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Author Topic: Why do my polarising sunglasses make things appear that I can't see normally?  (Read 7987 times)

Seth Faerber

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Seth Faerber  asked the Naked Scientists:

Hey guys! The podcasts are a dream come true!

Here's my question. Recently I bought a pair of polarized sunglasses.

They reduce glare as they should, but I can also see things with them on that I can't when they're off.
 
For instance, when I look at quite a few cars' back windows, a faint chequered pattern can be seen covering the window. I can't see anything of the sort when my glasses are off!

What is it about the construction of a car's rear window that forms this checker and why can I only see it with polarized glasses on?
 
Your fan,

Seth A. Faerber
San Antonio, Texas, USA


What do you think?


 

lyner

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The stresses which are caused by the treatment of the glass tend to polarise the light in patches as it passes through. The brightness is not affected so you won't see it with the naked eye. However, your polaroid glasses will reduce the level of one polarisation so some regions of the glass will appear dark.
This effect is often used by engineers as a way of studying stresses in transparent materials.
You will also have noticed how much easier it is to look below the surface ponds and lakes because the sunglasses reduce the level of light which is reflected from the surface. The good thing about polarising specs is that, although they only reduce the transmitted light by 50%, they significantly reduce the light reflected upwards from shiny horizontal surfaces, which is particularly annoying, without making a scene too dark to see anything useful.
Note that 'crossed polaroids' will cancel out pretty well all of the light; get together with a friend with a similar pair (don't chop up your brand new pair!).
« Last Edit: 15/05/2008 10:45:19 by sophiecentaur »
 

Offline syhprum

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Certain crystals will rotate the polarization of light when put under electrical stress, this technique was used in projection TV's in the thirties and survived well into the eighties when it was used in the HELL laser film recorders that I worked on 
 

lyner

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Offline LeeE

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IIRC, reflected light from surface mirrored objects i.e. polished metal or surface deposited mirrors, do not change the polarisation of the reflected light, so while polarised sun-specs help with reflections from water (and I guess with some air refractions) they don't help with shiney metal reflections.

 

lyner

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Brewster angle and all that.
Polaroids work pretty effectively, tho',in practice.
Most of the large area reflectors are of the non-conducting' variety - like, as you say, expanses of water, snow, plastic sheeting and, I suspect, paints. If you spent a lot of time over a shiny metal surface you'd have to use alternative types of 'shades'.
 

Offline techmind

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We did this one before, in November 2007.

See thread: http://www.thenakedscientists.com/forum/index.php?topic=11582.msg140779#msg140779

Quoting myself:
Quote
Light can be polarised (or partly-polarised) by reflection e.g. from the surface of glass or water.
This effect is strongest when the angle of reflection is close to the Brewster Angle (Wikipedia it!).
By arranging their axis of polarization perpendicular to the light scattered/reflected off horizontal surfaces, polarising sunglasses can dramatically cut down the glare of light reflected from horizontal surfaces such as the sea or road - while only reducing the brightness of everything else by about 50%.

If you put two polarizers in the path of light, with their axes crossed, then you can block the light completely.
However, many plastics can rotate the polarization of light, and typically rotate different wavelengths (colours) by different amounts. The amount of rotation can also be affected by applied or residual stresses in the plastic or glass. Consequently if you put a piece of plastic (or pre-stressed glass - glass that shatters into thousands of tiny pieces when you break it) between crossed polarizers you can see lots of pretty colours.

See my Kitchen Science experiment:

http://www.thenakedscientists.com/forum/index.php?topic=6730.msg129590#msg129590


Since the ambient light on a clear day can be partially polarised from the sky or other reflections you can sometimes see weaker versions of these effects even without deliberately/explicitly polarizing the light to start with.

In certain scenes the light can be partially polarised, and for various reasons the predominant polarisation may vary with wavelength (colour). Usually we don't notice these effects because our eyes are not polarisation-sensitve. When you wear polarising glasses however the polaroid material "analyses" the polarisation, converting polarisation effects to intensity effects... and revealing hidden colours!


See also my kitchen science experiments (link in the quote) for more things to try with polaroid sunglasses.
 

lyner

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Most scenes have more light from above the horizontal than from below. Your 'automatic exposure' system will present your retina with an appropriate amount of light (adjusts the pupil). In bright conditions - blue, very clear sky and, particularly, with snow / seascapes, there will be relatively less light from overhead and more from below. Your automatic system will tend to underestimate the available light and, as a consequence, it may overexpose your retina (pupil open more than). This is probably not good for your sensors - giving 'snow blindness' in the extreme. 
The principle benefit of polaroid glasses is that they tend to maintain a distribution in which the level from below the horizontal is more like 'normal' under these bright, clear conditions. This should give you a more appropriate pupil aperture.
 

Offline syhprum

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There have been numerous attempts to introduce polarized car headlights in combintion with polarizing windscreens so that one only mainly sees the randomly scattered light from the roadside while the light from the headlights of oncoming cars is much reduced eliminating the dazzle problem.
As yet no such scheme's have made it into production but the idea recurs every few years.
 

lyner

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I should imagine that a rain spattered windscreen could really upset a system like that - adding, in fact, to the flare effect.
 

Offline LeeE

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I think you'd lose a lot of the light intensity too, so for the same level of brightness you'd need higher output level lightbulbs.
 

Offline syhprum

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Modern cars use mini arc lights so the loss of intensity is not a big problem although the disturbance of polarization by rain drops might be.
Of course instead of a polarizing windscreen the driver could wear spectacles for night driving.
 

lyner

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Modern cars use mini arc lights
? Not Halogen? What sort of power supply is used? An inverter and all that?
 

Offline syhprum

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they run off the normal 12v supply via transistor current control electronics with a 10kV pulse generator to start them up.
They are completely sealed with Tungsten electrodes in a compressed gas envelope and a built in reflector.
I am not sure of their life but if my experience with them in scanners is anything to go by they should last at least 5000 hrs.
Further checking shows that this account from memory has some errors, see HID lamps in
http://en.wikipedia.org/wiki/Headlight
« Last Edit: 18/05/2008 08:09:41 by syhprum »
 

lyner

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Are they going to be even more dazzling than the flashy ones I normally see around?
 

Offline syhprum

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Mine don't dazzle, the have a very well defined beam and servos to compensate for the vertical movement of the car
 

Offline turnipsock

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Mine don't dazzle, the have a very well defined beam and servos to compensate for the vertical movement of the car

Are you talking about the little wheel that starts at 0 and goes to three? If so, this requires the driver to appreciate that when people are flashing their lights at her, the setting is to high and they should not just flash their lights back and wave.

My cars normal setting is 1, and I suspect this is the case with most cars. I often wonder why my car has this function as it also has rear self leveling suspension.
 

lyner

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Mine don't dazzle, the have a very well defined beam and servos to compensate for the vertical movement of the car
Cooh! It's a wonder you can bring yourself to talk to ordinary people like us with regular motor cars. I thought I was doing well when my latest car (bought about 4 years ago) had air conditioning and Halogen light bulbs. The previous one had pumped up tyres, I believe!

I must try to spot some of your type when I'm out and about. How would I recognize them?
 

Offline LeeE

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Although I wouldn't say that these lights are more dazzling in terms of brightness, they do seem to produce some colourful off-axis refraction effects, which can be quite distracting.
 

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