analyser axis. 

Law of molus

It States that the intensity of polarised light transmitted through the analyser varies as the square of cosine of the angle between the plane of transmission of analyser and polariser. 

                             Or

Angle between optical axis of analyser and polariser. 

I is directly proportional to cos(theta) × cos(theta) 

Proof - 

If 'a' be the amplitude of polarised light of polariser

Only "acos(theta)" passes through the axis of analyser

W.K.T

I is directly proportional to Amplitude × Amplitude

I is directly proportional to "acos(theta) × acos(theta)"

I = k × a × a × cos(theta) × cos(theta) 

I = Io × Io × cos(theta) × cos(theta) 

Where, Io = k × a × a is the intensity of incident light from polariser

Polarised light by reflection

Brewster's law - 

It states that when a light is incident at the Polarising angle, the reflected and refracted rays are perpendicular to each other. 

At the angle at the which Unpolarised light becomes plain polarised is known as Polarising angle, it is denoted by 'ip'. It is found that the sun of Polarising angle 'ip' and reflecting angle is 90°. 

We have, 

ip + r = 90°

r = 90° - ip

Now from snell's law 

This is known as Brewster's law.