# Sky colour explanation by scattering

• lockerman2007

#### lockerman2007

I have done a little research on the internet and I would like to ask my concept is correct or not.

1) Sky is blue
As the effect of rayleigh scattering is more effective for shorter wavelength, blue light scatter more than others (e.g. red). Furthermore, our eyes are more sensitive to blue than violet, so the sky is appeared to be blue.

2) Sky is at sunset
At sunset, the sun is far away and sunlight has to travel for a long distance to our eyes, most of the blue light scattered away and the intensity of blue light become very low, while red is less scattered, so red colour is dominant.

3) space is dark
As space is vacuum, no molecules responsble for scattering, so it is dark.

4) cloud is white
cloud contains large water molecules and mie scattering dominates. As mie scattering is not strongly dependent on waveleght, so the water molecules just scatter the white light of different wavelength in similar extent, so it is white.

By the way, I would like to ask what is the meaning of "scattering is more effective" ? It means the light intensity is larger after scattering or the light scattering in a "wider direction" ?

Thank you

Item 2 is somewhat in error. The distance to the sun is not a factor (compared to 1). The effect is primarily the result of the distance of sunlight through the atmosphere is greater at sunset or sunrise than at noon.

By the way, I would like to ask what is the meaning of "scattering is more effective" ? It means the light intensity is larger after scattering or the light scattering in a "wider direction" ?

Thank you

I think "scattering is more effective" means the shorter wavelengh light beams will scatter more than the longer ones. You can see the color of the sky is due to the scattering phenomenon. When a light beam scatters, some of it changes to many directions and the rest still goes in a straight line. The scattering lights is what you can see in the sky

Feynman's Lectures Book one has an excellent discussion of this subject. See pages 32-6 to page 32-9. You can find this book in most good libraries.

For 3) The sky appears dark simply because the quantity of light being scattered in the atmosphere has been vastly reduced, not because the scattering centres (molecules) have been removed.
lockerman2007 said:
By the way, I would like to ask what is the meaning of "scattering is more effective" ? It means the light intensity is larger after scattering or the light scattering in a "wider direction" ?
I would venture that the "scattering more effective" means a greater proportion of incident light is scattered.

Claude.

2) ... Furthermore, our eyes are more sensitive to blue than violet, so the sky is appeared to be blue.
I don't know what violet has to do with anything.

Thank you for the explanation and correction!

>>I don't know what violet has to do with anything.
If scattering is more effective for shorter wavelength, violet is scattered more than blue and the sky should be violet.

As for "scattering more effective", is greater proportion is scattered means greater intensity is scattered ?
That means for shorter wavelegth, the incident light will evenly distributed in all directions and for longer wavelegth, the intensity of light in the incident direction is larger than other directions ?

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If scattering is more effective for shorter wavelength, violet is scattered more than blue and the sky should be violet.

The sky color is a mixture of many colors. Furthermore, the sun spectrum is not flat- I believe its peak is the yellow. Therefore the blue intensity is greater than violet. Finally our eye sensitivity is not unform over all colors.

Isn't the red light refracted less than the blue light?
Blue light hitting the atmosphere is refracted downwards while the red light can follow a straighter path.

As for "scattering more effective", is greater proportion is scattered means greater intensity is scattered ?

Nope, what I said has nothing to do with intensity, I'm talking about the overall quantity of light. Think of it as a scattering efficiency Scattered Light/Non-scattered light.

Claude.