Polarisation

If you step out of your house on a hot sunny day, then you would definitely use sunglasses. Also, if you are watching a 3D movie, then you would use 3D glasses. In both cases, polarizationÂ plays a very important role. Let us see what polarisation is and study it in detail.

Polarisation MeaningÂ

Polarisation is nothing but transforming unpolarised light into polarized light. Unpolarised light is the light in which particles vibrate in all different planes.

Â  (Â Source: Wikipedia )Â

Ways of PolarisingÂ the Light

Polarisation by Polaroids

Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â (Â Source: s-cool )

In the above figure, we see that there is a planeÂ of vibrationÂ parrel to the plane. Also, there is a plane of vibrationÂ perpendicular to the plane. Above, the first image is of unpolarised one. The second image is polarized which is either perpendicularÂ or parallel. So let us start understanding polarization by polaroids.

Polaroids are the polarising materials consisting of molecules aligned in a particular direction. Every Polaroid has a pass axis. It will allow light to pass only through the pass axis. A polaroid can have horizontal pass axis as well as the vertical pass axis. These determine how the light will pass through it. So when an unpolarized light passes through a polaroid, it gets polarized.

Polarisation by Scattering

When light incidents on a molecule or an atom, it absorbs the light energy and it re-emits it in different directions. This is scattering is due to polarization. Also, the emitted light travels in different directions.

Furthermore, we get scattered light when unpolarized light is incident on a particle. So when unpolarized light passes through a molecule, it polarises the light in the direction perpendicular to the incident ray. Thus polarisation of light happens along this direction.

And this is how polarisation occurs by scattering of light. The scattered light is in a direction perpendicular to the direction of the incident ray. Also, polarization is complete in scattered light whereas it is partial in light passing through molecules.

Polarisation by Reflection and Refraction

Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  Â  (Â Source: Wikipedia )

In the above figure, we can see the incident ray, reflected ray and the refracted ray. On the incident ray, we see the unpolarized light. The unpolarized light is denoted as shown in the above figure. Here the dot represents perpendicular directions and lines indicate the parallel direction.

It is observed that most of the light in the reflected ray is polarised parallel to the plane with a very few unpolarised ones. Whereas in the refracted ray, most of the light is unpolarised with one or two polarized components. Thus we see that both the reflected and the refracted ray are partially polarised.

Brewster’s law

The law states that at a particular angle of incidence, the reflected ray is completely polarized. Also, the angle between the reflected ray and refracted ray is 90Â°. If i = iB, i.e. when the angle of incidence is equalÂ to Brewster’s angle, then total angle = 90Â°.

Then by Snell’s law, we say thatÂ $$\frac{Sin i }{Sin r }$$ =Â Î¼

âˆ iBÂ + rÂ  =Â Â 90Â°

r =Â 90Â°-Â iB

â‡’Â $$\frac{Sin iB}{cos iB}$$ =Â Î¼

â‡’ tanÂ iB =Â Î¼

Solved Questions For You

Q1.Â A ray of light is incident on the surface of the plateÂ of glass of refractive index 1.5 at the polarisingÂ angle. Then the angle of refraction of the ray will be.

1. 33.7Â°
2. 43.7Â°
3. 23.7Â°
4. 53.7Â°

Answer: A. If iPÂ be the polarising angle, then, tanÂ  iPÂ =Â Î¼ = 1.5 or iPÂ = 56.3Â°

Angle of refraction, r = 90Â° –Â  iPÂ = 90Â° –Â 56.3Â° = 33. 7Â°

Q2.Â In case of linearly polarised light, the magnitudeÂ of the electric field vector.

1. Varies periodically with time
2. Does not change with time
3. Increases or decreases with linearly with time
4. Is parallel to the direction of propagation

Answer: A. In any type of light whether polarised or unpolarized, the magnitude of electric field vector always varies periodically with time.

Share with friends

Customize your course in 30 seconds

Which class are you in?
5th
6th
7th
8th
9th
10th
11th
12th
Get ready for all-new Live Classes!
Now learn Live with India's best teachers. Join courses with the best schedule and enjoy fun and interactive classes.
Ashhar Firdausi
IIT Roorkee
Biology
Dr. Nazma Shaik
VTU
Chemistry
Gaurav Tiwari
APJAKTU
Physics
Get Started