Polarised light is light for which the oscillation of the electric field occurs in one plane only. A polariser is a device that filters out the component of the oscillation that is perpendicular to its axis of polarisation (only the parallel component passes through).

I will show in this video, that incident Light for which the plane of polarisation is not aligned with the polarisers’ axis, sees its amplitude decrease when passing through the polariser. The amplitude of a wave is related to the energy it carries, so the energy carried by that light also decreases when it passes through a polariser.

Malus law quantifies this phenomena. After explaining that “Intensity” is the quantity used to express the energy carried by a wave, I derive Malus Law. I show how Malus law relates the output intensity of the light passing through a polariser to the incident intensity and the angle between the incident light’s polarisation plane and the axis of polarisation (I = I0 x cos^2(teta)). This is followed by a short exercise to show how to use Malus law in practice.

Then, I look at what happens to the energy of light passing through a polariser when the incident light is not polarised. The intensity is cut if half. The real proof will be subject of another video (Why does a polariser cut the intensity of unpolarised light in half? (Polarisation Part 3 Waves Physics).

I conclude with an application example where unpolarised light passes through two polarisers.

*** Content ***
00:00 What is polarised light?
00:44 Effect of a polariser on polarised light
02:30 Effect of a polariser on the amplitude (A = A0 * cos (teta)) and the energy carried by the wave.
04:23 What is Intensity?
05:23 Deriving Malus Law (starts from the relation between amplitude and Intensity of a wave).
06:47 Example of application of Malus Law (polarised light passing through a polariser).
07:42 Effect of a polariser on unpolarised light.
09:51 Example of application of Malus Law (unpolarised light passing through a couple of polarisers in series).

*** Other Videos about Polarisation ***
What is Polarised Light? (Polarisation Part 1 Waves Physics)
Link to Video:    • What is Polarised Light? (Polarisatio...  

Why does a polariser cut the intensity of unpolarised light in half? (Polarisation Part 3 Waves Physics)
Link to Video: To be announced


This video is produced and presented by Edouard Reny, Ph.D. in solid state chemistry and private tutor in Physical Sciences.

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