Rayleigh Scattering Conceptual Question about the Color of the Sky

In summary: Our eyes have receptors for red, green, and blue light, but not for violet light. So when violet light is scattered in the atmosphere, we don't see it as a distinct color, but rather as a mix of blue and red. This is why the sky appears blue rather than purple.
  • #1
PhDeezNutz
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Homework Statement
Why when we look at the sun directly is it red/orange? and why does the sky appear blue?
Relevant Equations
For Rayleigh Scattering we have the scattering cross section

##\frac{\ d \sigma}{d \Omega} = \left( \frac{k_0^2 \alpha}{4 \pi}\right)^2 \left( 1 - \left|\hat{r} \cdot \hat{e}_0 \right|^2 \right)##

Where ##\hat{r} = \hat{k}## (the scattered wave propagates radially). ##\hat{e}_0## is the unit vector of the incident electric field. For the sake of simplicity I'm going to assume normal incidence (see picture below)

The ##k_0^4## dependence in the numerator implies a ##\frac{1}{\lambda^4}## dependence of ##\frac{d \sigma}{d \Omega}##.
Image 5-27-20 at 7.58 AM.jpg


We know ##\lambda_{blue} < \lambda_{red}## so citing the formula above (in the relevant equations) it's apparent that blue light is scattered more than red light. But presumably this would hold for ##\theta = 0## as well but when we look at the sun head on we don't see blue, we see red/orange.

Am i interpreting this formula wrong?
 
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  • #2
I think it's because of the following. Even when ##\theta = 0## blue light is scattered more and red light is scattered less and that's why the "inner part" is red and the "outer part" is blue.

I think it's just that simple.
 
  • #3
What do you mean by inner part and outer part?
 
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  • #4
vela said:
What do you mean by inner part and outer part?

inner part - apparent angular size of the sun

outer part - the rest of the blue sky
 
  • #5
Your answer feels incomplete to me. In the first post, you said blue light is scattered more than red light in the forward direction and asked why this didn't mean the Sun appeared blue. In the second post, you said blue light is scattered more than red light in the forward direction and concluded this is why the Sun appears red. I don't really see an explanation in there.
 
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  • #6
vela said:
Your answer feels incomplete to me. In the first post, you said blue light is scattered more than red light in the forward direction and asked why this didn't mean the Sun appeared blue. In the second post, you said blue light is scattered more than red light in the forward direction and concluded this is why the Sun appears red. I don't really see an explanation in there.

yeah I think I was trying to reconcile it anyway I could...and it fell short. Still open to suggestions.
 
  • #7
It might be easier to think about the total cross section, ##\sigma = \int (d\sigma/d\Omega)\,d\Omega##. You can interpret ##\sigma_{\rm blue} > \sigma_{\rm red}## to mean that the air molecules are a "bigger" target than for red light. In other words, the probability of an interaction between the light and an air molecule is greater for blue light than for red light.

Now think of light along your line of sight coming from the Sun to your eyes. For simplicity, let's assume equal amounts of blue and red light are emitted by the Sun. If you were on the Moon, equal amounts of blue and red light would reach your eyes because there's nothing that gets in the way between you and the Sun. If you're on the Earth, however, the atmosphere is there. What's going to happen?
 
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  • #8
Not all the "forward" light is scattered. The stuff that is not scattered goes on through but diminished preferentially of its blue light. If you work it out, the total scattering of sunlight is about 10-5 per meter of air at sea level density. Depending on your interest you could calculate the total Rayleigh cross section (per atom) and figure what percentage of the sunlight is Rayleigh scattered. Of course the path gets longer as the sun sets but even at dusk it is only a fraction.
 
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  • #9
A good follow up question is: why doesn't the sky appear purple.
 
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  • #10
nrqed said:
A good follow up question is: why doesn't the sky appear purple.

That's an interesting question.

Based on the Rayleigh scattering cross-section formula

$$\sigma_s \sim \frac{1}{\lambda^4}$$

We indeed have ##\sigma_{violet} > \sigma_{blue}## so one would a priori expect to see the sky purple. The answer has to do with how we perceive violet
 
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Related to Rayleigh Scattering Conceptual Question about the Color of the Sky

1. What is Rayleigh scattering?

Rayleigh scattering is a phenomenon that occurs when light is scattered by small particles in the atmosphere. These particles, such as nitrogen and oxygen molecules, are much smaller than the wavelength of visible light, causing the light to scatter in all directions.

2. Why is the sky blue during the day?

The sky appears blue during the day because of Rayleigh scattering. The shorter, blue wavelengths of light are scattered more easily by the particles in the atmosphere, making them more visible to our eyes. This is why the sky appears blue when we look up during the day.

3. Why does the sky appear red during sunrise and sunset?

During sunrise and sunset, the sun's rays must pass through more of the Earth's atmosphere before reaching our eyes. This causes more scattering of the shorter, blue wavelengths of light, leaving the longer, red wavelengths to reach our eyes. This is why the sky appears red during these times.

4. Does Rayleigh scattering only occur in Earth's atmosphere?

No, Rayleigh scattering can occur in any medium that contains small particles, such as dust or water droplets. It is also responsible for the blue color of water and the red color of sunsets on other planets.

5. Can the color of the sky change?

Yes, the color of the sky can change depending on the amount and size of particles in the atmosphere. For example, on a hazy day, the sky may appear more white or gray due to larger particles scattering all wavelengths of light equally. Additionally, pollution and volcanic eruptions can also affect the color of the sky by introducing different types and amounts of particles into the atmosphere.

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