Class 12 Physics Wave Optics Polarization by Polaroid

Polarization by Polaroid’s

  • Polaroids are polarizing materials consisting of long chain of molecules aligned in a particular direction.
  • Polaroid’s are in the form of thin sheets.
  • Every Polaroid has pass axis. Pass axis is like a gate of aPolaroid which determines how the light will pass through it.
  • There is horizontal as well as vertical pass axis.
  • Unpolarized light on passing through a Polaroidit gets polarized.


 Experiment 1:-Polarization by a single polaroid (P1)

  • When a beam of light passes through Polaroid was a polarized light.


  • The intensity was almost halved. As half of light rays were not allowed to enter only few were allowed to pass through it.
  • Rotation of thePolaroid P1 has no effect on the intensity of transmitted light.
  • There will be many different orientations but all are present in equal amount.
  • Whatever is the direction of the Polaroidbut always the intensity of the lightcoming out of the Polaroid will be same.
  • Sometimes it will allow vertically polarized light to pass and sometimes horizontal.


Experiment 2:-Polarization by a two polaroid (P1,P2)

  • One more Polaroid(P2) was introduced in between the unpolarized light and Polaroid(P1).


  • Intensity of the light is almost halved after P2.
  • When the polarized light enters the second Polaroid (P1) then
    • If the pass axis of the polarizer P1 is along the same direction as P2 then the light will come out of the Polaroid P1.
    • Intensity will remain same.
  • Rotation of the Polaroid P1 has effects on intensity of transmitted polarized light.
  • When the two axes are not oriented in the same direction i.e. if the axis of the P1 is perpendicular to the axis of the P2.
  • Then P1 won’t allow any light to pass through it as a result net intensity will be 0.
  • It concludes that :-
    1. Transmitted intensity=0 when pass axis of P1 is perpendicular to P2.
    2. Transmitted intensity=maximum when pass axis of P1 is parallel to P2.

Polaroid Experiment:Conclusion

  1. Intensity of the polarized light varies with the angle between the pass axes of the two Polaroid.
  2. I =0 whenθ =900.
  3. I =max when θ=00.
  4. As the angle increases from 00 to 900 the intensity keeps on decreasing.


  • Malus’ Law: Malus’ law explained when the angle between the pass axes be some arbitrary angle θ.
  • Experimentally it was found that intensity varies as cos2θ ;
    • I=I0 cos2θ.
    • Where θ =angle between the pass axes of two Polaroids, I0=Intensity of polarized light after passing through P1 and I =new intensity after rotation.


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