Wave Optics

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, polarization plays a very important role. Let us see what polarisation is and study it in detail.

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Polarisation Meaning 

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

Polarisation

  ( Source: Wikipedia ) 

Ways of Polarising the Light

Polarisation by Polaroids

Polarisation

                                                                                                                                     ( Source: s-cool )

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

Polarisation

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.

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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

Polarisation

                                                                                                                      ( 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 90°. If i = iB, i.e. when the angle of incidence is equal to Brewster’s angle, then total angle = 90°.

Then by Snell’s law, we say that \( \frac{Sin i }{Sin r } \) = μ

∠i+ r  =  90°

r = 90°- iB

⇒ \( \frac{Sin iB}{cos iB} \) = μ

⇒ tan iB = μ

Solved Questions For You

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

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

Answer: A. If ibe the polarising angle, then, tan  i= μ = 1.5 or i= 56.3°

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

Q2. In case of linearly polarised light, the magnitude 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.

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