What is the Difference Between Mie Scattering and Rayleigh Scattering?

[ZealScience]

I learned something about mie scattering in another thread. But I'm still perplexed, as what makes it different from scattering by air molecules (called Rayleigh scattering). But why doesn't the pattern cause polarization of light?

 

[Dale]

The difference between Rayleigh and Mie scattering is simply the size of the scattering particle relative to the wavelength. I don't know about the polarization.

 

[ZealScience]

The difference between Rayleigh and Mie scattering is simply the size of the scattering particle relative to the wavelength. I don't know about the polarization.

Linear polarization of light is because when scattering happens, observer at certain angle only receive the wave that is oscillating perpendicularly to the path of the light which means light is polarized (it is actually well explained by quantum mechanics, where probability is determined by angle of oscillation). But I see similar pattern in mie scattering, but there is no polarization at all, so I can't understand it well.

 

[Andy Resnick]

Mie scattering is indeed polarization dependent- why do you say it is not?

http://farside.ph.utexas.edu/teaching/jk1/lectures/node103.html

"However, the differential scattering cross section in any particular direction is, in general, different for different circular polarizations of the incident radiation. This implies that if the incident radiation is linearly polarized then the scattered radiation is elliptically polarized. Furthermore, if the incident radiation is unpolarized then the scattered radiation exhibits partial polarization, with the degree of polarization depending on the angle of observation. "

 

[ZealScience]

Mie scattering is indeed polarization dependent- why do you say it is not?

http://farside.ph.utexas.edu/teaching/jk1/lectures/node103.html

"However, the differential scattering cross section in any particular direction is, in general, different for different circular polarizations of the incident radiation. This implies that if the incident radiation is linearly polarized then the scattered radiation is elliptically polarized. Furthermore, if the incident radiation is unpolarized then the scattered radiation exhibits partial polarization, with the degree of polarization depending on the angle of observation. "

In Rayleigh scattering, there's a preference of polarizing blue light when we are at large angle to light of incidence, but in mie scattering like the case of cloud, there's no sign of preference of any color, so I'm confused.

 

[Andy Resnick]

In Rayleigh scattering, there's a preference of polarizing blue light when we are at large angle to light of incidence, but in mie scattering like the case of cloud, there's no sign of preference of any color, so I'm confused.

I don't follow-light scattering from spheres is parameterized by 'ka', where k is the wavenumber and 'a' the particle radius. Why do you say 'in mie scattering like the case of cloud, there's no sign of preference of any color'?

 

[ZealScience]

I don't follow-light scattering from spheres is parameterized by 'ka', where k is the wavenumber and 'a' the particle radius. Why do you say 'in mie scattering like the case of cloud, there's no sign of preference of any color'?

Because clouds are white, white is not a monochromatic light, thus white means all monochromatic lights are equally polarized that shows the color of white, rather than air molecules that polarize blue light more at large angle that makes sky blue, how to explain that?

 

[Redbelly98]

For a cloud, light can scatter many many times off of the drops of water. This effectly randomizes the polarization, so the light you see is unpolarized. Also, the scattering probability is high for all visible wavelengths, so the light is white.

For the blue sky, you are mostly seeing light that has scattered just one time, so the polarization resulting from a single scattering event is maintained.

 

[ZealScience]

For a cloud, light can scatter many many times off of the drops of water. This effectly randomizes the polarization, so the light you see is unpolarized. Also, the scattering probability is high for all visible wavelengths, so the light is white.

For the blue sky, you are mostly seeing light that has scattered just one time, so the polarization resulting from a single scattering event is maintained.

Sounds reasonable, thanks

 

[Andy Resnick]

Because clouds are white, white is not a monochromatic light, thus white means all monochromatic lights are equally polarized that shows the color of white, rather than air molecules that polarize blue light more at large angle that makes sky blue, how to explain that?

I still don't understand you- polarization has nothing to do with color.

Note also, clouds are optically dense (multiple scattering), which results in an overall depolarization. Ah- I see Redbelly98 noted this as well.

 

[ZealScience]

I still don't understand you- polarization has nothing to do with color.

Note also, clouds are optically dense (multiple scattering), which results in an overall depolarization. Ah- I see Redbelly98 noted this as well.

No, this is not correct. Why the sky is blue? Why the twilight is red? Because at large angles polarization prefers blue which means more blue color is polarized. Twilight or aurora or even the color of the far side of the sky is red is because bluer colors (higher frequencies) are polarized and reflected at large angles, but redder colors are not polarized and reflected which means they would still reach you at smaller angles.

The mentor's explanation showed the reason.

 

[Redbelly98]

Zeal, you seem to be confused about the role of polarization in Rayleigh scattering causing the blue sky. While it's true that the scattered light is polarized when viewed from certain angles, it is not the polarization that causes the blue color. Instead it is the relatively higher scattering probability for shorter wavelengths that gives the sky this color. The fact that there is polarization does not really enter into the explanation of the color.

Hope that makes sense.

EDIT: here is a figure from Wikipedia. A blue photon has roughly 15-20% chance of being scattered, while this probability is only about 5% for red photons:

(From http://en.wikipedia.org/wiki/Rayleigh_scattering )

 

[ZealScience]

Zeal, you seem to be confused about the role of polarization in Rayleigh scattering causing the blue sky. While it's true that the scattered light is polarized when viewed from certain angles, it is not the polarization that causes the blue color. Instead it is the relatively higher scattering probability for shorter wavelengths that gives the sky this color. The fact that there is polarization does not really enter into the explanation of the color.

Hope that makes sense.

EDIT: here is a figure from Wikipedia. A blue photon has roughly 15-20% chance of being scattered, while this probability is only about 5% for red photons:

(From http://en.wikipedia.org/wiki/Rayleigh_scattering )

Yes, I know that, but I mean if there is no polarization then there won't be difference in color between sky and twilight because there is polarization. If there is polarization in mie scattering then your explanation might be appropriate.

 

[Andy Resnick]

Yes, I know that, but I mean if there is no polarization then there won't be difference in color between sky and twilight because there is polarization. If there is polarization in mie scattering then your explanation might be appropriate.

There will still be a difference in color because different wavelengths scatter differently: shorter wavelengths scatter more, meaning the angle between incident and scattered waves is larger, than longer wavelengths. This is the origin of blue skies and red sunsets- not polarization effects.

The only evidence (AFAIK) that our eyes can detect polarization is Haidinger's brush:

http://en.wikipedia.org/wiki/Haidinger's_brush