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Author Topic: Why can I see a mirror reflection despite polaroid sunglasses?  (Read 11012 times)

Offline Mark Severn

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Mark Severn  asked the Naked Scientists:

Hi All

newbielink:http://www.thenakedscientists.com/HTML/podcasts/ [nonactive] and listen to it every week via newbielink:http://www.thenakedscientists.com/HTML/podcasts/ [nonactive].
 
I have a question about polarising sunglasses. I am a keen fisherman and use polarising sunglasses to help me see into the water and spot fish. I understand that they work because the light reflected off the water is polarised horizontally. The sunglasses block this horizontally polarised light. The same effect can be seen with the windscreen of a car and when viewed with polarising lenses it makes it possible to see an oncoming drivers face more clearly.

So why can I still see a reflected image in a mirror? If all reflected waves are horizontally polarised, that is parallel to the reflected surface, should not the light be blocked by the sunglasses as well?

Regards
 
Mark Severn
Melbourne
Australia

What do you think?


 

lyner

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Water is, essentially, an insulator. Light is reflected at an air / water interface due to the change in refractive index. The amount of reflection for normal and parallel polarisation is different (particluarly at oblique angles)  which is why polaroid glasses work. You've already lost one polarisation component (50% ) before the glasses get in the way -they just get rid of the other component.
The reflection in a mirror is due to the metal silvering having very high  conductivity. Reflection of all polarisations at a highly conducting surface is the same , giving you  almost 100% reflection.
 

Offline lightarrow

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Mark Severn  asked the Naked Scientists:

Hi All

Love the show and listen to it every week via podcast.
 
I have a question about polarising sunglasses. I am a keen fisherman and use polarising sunglasses to help me see into the water and spot fish. I understand that they work because the light reflected off the water is polarised horizontally. The sunglasses block this horizontally polarised light. The same effect can be seen with the windscreen of a car and when viewed with polarising lenses it makes it possible to see an oncoming drivers face more clearly.

So why can I still see a reflected image in a mirror? If all reflected waves are horizontally polarised, that is parallel to the reflected surface, should not the light be blocked by the sunglasses as well?

Regards
 
Mark Severn
Melbourne
Australia

What do you think?
Not all reflected waves are horizontally polarised, but only those reflected in an almost horizontal plane. When light from the sun is reflected off a windscreen or a water surface, it is polarized with the same plane of that surface; if you have, instead, a mirror or another reflecting surface near the road, for example, which is put in vertical and a source of light which doesn't come from above but from the road, then light is polarized vertically and you would see the reflection completelly; but you understand that such a situation is very unusual, you should have, for example, lights from a car's lamps reflected off a window (during the night) or something of that kind.
Anyway, remember that even if the surfaces are horizontal, reflected light is not completely polarized; this would happen only at a specific reflection angle: "Brewster's angle", which, for glass is about 56:
http://en.wikipedia.org/wiki/Brewster's_angle
« Last Edit: 22/11/2008 18:31:46 by lightarrow »
 

Offline Mark Severn

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Water is, essentially, an insulator. Light is reflected at an air / water interface due to the change in refractive index. The amount of reflection for normal and parallel polarisation is different (particluarly at oblique angles)  which is why polaroid glasses work. You've already lost one polarisation component (50% ) before the glasses get in the way -they just get rid of the other component.
The reflection in a mirror is due to the metal silvering having very high  conductivity. Reflection of all polarisations at a highly conducting surface is the same , giving you  almost 100% reflection.

Hi Sophiecentaur


I am not sure that the effect is due to whether or not the reflective surface is conductive or not. I believe that it is more to do with Lightarrow's explanation.

Cheers

Mark 7
 

Offline Mark Severn

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Mark Severn  asked the Naked Scientists:

Hi All

newbielink:http://www.thenakedscientists.com/HTML/podcasts/ [nonactive] and listen to it every week via newbielink:http://www.thenakedscientists.com/HTML/podcasts/ [nonactive].
 
I have a question about polarising sunglasses. I am a keen fisherman and use polarising sunglasses to help me see into the water and spot fish. I understand that they work because the light reflected off the water is polarised horizontally. The sunglasses block this horizontally polarised light. The same effect can be seen with the windscreen of a car and when viewed with polarising lenses it makes it possible to see an oncoming drivers face more clearly.

So why can I still see a reflected image in a mirror? If all reflected waves are horizontally polarised, that is parallel to the reflected surface, should not the light be blocked by the sunglasses as well?

Regards
 
Mark Severn
Melbourne
Australia

What do you think?
Not all reflected waves are horizontally polarised, but only those reflected in an almost horizontal plane. When light from the sun is reflected off a windscreen or a water surface, it is polarized with the same plane of that surface; if you have, instead, a mirror or another reflecting surface near the road, for example, which is put in vertical and a source of light which doesn't come from above but from the road, then light is polarized vertically and you would see the reflection completelly; but you understand that such a situation is very unusual, you should have, for example, lights from a car's lamps reflected off a window (during the night) or something of that kind.
Anyway, remember that even if the surfaces are horizontal, reflected light is not completely polarized; this would happen only at a specific reflection angle: "Brewster's angle", which, for glass is about 56:
newbielink:http://en.wikipedia.org/wiki/Brewster's_angle [nonactive]

Hi Lightarrow

Are you saying that the only light reflected from a transparent surface is likely to be polarised in the same plane as the surface, and any other polarisations are likely to pass through the surface or be absorbed? Where as a Mirrored surface will reflect all polarisations? I did check the Wiki entry but don't quite understand what is going on in relation to my question.

Cheers

Mark 7

 

Offline lightarrow

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Hi Lightarrow

Are you saying that the only light reflected from a transparent surface is likely to be polarised in the same plane as the surface, and any other polarisations are likely to pass through the surface or be absorbed? Where as a Mirrored surface will reflect all polarisations? I did check the Wiki entry but don't quite understand what is going on in relation to my question
Yes, semplifying it's that way. What you say however happens only for a specific incidence angle, as I said, the Brewster's angle; at other angles, the light is not totally polarized (along the plane of the surface) but however is prevailingly polarized in that direction.
http://webphysics.davidson.edu/physlet_resources/bu_semester2/c27_brewster.html

In the following video the reflecting surface is in a vertical plane, so light reflected off it is prevailingly vertically polarized: a laser emits horizontally polarized light and the reflected light is very low, near brewster's angle:
http://video.google.com/videoplay?docid=-1279506019555091221
 

lyner

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Mark Severn
Light of all polarisations is reflected almost completely by a (shiny) metal, highly conductive surface for all angles of incidence. Polaroid glasses will reduce the brightness by 1/2.

A, possibly, small fraction of light is reflected at transparent surfaces, the rest is transmitted. At the Brewster Angle, only parallel polarisation is reflected and no normal polarisation is reflected. Your view of a scene includes a small proportion of surfaces for which this applies totally but, for low angles, you can expect a majority of reflected light to be parallel polarised - the polaroid glasses will eliminate this - reducing the total brightness of the HP component by much more, of course.

There is no 'change' of polarisation' on reflection - you just get a selection of one component.

Logically, you get less and less polarisation as the angle of incidence approaches zero. (Which polarisation would be which for normal incidence?)

You get much brighter reflections from a highly conducting surface, whatever.
« Last Edit: 26/11/2008 23:11:06 by sophiecentaur »
 

Offline Mark Severn

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Hi All

If only light hitting the surface of water is reflected only at a specific angle then why can I see a full reflection of the sky to horizon when looking at a very still lake for instance. I still don't think it has anything to do with a conductive surface.( I assume by conductive you mean electrically) I must admit I havent been wearing my polariods whilst looking at such a scene so dont know what polarisations were there.

Mark 7
 

lyner

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You will get reflections at all angles - just like with an ordinary mirror.  One particular ray will be reflected at one particular angle (reflection laws). You see just one part of the image when you look in one particular direction but you see lots of rays at all different angles - so you see a full image if you look over the whole surface.
Whilst the Physics of reflection is the same for all interfaces, the detailed properties of transparent substances and of 'shiny' metals are different so you get two distinctly different behaviours.

The conductivity and dielectric constant of water are different from that of air so you get reflection at the discontinuity.  For low angles there is much more HP light reflected than VP light (most of the light goes right into the water, remember - perhaps 5% is reflected). The polaroid glasses eliminate the  HP component entering your eye so the glare from the Sun / sky is reduced.

A mirror uses a metalised layer as the reflector; they used to use mercury. A conductive surface (yes, electrically), of a metal, produces a much stronger reflection (just short of 100%) and is not polarisation sensitive. So polaroids don't work for that class of reflection.
« Last Edit: 11/12/2008 11:33:21 by sophiecentaur »
 

Offline Vitali

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Hello,
Your debates have inspired me to beg the question. So, let a beam of infrared light with linear polarization be almost directly reflected from the hood of your car (not too dirty). Then, let this beam be backscattered at the nearby (more or less dry) asphalt. Can we still distinguish between these cases using just Polaroid glasses and a night vision device? Thank you.
Vitali-
newcomer 
 

lyner

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Why can I see a mirror reflection despite polaroid sunglasses?
« Reply #10 on: 12/12/2008 10:36:15 »
You have to assume that the paint doesn't absorb the IR and that the polaroids still work at IR. Also, you assume the reflection is at the paint surface and not the metal and that the reflection is actually polarized, specular and not diffuse.
It still depends on having an appropriate Brewster angle for the air / paint interfaceat IR.
(party pooper, I'm afraid)

Taking that lot into account, would you like to modify / clarify  the question?

 

Offline Vitali

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Why can I see a mirror reflection despite polaroid sunglasses?
« Reply #11 on: 13/12/2008 06:55:51 »
You have to assume that the paint doesn't absorb the IR and that the polaroids still work at IR. Also, you assume the reflection is at the paint surface and not the metal and that the reflection is actually polarized, specular and not diffuse.
It still depends on having an appropriate Brewster angle for the air / paint interfaceat IR.
(party pooper, I'm afraid)

Taking that lot into account, would you like to modify / clarify  the question?


  :) Thanks to your hints the problem could, seemingly, be solved < newbielink:http://ieeexplore.ieee.org/Xplore/login.jsp?url=/stamp/stamp.jsp?arnumber=883239&isnumber=19094 [nonactive] > Actually, in my R&D project I plan to use crossed polarizers at 800 nm to better distinguish between backscatter from asphalt and specular "back" reflection from automotive paint, the angle between transmitter and receiver is assumed sufficiently small.
 

lyner

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Why can I see a mirror reflection despite polaroid sunglasses?
« Reply #12 on: 13/12/2008 13:18:49 »
So you are trying to improve the contrast of a straightforward IR picture?
It could be worth trying but the backscatter from targets and clutter is not likely to be polarised because the reflections would be about 'normal', would they not? At normal incidence, there will be no selection between polarisation planes. Effectively, all you see will be specular reflections because there is only one source of illumination.  (Reflection laws)
 

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Why can I see a mirror reflection despite polaroid sunglasses?
« Reply #12 on: 13/12/2008 13:18:49 »

 

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