# I Polarisation of light

1. Sep 18, 2016

### Ankush GK

W.k.t light gets polarised when incident from rarer to denser medium.(gets polarised by reflection at a certain angle ie: angle of polarisation)
But if the light is sent from a denser medium to rare mediym, will there be any polarisation??
If there is, will it be due to reflection??
And can the angle of polarisation become equal to critical angle in that case, leading to total internal teflection??
Ultimately my doubt is that can light be polarised by 'total internal reflection??'

2. Sep 18, 2016

### drvrm

If the Brewster angle for polarisation by reflection equals to the angle for Total internal reflection becomes identical one can think of polarisation by TIR....
but the above statement seems to be logically 'incorrect' as polarisation by reflection means part of the incident light gets polarised in a particular manner and another part gets transmitted but a TIR will mean whole of it being reflected back so all types of vibrations will be /should be there.
one should analyse further as to "how this selection of polarised states occur and why at a particular angle?

3. Sep 18, 2016

### jfizzix

If you want to see how things ought to work out for yourself, you should look up the Fresnel equations. These equations describe the amplitudes of transmitted and reflected light given the indices of refraction of the two materials, the polarization of the incident light, and the angle of incidence of that light.

They're a pretty comprehensive and nifty consequence of Maxwell's equations of electromagnetism.

From a high index to a low index, you should still see the reflected component of light be partially polarized, and at the critical angle, called Brewster's angle see the reflected component be perfectly polarized. Depending on the polarization of the incident light, you could get a bright reflection, or no reflection at all at Brewster's angle.

The polarization won't be "due to reflection", so much as the properties of the materials that give rise to the Fresnel equations.

4. Sep 18, 2016

### Ankush GK

Is there any known polaroid/crystal which can polarise the light incident on it, twice?(once when incident and the other when the refracted light emerges)

5. Sep 18, 2016

### drvrm

i can not say off hand but one can check
see https://www.uwgb.edu/DutchS/Petrology/xls-pol.htm

6. Sep 18, 2016

### sophiecentaur

To get polarisation from a crystal, you need birefringence. Air is not birefringent so you (I) would not expect any polarisation of the two emerging polarised rays from a birefringent crystal.

7. Sep 18, 2016

### Ankush GK

What is birefringence??

8. Sep 19, 2016

### sophiecentaur

I thought, from your question, that you would know about birefringence because that is how crystals produce polarisation. If you google the term you will find many hits so choose one that suits your particular level of knowledge. This wiki link may be of interest.
If you get two rays out of a birefringent crystal then you can pass one of the (plane polarised) rays into another crystal, orientated at 45° and that can produce two new rays which are polarised at +/- 45° to the first polarise ray. The second crystal resolves the E field of the first polarised ray into E field components alright angles to each other. You can carry on doing this 'for ever' and with as many different polarisation angles as you want if you choose appropriate orientation angles for the subsequent crystals. (But there is always loss through the crystals and the Energy gets divided up each time a ray is split this way.

9. Sep 19, 2016

Thank you