Why does a polariser cut the intensity of unpolarised light in half? (Polarisation Part 3 - Physics)
Physics Made Easy
Malus’s Law relates the intensity of light coming out of a polariser to the intensity of the polarised light coming in. But what if the light coming in the polariser is unpolarised?
The answer given in text books is straightforward. The intensity gets halved (I = Io/2). The reason why is not often mentioned… Even googling it or researching Youtube does not provide a clear insight. So I scratched my head and derived it for you!
But first, one needs to clearly understand what it means for light to carry energy by defining a commonly used quantity to quantify it (Intensity), and then, to clearly understand what is unpolarised light vs. Polarised light.
After a quick reminder of what a polariser does to the intensity of a polariser beam of light, we can finally enter the meat of the video:
By deriving the average of Malus Law (taken here as a function) over all the possible angles of polarisation, we mathematically arrive to the conclusion that when the incident light is unpolarised, the intensity coming out of a polariser will always be half of what it was before entering the polariser.
Enjoy the ride!
**** Content **** 00:16 What is the energy carried by an EM wave? 01:44 Quantifying the Energy of an EM wave (defining intensity). 03:20 Understanding what is unpolarised light. 05:40 Effect of a polariser on the energy of a polarised wave (Malus Law) 05:23 Deriving Malus Law (starts from the relation between amplitude and Intensity of a wave). 06:54 Effect of a polariser on the energy of an unpolarised wave: Proof that I = I0/2 (by averaging Malus Law over all polarisation angles of Malus law).
**** Other Videos about Polarisation **** What is Polarised Light? (Polarisation Part 1 - Waves - Physics) Link to Video: https://youtu.be/U8FanZu4X1I
Malus Law (derivation and use) + What is Intensity (Polarisation Part 2 - 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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Edouard provides one-on-one private tuition in Physics (face to face or by Skype). If you wish to contact him, visit his website: https://www.physics-tutor.nl/
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