How Does Rotating a Polarizing Filter Affect Sky Photography?

[MrJimH]

Why is the sky polarized?

 

[K^2]

It is? I never noticed any change when playing with polarized glasses and glare.

 

[VanOosten]

Both the polarization and the colors of the sky are created by light scattering off of random particles

 

[Phyisab****]

Are you sure light from the sky is polarized? I'm not sure how this would happen given the isotropic and random nature of the atmosphere.

 

[VanOosten]

Are you sure light from the sky is polarized? I'm not sure how this would happen given the isotropic and random nature of the atmosphere.

yes try it out with some polarized sun glasses on a clear summer day its quite neat. the full explanation of how is described quite well here http://www.polarization.com/sky/sky.html

 

[atyy]

http://farside.ph.utexas.edu/teaching/em/lectures/node97.html

 

[Phyisab****]

Interesting, you learn something every day. Someday I will get around to reading the chapter in my QM book on scattering, which we skipped over.

 

[Redbelly98]

yes try it out with some polarized sun glasses on a clear summer day its quite neat.

Or any clear day, year round. Just look in a direction 90 degrees away from the sun to get the maximum effect.

 

[DaveC426913]

Well, if the sky weren't polarized, then a polarizing filter on your SLR would not be much use in darking the sky.

 

[Integral]

Well, if the sky weren't polarized, then a polarizing filter on your SLR would not be much use in darking the sky.

This is not an indication of polarization. You will get that effect with circularly polarized light. Read Redbelly's reply.

 

[MrJimH]

yes try it out with some polarized sun glasses on a clear summer day its quite neat. the full explanation of how is described quite well here http://www.polarization.com/sky/sky.html

Thanks

 

[kgbgru]

I've had grasses as long as I can remember. I have always loved to look at the sky with my prescritpion sun glasses and turn my head to watch the colors and intensity change. No matter what angle I look through there is always a view of the sky. It has light polarized in many directions, from what I can tell all. Try looking at a monitor with polarized glasses. Many are polarized in a single direction. You will find an angle where the screen is completely black. That happens when the light is polarized normal to your lenses. I love to stare at the angles that makes the sky a dark blue. However, that angle almost never darkens the whole sky the same.

 

[Andy Resnick]

Why is the sky polarized?

I can't tell you 'why', but I can tell you the mechanism- Mie scattering.

http://www.polarization.com/sky/sky.html

(looks like someone else found that link also)

The degree of polarization is greatest 90 degrees (right angles to) the sun, is greatest on a clear day, and some people can see the effect (Haidinger's brush). Bees navigate using polarization.

 

[DaveC426913]

This is not an indication of polarization. You will get that effect with circularly polarized light. Read Redbelly's reply.

How is this not an indication of polarization?

I read RedBelly's reply.

Or did you mean this is not an indication of a specific kind of polarization? The OP did not specify.

 

[Integral]

Circularly polarized light means it has no specific constant component of polarization. If When a beam of sun light passes through a polarizing filter 50% of the light is adsorbed by the filter, 50% passes through and emerges as polarized light. It is this 50% loss that you see in your pictures.

If you rotate your cameras polarizing filter by 90deg you will not see a change in the intensity of the light emerging.

If you, on a bright sunny day stand facing your shadow, and look at the sky through a polarizing filter you can orient it to any angle and not observe a change in intensity. Now do a 90 deg turn (for the military guys , Right face!) now when you observe the sky through a polarizing filter you WILL observe a change in intensity by changing the orientation of your filter. This is because you cannot observe the axis of the beam oscillating directly toward you, but see the transverse portion of the beam.

I am not real happy with my wording on that. Perhaps someone else can chime in.

 

[Redbelly98]

Camera polarizing filters do offer variable attenuation of linearly polarized light, and the transmission will change with the orientation of the polarizer. They are helpful in reducing the glare in reflections, and as DaveC noted help with darkening the sky. I have observed these effects personally.

That being said, Integral is correct in that they are circular polarizers. (Or at least that is what they are called.) What I think they are is a linear polarizer + a quarter-wave plate; the wave plate converts the linearly polarized light that is transmitted by the polarizer into circularly polarized light. (*** see comment below)

The linear polarizer would reduce reflected glare, and help with making the sky darker. The only benefit of the circular polarizer that I can think of is that it creates an even mix of the two linear polarizations, so that the reflectivity and transmissivity of the SLR optics (which are polarization dependent) would be the same as for unpolarized light, i.e. what the camera optics are designed for.

*** What I don't understand is, how does one create circularly polarized white light? Quarter-waveplates should only work for specific wavelengths, not a broad range. But that is getting off-topic from this thread, so I may just start a new thread (after some googling) to pose that question.

 

[Dadface]

Light scattered away from any incident beams is partially plane polarised and light scattered through 90 degrees is completely plane polarised.Here the light source is the sun and the wavelengths most strongly scattered are closer to the blue region of the spectrum.On a cloudless(ideally) day look at the sky at different angles through a piece of polaroid which you rotate.Looking vertically upwards is quite effective.

 

[KBT]

Let's see if I can remember this from my EM class. It's been a while.

Well, imagine you are looking at the sky 90 degrees from the direction the sun's rays are traveling. The light that reaches your eyes was scattered from something (dust, air, etc.) in the sky. If we treat this something as an electric dipole, we can see why the light must be polarized.

First, if the dipole is aligned in the direction the sun's rays are traveling, it will not radiate. EM-waves are transverse, so the electric field can't oscillate the dipole.

Second, if the dipole is aligned with your line of sight, it will radiate, but not toward you. This is because of the angular dependence of dipole radiation. See reference below or http://en.wikipedia.org/wiki/Dipole_antenna

Lastly, we are left with one final direction perpendicular to your line of sight as well as the direction of the sun's rays. Dipoles aligned in this direction will radiate strongly toward you. The light scattered from these dipoles is polarized in the direction of the dipoles.

A little thought will show that light scattered from other directions will be less and less polarized until, when looking directly at the sun (erhm, well, to the side of the sun; don't look right at it :tongue2: ) the light is not polarized at all.

For more info, check out David J. Griffiths: "Introduction to Electrodynamics" Third Edition Chapter 11 pg. 449 (Actually, the whole chapter's pretty interesting. )

 

[Andy Resnick]

If you rotate your cameras polarizing filter by 90deg you will not see a change in the intensity of the light emerging.

That is not true- rotating linear polarizer while imaging the sky will indeed produce very profound optical effects. These are very visible using wide-angle lenses; there is a dark band in the sky that appears.