Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: katieHaylor on 05/07/2017 19:06:06
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Tony says:
I know polarised sunglasses are best for blocking most UV light in nature, because UV tends to be polarised in one direction (maybe up-down?) while the sunglasses are polarised at 90 degrees to that (so left-right?) and block it out. But why is UV light polarised to begin with? And how can UV light be polarised in the same direction worldwide, given that "up" and "down" changes depending on where you are in the world?
What do you think?
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Light emitted by the sun oscillates in all directions perpendicular to the direction it travels. When reflected on a surface such as a lake, it becomes polarized parallel to the surface of the lake. This means that the light reflecting off the lake, or any reflective surface, only oscillates along the axis parallel to the surface in the plane perpendicular to the direction it travels. Polarized sunglasses block horizontally polarized light, in order to protect your eyes from these strong reflections. They do so by facilitating a type of venietian blind type array wthin the glass, and work in the same way as one might use a venetian blind to block direct sunlight by changing the angle. The angle of the 'venetian blind' type array within the lenses of the polarized sunglasses is oriented with the intention of blocking out reflected light.
UV filtering for sunglasses is something entirely different. Sunglasses that meet the requirement of blocking ultra violet wavelengths are often labelled as UV 400 because they filter wavelengths up to 400 nanometers, but EU standards require just 380 nanometers.
Ultra violet rays propagate in the same way as any other frequency of light but they have a shorter wavelength than most.
UV filtering works because certain molecules can absorb different wavelengths of light, UV, IR etc. and sunglasses manufacturers (and sun screen manufacturers) include the right molecules in their lenses so that these molecules absorb and block much of the frequency in order to meet UV filtering requirements.
Light of any frequency can become polarized by reflection or scattering and polarized sunglasses can block these polarized rays. (I love Feynman's description of men holding spears pointed upwards trying to get through a doorway)
Filtering of harmful UV or IR is a different process. I'm sure someone else here at the forum can describe filtering better than I have.
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Adding to Timey's description...
But why is UV light polarised to begin with?
Sunlight traveling through the atmosphere is scattered by dust in the atmosphere. The scattered light is polarized.
Short wavelength (blue) light is scattered more than long-wavelength (red) light. This is why the daytime sky appears blue.
Ultraviolet light is scattered even more than blue light.
So sheltering under a tree provides little protection against UV, since it is scattered from all directions in the sky. Better to use sunscreen on your exposed skin. This blocks UV, regardless of its polarization.
See: https://en.wikipedia.org/wiki/Scattering#Electromagnetic_scattering
And how can UV light be polarised in the same direction worldwide
The angle of polarization is related to the light source (the Sun), and where you are viewing the Sun.
The angle of polarization is strongest at around 90 degrees from the Sun, and "faces towards" the Sun.
Take some polarized glasses, look about 90 degrees away from the Sun, and rotate the glasses through 360 degrees. You will see the sky get alternately light and dark. Then try it at 90 degrees from the Sun in a different direction. You will find that the dimmest angle is different from before.
polarised sunglasses are best for blocking most UV light in nature
This would be true if the light had a consistent polarization, like light reflected from a road.
But polarization is different in different parts of the sky, so polarized glasses won't help you.
It is best to use sunglasses that filter out all UV, regardless of its polarization.
In Australia, anything that looks like sunglasses must pass stringent tests for blocking UV, since it is dangerous and invisible.
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Ah yes, I didn't say something quite correctly. Thanks for bringing my attention to it. Light of all frequencies doesn't polarize to the same degree. Shorter wavelengths polarize to a greater degree.
When I was in Western Austrailia many years ago I noticed that sunglasses are a must. It actually hurts your eyes not to wear them. In UK, sure sunglasses are nice to wear, a comfort for a long drive, advisable for being out on the water, and essential obviously for snow, but in Austrailia the sunlight feels really harsh. I arived in Australia having lost my sunglasses in Bali, and geeze, you can get snow blindness from walking down the street off the concrete (almost).
Also one of the things that really struck me about Australia is that the sky seems so much higher than in the UK. I do have a little understanding of the physics of why the sun is harsher and the sky seems higher in Australia than in UK, but as you are a native Australian (or at least living there) I wonder if you might shed some light on what the physics are?
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the sky seems higher in Australia than in UK
It might have to do with the low-hanging clouds that seem so common in the UK (and in Belgium, where I lived for a while)? ;)
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Light of any frequency can become polarized by reflection or scattering and polarized sunglasses can block these polarized rays ...
Polarizing filters are not one-size-fits all, e.g. polarizers made for visible light, (like sunglasses), would not function as polarizers for wavelengths which are much shorter or longer.
If you cross optical polarizers at 90o some blue light gets through : if optical polarizers worked equally well at all wavelengths the background of this cross-polarized image should be black, rather than blue-grey ...
(https://apm.iitm.ac.in/smlab/kramesh/images/sp1.jpg)
https://apm.iitm.ac.in/smlab/kramesh/t7.html (https://apm.iitm.ac.in/smlab/kramesh/t7.html)
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the sky seems higher in Australia than in UK
It might have to do with the low-hanging clouds that seem so common in the UK (and in Belgium, where I lived for a while)? ;)
Lol Evan - I had been thinking more on the lines of the quote below, but I'm quite liking your explanation.
http://www.theweatherprediction.com/habyhints/276/
:link
With a low relative humidity throughout the troposphere, water vapor can not condense (even on condensation nuclei which can support condensing when RH is somewhat less than 100%). This relative low abundance of water vapor and lack of condensation takes the white hue out of the sky. It makes the sky appear bluer and increases horizontal and vertical visibility.
The reason I asked is that a low abundance of water vapor in the air will reduce polarization of light won't it. I have heard that the lacking in ozone layer over Australia might contribute to the harshness of the sun ... (btw, how is the ozone layer doing these days? I heard rumor it was growing back (I could google, however that is such a conversation killer)) ...but could a reduction in polarization be responsible for a more harsh sunlight?
*
RD - Yes wavelengths. Some men carry longer spears than others. Doorways to allow one type or another of these particular men with their particular length spears through, or keep them out, must be differently aligned in spatial dimensions.
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There are a few kinds of polarizing glasses used for various purposes these days:
- Sunglasses reducing glare from the water and road have both lenses with the same linear polarization
- "Old Fashioned" 3D glasses have linear polarization, with one lens horizontal, and the other vertical. These suffer from the problem that if you tilt your head, you start to see double
- Modern 3D glasses have circular polarization, with one lens clockwise, and the other anti-clockwise. It doesn't matter if you tilt your head; the circular polarization is not affected.
I had the impression that the 3D glasses used in regular cinemas has the opposite polarization from 3D IMAX, so you couldn't use one set of glasses in the other cinema. (Laser IMAX uses a totally different mechanism.)
Warning: Cinema-style polarized glasses don't have the UV-blocking filter that you need in sunglasses.
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To further answer the OP's question about 'how' light becomes polarized:
We have covered the fact that light becomes polarized by reflection, and that polarized sunglasses are designed not to allow these reflective glares to pass through the lens of the sunglasses, however...
:sciencenews.org
According to quantum electrodynamics, the theory describing how light interacts with charged particles such as electrons, empty space isn’t really empty. It is filled with a roiling soup of ethereal particles, constantly blipping into and out of existence. As light passes through the void, its wiggling electromagnetic waves interact with those particles. Under strong magnetic fields, light waves that wiggle along the direction of the magnetic field will travel slightly slower than light oscillating perpendicular to the direction of the magnetic field, which rotates the overall polarization of light coming from the star.
A similar effect commonly occurs in a more familiar situation, in what are known as birefringent materials. The liquid crystals in computer monitors similarly rotate the polarization of light. Horizontally polarized light, for example, is sent to each pixel, but a filter lets only vertically polarized light escape. To switch on a pixel, the liquid crystals twist the light waves 90 degrees so the waves will pass through.
So light arriving from our sun will already be polarized to some extent by the conditions of it's journey before it reaches our atmosphere.
:physics.bu.edu
Light scattering off atoms and molecules in the atmosphere is unpolarized if the light keeps traveling in the same direction, is linearly polarized if at scatters in a direction perpendicular to the way it was traveling, and somewhere between linearly polarized and unpolarized if it scatters off at another angle.
:physics.bu.edu
If unpolarized light passes through a polarizer, the intensity of the transmitted light will be 1/2 of what it was coming in. If linearly polarized light passes through a polarizer, the intensity of the light transmitted is given by Malus' law
So back to the harshness of the sun in Australia. (Western Australia being my experience) Clearly if there is reduced humidity in the air (the causes of which I posted in a link previously), then light intensity will be greater, as well as the sky appearing to be higher with the increased visibility, caused by the fact of the reduction in light scattering due to the reduced humidity.
So Evan is indeed quite correct that the low hanging clouds in the UK (and Belgium) have something to do with the appearance of lower skies.
The ozone layer acts as a natural filter to protect the Earth from ultra violet rays. The good news is that it is on the mend.
http://time.com/4391034/antarctic-ozone-layer-healing/
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So light arriving from our sun will already be polarized to some extent by the conditions of it's journey before it reaches our atmosphere.
There are mechanisms to generate polarized light which apply in strong magnetic fields, such as locally on the Sun in sunspots. It occurs more often in stars that have stronger magnetic fields than the Sun.
Polarization can also be seen with interstellar dust clouds.
However, the path between Sun and Earth is a pretty good vacuum; it consists of the solar wind, ie a low density plasma of hydrogen & helium ions, plus free electrons. These do not provide the surfaces necessary to scatter and polarize light.
It is only when the sunlight reaches the Earth's atmosphere that it encounters dust particles comparable to the size of the wavelength of light (or larger) that form effective sites for scattering and polarization of light.
See: https://en.wikipedia.org/wiki/Polarization_in_astronomy
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According to quantum electrodynamics, the theory describing how light interacts with charged particles such as electrons, empty space isn’t really empty. It is filled with a roiling soup of ethereal particles, constantly blipping into and out of existence. As light passes through the void, its wiggling electromagnetic waves interact with those particles. Under strong magnetic fields, light waves that wiggle along the direction of the magnetic field will travel slightly slower than light oscillating perpendicular to the direction of the magnetic field, which rotates the overall polarization of light coming from the star.
Does this effect not occur in the vacuum between the earth and the sun?
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In astronomy, the Faraday effect is used to measure magnetic fields between polarized radio-frequency sources like pulsars and the Earth.
As I understand it, astronomers time the arrival of pulses of right- and left-circularly polarized light from distant pulsars.
The effect is significant if the magnetic field is very strong, or the path length is very long. This effect has been used to measure the Sun's magnetic field as the line of sight to a distant polarized radio source passes through the Sun's corona.
The linear polarization of ultraviolet light on Earth is due to scattering, and is highest at large angles from the Sun. The Faraday effect applies to a difference in velocity of circular polarization, and applies along the line of sight to the source.
See: https://en.wikipedia.org/wiki/Faraday_effect#Faraday_rotation_in_the_interstellar_medium
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rotate the glasses through 360 degrees. You will see the sky get alternately light and dark.
I tried this experiment yesterday when I was out on the water.
The effect is clearer if there are patches of white clouds and blue sky.
The white clouds reflect unpolarized light from the Sun, and their brightness is unchanged as you tilt your head.
But the polarized light from the blue sky changes intensity greatly as you tilt your head.
The change in contrast is quite dramatic.
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To further answer the OP's question about 'how' light becomes polarized:
We have covered the fact that light becomes polarized by reflection, and that polarized sunglasses are designed not to allow these reflective glares to pass through the lens of the sunglasses, however...
Light can also becomes polarized by refraction, as stated in Fresnel's equation.
https://en.wikipedia.org/wiki/Fresnel_equations
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The "polaroid" film in polarising sun glasses consists of iodine molecules aligned in a stretche polyvinylalcohol matrix.
It absorbs UV light strongly- regardless of the polarisation.
So it provides a useful level of UV protection.
It also blocks polarised visible light (if the polarisation angle is correct) which means it cuts the glare from reflective surfaces like water, snow or sand.
And, because roughly half of "non polarised" light isn't at the right angle, it doesn't get through so they drop the intensity of the light reaching the eye by about 50% (A bit more due to other losses)- which is more comfortable.
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You asked a very interesting question :)... I think now the tinted glasses are slightly "behind" scientific laws. Of course they can not filter the corners of the ultraviolet. From your question I had my own: protective goggles, which are assigned to delay blue, they, too, can not fix the color at all angles (although they completely cover the eye-bag). It seems to me that here there is a distortion of the surface ... that is, if the earth were square, then all the angles of ultraviolet penetration would be extremely simple ... But since the Earth is round, the perimeter is everywhere different and at the same time the same ... paradox