Class 12 Physics Polarization Notes

Class 12 Physics Polarization Notes
UNIT 3
CLASS 12 PHYSICS • WAVE AND OPTICS

Polarization

Chapter 13
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Polarization

The phenomena of interference and diffraction shows that light is wave motion but they don’t prove whether the waves are transverse or longitudinal. The phenomena of light on which the vibration of light are restricted in a particular plane is called the polarization of light and such a light is called plane polarized light.

In other word the phenomena which study the transverse nature of light is known as polarization.

Difference Between Polarized and Unpolarized Light

Unpolarized Light Polarized Light
The vibration of light is possible in all direction with equal amplitude. The vibration of light is confined to one plane only.
It has more intensity. It has less intensity.
It is represented by vibrations in many directions. It is represented by vibrations in one plane only.
Comparison of unpolarized and plane polarized light Unpolarized light Many vibration planes Polarized light One vibration plane
Unpolarized and plane polarized light

Analyzer and Polarizer

Analyzer

Analyzer is a device which analyzes whether the light is polarized or not.

Polarizer

Polarizer is a device which polarizes the light.

Polaroid

A device used to produce plane polarized light is called Polaroid.

Polarizing Angle

The angle of incidence at which the reflected light is completely polarized is called polarizing angle.
Polarizing angle at a reflecting surface Incident light Polarized reflected light Refracted light θₚ
Polarizing angle

Brewster’s Law

It states that the refractive index of a medium is equal to the tangent of the polarizing angle for the given medium.
μ = tanθp
μ = refractive index
θp = polarizing angle

Proof of Brewster’s Law

Brewster law geometry showing unpolarized incident light, polarized reflected light and refracted light Unpolarized incident light Polarized reflected light Refracted light θₚ r 90° Surface MN
Brewster’s law geometry

Let AB be the unpolarized incident light at angle θp which incident at point ‘B’ on MN surface whose refractive index is ‘μ’ and some portion of ray get reflected along BC which is polarized and remaining portion get refracted along BD with angle of refraction ‘r’.

Then from Snell’s law:

μ = sin i / sin r
μ = sinθp / sin r   — (i)

Brewster found at polarizing angle θp, the angle between reflected light and refracted light is 90°.

θp + 90° + r = 180°
r = 90° − θp

So equation (i) becomes:

μ = sinθp / sin(90° − θp)
μ = sinθp / cosθp
μ = tanθp

which proves Brewster’s law.

Transverse Nature of Light

Demonstration of transverse nature of light using two polarizing crystals with parallel and crossed axes P₁ and P₂ parallel P₁ P₂ Maximum P₂ rotated by 90° P₁ P₂ No light This shows the transverse wave nature of light.
Demonstration of transverse nature of light

When ordinary light is passed through a pair of crystal P1 and P2 with their plane at right angle to the direction of light as shown in figure (1), the intensity is maximum in this position.

But when the plane of ‘P2’ is rotated through plane as shown in figure (2), the intensity is minimum in this case. This shows that light is transverse wave nature.

Some Useful Terms

Brewster’s Law
μ = tanθp
At Polarizing Angle
θp + 90° + r = 180°
Refractive Index and Velocity
μ = c/v
c = velocity of light = 3×108 m/s
Critical Angle Relation
μ = 1/sin C
C = critical angle
Relative Refractive Index
wμg = μg/μw
Relation Written in the Source
wμg ≈ tanθp
Therefore, μg/μw = tanθp
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