Polarization
The vibration of the light waves in the single plane is called polarization.
The light wave shows different property in different direction perpendicular
to the propagation of light due to which light is transverse wave. The
transverse nature of the light is polarization of light. This phenomenon was discovered by Huygens
in 1960.
Why light is a transverse wave not longitudinal wave?
Logic: In light, particle displacement is perpendicular to the
direction of wave propagation.
Experimental Reason:
When the light passes(incident) through the tourmaline crystal and
on further rotating that crystal, there is no any remarkable change is noticed.
When Two tourmaline crystal are placed parallel to each other and one is
rotated making another fixed, the intensity of light gradually decreases and no
light comes out if both are perpendicular.
By this experiment we came to know that light is not a longitudinal
wave but a transverse wave.
Tourmaline Crystal: It is a silicate compound which has a special feature i.e., any
electric field that is perpendicular to axis of crystal is completely absorbed.
Difference Between Polarized and Unpolarized Light:
|
Polarized
Light |
Unpolarized
Light |
|
The light wave having vibration along single direction is called polarized light. |
The light wave having vibration along different direction is called polarized light. |
|
It is unsymmetrical about the direction of propagation of light. |
It is symmetrical about the direction of propagation of light. |
|
When it passes through tourmaline crystal, there is change in intensity. |
When it passes through tourmaline crystal, there is no change in intensity. |
|
Diagram: |
 |
Polaroid: Polaroid is a crystal which contains long chain of molecule aligned in particular direction, used to produce a plane polarized light.
They are the crystal of iodosulphide of quinine known as herapathite.
Note: When light incident on two polaroid one after another kept
perpendicular to each other, there is no transmission of light through second
polaroid. But when light is incident on two polaroid one after another kept
parallel to each other, there is transmission of light through both polaroid.
Fig: Two Polaroid’s with transmission axis Parallel
Fig: Two Polaroid’s with transmission axis Perpendicular
Day to day use of Polaroid
i. Use of Sunglass while viewing bright surface: The bright
surface produce glare which might be harmful to our eyes, which can be avoided
by wearing sunglass made of polaroids with vertical transmission axis. Those
sunglasses help to reduce the intensity of light.
ii. Polaroid are used to record three dimensional Pictures.
iii. Polaroids are used in photo-elasticity.
iv. They are used in LCD displays of watches.
Brewster ‘Laws:
In 1811 Brewster developed the relation between the polarizing
angle and refractive index (μ) of the transparent substance with respect to the
surrounding medium.
Simply, the mathematical relation between polarizing angle θp
and refractive index μ of substance and surrounding is called Brewster’s Law.
i.e., μ=tanθp
Derivation: Let
us consider unpolarized beam of light PQ incident on the transparent substance
at an incident angle θp. Which gets reflected along QR and refracted
along QS as shown in figure below. Let r be the refracting angle.
Note:
PQ: Unpolarized Light
QR: Polarized Light
From Snell Law:
..................(i)From Brewster Law:
........(ii)From i and ii:
Or, Cosθp = Sin r
Or, Sin (90-θp) = Sin r
Or, 90 - θp=r
Or, r + θp =90
As, i = θp,
Then, i + r = r+ θp =90 à[Where i is incident angle]
Notes:
1.
Polarization is only shown by transverse wave.
2.
Unpolarized light are symmetrical whereas
polarized light is unsymmetrical.
3.
Tourmaline crystal is used to polarize light.
4.
Ordinary source of light are polarized.
5.
Polarizing angle is dependent of color of
light.
6. Our necked eye is unable for polarization detection.