IEMDaily - Video Lecture Notes

Definition

Polarization of light waves

The phenomena of restricting the vibrations of light in a particular direction is called polarization of light.

Definition

Polarization by double refraction

When the unpolarized light ray falls on certain crystals like calcite, quartz etc. then it is found that 2 refracted rays are produced. This phenomena is called double refraction.
Out of 2 refracted ray
1. One is ordinary ray which obeys laws of refraction
2. Other is extra ordinary ray which does not obey laws of refraction

Definition

Polarization by dichroism

When a light is incident on strip of tourmaline crystal it splits in two polarized refracted rays due to double refraction.
Out of 2 refracted rays one is ordinary and other is extra ordinary/
In ordinary rays the vibrations perpendicular to principal section of the crystal is completely absorbed by the crystal and extra ordinary ray which has vibrations parallel to principal section of crystal is transmitted through the crystal.
The emergent ray is plane polarized with yellow green color.
This phenomenon of selective absorption of the crystal is known as dichroism.
The polaroid are based on this principle

Definition

Polarisation by scattering

When unpolarized light is scattered by small particles, the scattered light is partially polarized. The blue light received from the sky is accordingly partially polarized.

Example

Definition and use of Malus' law

Malus's Law. According to Malus, when completely plane polarized light is incident on the analyzer, the intensity

I

of the light transmitted by the analyzer is directly proportional to the square of the cosine of angle between the transmission axes of the analyzer and the polarizer.

Example: Two polaroid

A

and

B

are kept with their transmission axes at an angle

θ

with respect to one another. If the transmitted intensity of light

I_{t} = 0.75 I_{0}

where

I_{0}

is the intensity of light incident on the system then find

θ

.

Solution:

I_{t} = I_{0} \cos^{2} θ

\Rightarrow 0.75 I_{0} = I_{0} \cos^{2} θ

\Rightarrow \cos^{2} θ = \frac{3}{4}

\Rightarrow θ = 30^{0}

Definition

Polaroids

A polaroid is a type of synthetic plastic sheet used to polarize light. It consists of long chain molecules aligned in a particular direction. The electric vectors (associated with the propagating light wave) along the direction of the aligned molecules get absorbed. Thus, if an unpolarised light wave is incident on such a polaroid then the light wave will get linearly polarized with the electric vector oscillating along a direction perpendicular to the aligned molecules.

Definition

Light through two polaroids

When light passes normally through a polaroid, electric vector oscillating in the direction of the aligned molecules of the polaroid get absorbed and the electric field vector perpendicular to it is passed and hence then intensity is halved. When a second polaroid is introduced in front of the first polaroid the same phenomenon occurs. So, if the transmitted wave from the first polaroid has electric field vector perpendicular to the molecules of the second polaroid, then a full intensity of light comes out and if they are perpendicular, no light comes out. Hence, rotation of the second polaroid causes variation of intensity of the output light intensity.

Example

Uses of Polaroids

Polaroids polarize light: A number of needle shaped crystals of quinine iodosulphate with their axes parallel to one another are packed between two sheets of plastic. This arrangement serves as the polaroids.

Important uses are:

These reduce excess glare and hence sun glasses are fitted with Polaroid sheets.
These are also used to reduce headlight glare of cars.
They are used to improve color contrast in old oil paintings.
These are useful in 3-D motion pictures i.e., holography.
Wind shields of automobiles are also made of Polaroid sheets.