Polarized Light....

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sps27

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I have some doubts concerning Polarized light and have read some of the other threads on this issue but wasn't quite convinced with the explanation mentioned therein so I've tried to articulate my doubts etc., and let me know if I am way off track.

The specific example I am alluding to is on page 236 of TBR Physics part 2, example 10.2a. In the example, a vertical polarized light is sent through an empty sample cell first and it then falls on a horizontally oriented polarizer. So nothing comes through out of the polarizer. Next the sample cell is filled with D glucose and one quarter of incident light intensity gets through. This example uses Malu's law which is I = I(original)Cos^2theta where I is the intensity of light coming out of polarizer and I(original) is the intensity of incident light and theta is the angle of incident light to polarizer.

So using Malu's law they've calculated that incident light should be at a 60 deg angle to the polarizer, in which case 25% of it will fall through the polarizer. So therefore, the D glucose must be rotating the Vertically polarized light by 30 deg. And all of that makes sense to me.

What confuses me is this. So if an unpolarized light incident at 30 deg angle to the polarizer will pass through with 75% intensity -- at 45 deg angle will pass through with 50% intensity -- at 60 deg will pass through with 25% intensity -- at 90 deg will not pass through. So the idea that polarizer allows light vibrating in just 1 plane (parallel to the plane of polarization) does not hold. It will allow all planes at varying intensity except the one which is oriented perpendicular to the plane of polarization. And the maximum intensity will be for the one which is parallel to the plane of polarization. Is that correct?

OR do the molecules in the polarizer take that 60 deg incident light, rotate 25% of that light intensity by 60 deg and out comes 25% intensity but rotating only in the plane of the polarizer?

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After a bit of search on the Internet I think I understand this now. And for what's its worth -- Malu's law is applicable only for linearly polarized light falling on polarizer, not for an un-polarized light. So for an un-polarized light, as the theory suggests, only a single plane of vibration of light parallel to plane of polarization passes through. I don't think TBR mentioned this fact in the book, but in any case, I hope this helps others as well.....
 
Im not sure if I understand your question. Are you asking why its only light that is polarized perpendicular to a polarizer that is 100% blocked? And why light at other degrees to the polarizer are allowed through to varying extents?

If so, its quite simple actually. The thing we are describing when we talk about polarized light is the direction of oscillation of the electric field of light. Electric field is a vector, and thus has components. So even though light polarized, say, 60 degrees to the horizontal, is not aligned with a horizontal polarizer, it still has a horizontal vector component that IS aligned with it. That is what passes through.

The greater the parallel component, the greater the intensity of light that gets through. Only in the case of light polarized perpendicular to the polarizer is there no parallel vector component and thus complete blocking of all incident light.

Hope that helps
 
Im not sure if I understand your question. Are you asking why its only light that is polarized perpendicular to a polarizer that is 100% blocked? And why light at other degrees to the polarizer are allowed through to varying extents?

If so, its quite simple actually. The thing we are describing when we talk about polarized light is the direction of oscillation of the electric field of light. Electric field is a vector, and thus has components. So even though light polarized, say, 60 degrees to the horizontal, is not aligned with a horizontal polarizer, it still has a horizontal vector component that IS aligned with it. That is what passes through.

The greater the parallel component, the greater the intensity of light that gets through. Only in the case of light polarized perpendicular to the polarizer is there no parallel vector component and thus complete blocking of all incident light.

Hope that helps

So if an un-polarized light vibrating in several different planes were incident on a polarizer, the horizontal component (Cos(theta)) of all those vibrations in the plane of polarizer will pass through, so essentially what comes out of the polarizer is just one component. Ok, that makes a whole lot of sense to me. Thanks bud. So Malu's law is applicable in all situations, for polarized and un-polarized light.......and not just for incident polarized light as some of the posts on the Internet seem to claim.....

Thanks!
 
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So if an un-polarized light vibrating in several different planes were incident on a polarizer, the horizontal component (Cos(theta)) of all those vibrations in the plane of polarizer will pass through, so essentially what comes out of the polarizer is just one component. Ok, that makes a whole lot of sense to me. Thanks bud. So Malu's law is applicable in all situations, for polarized and un-polarized light.......and not just for incident polarized light as some of the posts on the Internet seem to claim.....

Thanks!

Woah, slow down there buddy haha. I was simply describing why light at angles to a polarizer can still get through. Thats a qualitative explanation. Calculating their exact intensity is a quantitative task and likely not as simple. From my understanding, Malus' law (not malu's btw) does indeed only apply to plane-polarized light. Unpolarized light is a pretty complicated phenomenon and I am not familiar with any simple equation that can give you the intensity of light that results after polarizing it. Your rationale of what comes out of the polarizer essentially being one big component is fine though.

Anyway, something as complicated as calculating the intensity of polarized light from an unpolarized source is certainly beyond the scope of the mcat.
 
Woah, slow down there buddy haha. I was simply describing why light at angles to a polarizer can still get through. Thats a qualitative explanation. Calculating their exact intensity is a quantitative task and likely not as simple. From my understanding, Malus' law (not malu's btw) does indeed only apply to plane-polarized light. Unpolarized light is a pretty complicated phenomenon and I am not familiar with any simple equation that can give you the intensity of light that results after polarizing it. Your rationale of what comes out of the polarizer essentially being one big component is fine though.

Anyway, something as complicated as calculating the intensity of polarized light from an unpolarized source is certainly beyond the scope of the mcat.
I think the intensity of polarized light from an un-polarized source will always be 50%. At least that is what I have read. But as you said, for MCAT level I think this is good enough understanding and I will leave it at that. Thanks a bunch for your input though.
 
I think the intensity of polarized light from an un-polarized source will always be 50%. At least that is what I have read. But as you said, for MCAT level I think this is good enough understanding and I will leave it at that. Thanks a bunch for your input though.

It is correct that the transmitted intensity of unpolarized light incident through a polarizing filter will always be 50% of the incident intensity.
 
I think the intensity of polarized light from an un-polarized source will always be 50%. At least that is what I have read. But as you said, for MCAT level I think this is good enough understanding and I will leave it at that. Thanks a bunch for your input though.

Hmm, yeah now that I think about it, that does make sense. Due to the symmetry of all the planes of polarization that make up unpolarized light, a net 50% of the electric field vector components will be aligned with any one polarizer. This is why I love answering questions, even the answerer can learn. Thanks!
 
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