Polarisation(Introduction)
Polarisation of light has great importance in our life. We see that polarized glasses are used to reduce the glare of the headlights of cars. Similarly, the photographer uses filters to take clear photos. The light reflected from the surface of the water sometimes dazzles the eyes, making it difficult to see. We will see the reason for all in this chapter. So here we are going to discuss the topic Polarisation of light.
Light is a non-mechanical wave( means required no medium to propagate).
Light is also transverse wave (means vibration of wave and propagation of wave are perpendicular to each other)
Light is also an Electromagnetic Wave in which electric and magnetic field vectors vary sinusoidal perpendicular to each other as well as perpendicular to the direction of propagation of the light wave.
NOTE:-
It has been found experimentally that the electric component of light is mainly responsible for optical effects. i.e vibration of light means vibration of the electric vector of light.
The light emitted by the general source is called ordinary light or unpolarised light. In unpolarised light, the vibration of the electric field vector is in all possible direction perpendicular to wave propagation.
If we restrict this vibration in a particular direction in a plane in any way. Then ordinary light is said to be polarised and known as polarised light or plane polarised light.
” The phenomenon of restricting the vibrations of light in a particular direction in a plane perpendicular to the propagation of light is called polarisation of light“
ORDINARY LIGHT INTO POLARISED LIGHT BY POLAROID / POLARISER
Here allow passing only those vibrations which are along pass-axis. Other vibrations are absorbed by polariser.
Q. How we can show that light is a transverse wave nature by simple experiment?
Ans:- Here we use two polaroids P1 known as polariser and P2 known as an analyser.
Suppose that ordinary light of intensity incident on the polariser P1. The intensity of polarised light emerging from P1 is
. If the transmission axis of P1 and P2 are parallel as fig(1), the intensity of light emerging from P2 will be
.
However, if the transmission axis of P1 and P2 are perpendicular as fig(2). No light will be transmitted through the analyser P2.
The above experiment shows that light is transverse in nature. If light wave were longitudinal, then the intensity of light emerging from P2 would not have been affected due to rotation.
LAWS OF MALUS
According to the law of malus, when completely plane polarised light is incident on the analyser, the intensity I of the light transmitted by the analyser is directly proportional to the square of the cosine of the angle between the pass axis of the analyser and the polariser.
i.e
It is clear from fig, the analyser will transmit only that component which is parallel to its transmission axis i.e
Therefore, the Intensity I of light transmitted by the analyser is
now,
Here is the intensity incident on the analyser.
from (1) and (2)
PARTIALLY POLARISED LIGHT
When both electric components of the light ray are present, but one is stronger than the other(i.e amplitude is varying). There is no stable phase relationship between the two perpendicular components. When such light is viewed through a rotating analyser, one sees a maximum and a minimum of intensity but not complete darkness. This kind of light is called partially polarised.(in plane polarised light, the vibration is confined in one plane with full amplitude)
POLARISATION BY SCATTERING
When an unpolarised ray of light travels through a medium of tiny particles which are separated by a distance not small as compared with the wavelength of light.
Some parts of light are absorbed by these particles and the remaining part is scattered in all direction. The scattered light will have completely polarised, partially polarised and complete unpolarised depending upon its direction of scattering.
From fig, light is travelling along the x-axis. Here scattered light along y and z-axis is completely plane polarised but another scattered light is polarised by varying degree)
POLARISATION BY REFLECTION
Ordinary light may be polarised by the process of reflection. When an unpolarised light is reflected from a surface, the reflected light may be completely polarised, partially polarised or unpolarised depending on the angle of incidence.
If the angle of incidence is or
, then reflected light is unpolarised. But for the angle of incidence between
and
, the reflected beam is polarised to a varying degree.
For a particular angle of incidence, the reflected light is completely plane polarised. This angle is called Brewster Angle .
If the light is incident on transparent medium, it has been found experimentally that when light is incident at the Brewster Angle, the reflected and refracted rays are perpendicular to each other.
BREWSTER LAW
It states that when light is incident at Brewster Angle () at the interface of a transparent medium, the R.I of the medium w.r.t the surrounding medium is equal to the tangent of the Brewster Angle.
where is the R.I of medium w.r.t surrounding.
from fig
using Snell’s law
A SPECIAL CASE OF TOTAL TRANSMISSION
| Introduction | Huygens Principle |
| Refraction and Reflection of Plane waves using Huygens Principle | Coherent and Incoherent Addition of Waves |
| Interference of Light Waves and Young’s Experiment | Diffraction |
| Polarisation | |
IMPORTANT QUESTIONS FOR PRACTICE
1. Discuss the intensity of transmitted light when a polaroid sheet is rotated between two crossed polaroids?
2. Unpolarised light is incident on a plane glass surface. What should be the angle of incidence so that the reflected and refracted rays are perpendicular to each other?
3. What is the Brewster angle for air to glass transition? (Refractive index of glass = 1.5.)
4. Light of wavelength 5000 Å falls on a plane reflecting surface. What are the wavelength and frequency of the reflected light? For what angle of incidence is the reflected ray normal to the incident ray?