Why is the sky blue?
- Short answer: Rayleigh Scattering.
- Long answer: Rayleigh Scattering "is the (dominantly) elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light", for which air particles qualify. This scattering follows the fourth power of the radiation's frequency. So, for lower frequencies (red side of the visible spectrum), any incoming light goes through the atmosphere. For high frequencies (blue and violet), the light from the sun gets scattered in the atmosphere, instead of going straight down.
Since the light from the sun is practically a continuum in the visible spectrum, we see the color that comes up when integrating the spectrum raised to the power of 4, and that yields something close to violet (the last color we can see before ultra-violet), but not exactly violet..., it's pulled a bit to lower frequencies... so we get blue.
Here's a picture:
How about that reddish band at sun-set?
You can only see it near the horizon, and the sun is setting, because light from the sun's light has to go through more atmosphere near the horizon and the effect is noticeable, due to the low light caused by the setting of the sun. All blue light gets scattered and the only remaining frequencies are the lower, closer to red (frequencies lower than red, you get infra-red... that's sort of what "infra" means.... below. That's why I like to speak of these things in terms of frequency instead of the more common approach which uses the wavelength and makes us invert the whole reasoning, but then the nomenclature infra-red and ultra-violet stop making sense).
An image is worth a thousand words:
You see this effect better in the afternoon than in the morning because a warm atmosphere rises and provides a wider reddish band... and in the afternoon, the atmosphere has been warming up for some 12 hours.... in the morning, it's been cooling off for the other 12 hours and the reddish band is smaller.
- Short answer: Rayleigh Scattering.
- Long answer: Rayleigh Scattering "is the (dominantly) elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light", for which air particles qualify. This scattering follows the fourth power of the radiation's frequency. So, for lower frequencies (red side of the visible spectrum), any incoming light goes through the atmosphere. For high frequencies (blue and violet), the light from the sun gets scattered in the atmosphere, instead of going straight down.
Since the light from the sun is practically a continuum in the visible spectrum, we see the color that comes up when integrating the spectrum raised to the power of 4, and that yields something close to violet (the last color we can see before ultra-violet), but not exactly violet..., it's pulled a bit to lower frequencies... so we get blue.
Here's a picture:
![[Image: bluesky.gif]](https://images.weserv.nl/?url=home.comcast.net%2F~vinelandrobotics%2Fbluesky.gif)
How about that reddish band at sun-set?
You can only see it near the horizon, and the sun is setting, because light from the sun's light has to go through more atmosphere near the horizon and the effect is noticeable, due to the low light caused by the setting of the sun. All blue light gets scattered and the only remaining frequencies are the lower, closer to red (frequencies lower than red, you get infra-red... that's sort of what "infra" means.... below. That's why I like to speak of these things in terms of frequency instead of the more common approach which uses the wavelength and makes us invert the whole reasoning, but then the nomenclature infra-red and ultra-violet stop making sense).
An image is worth a thousand words:
![[Image: as-figure2.png?]](https://images.weserv.nl/?url=www.scratchapixel.com%2Fassets%2FUploads%2FAtmospheric%2520Scattering%2Fas-figure2.png%3F)
You see this effect better in the afternoon than in the morning because a warm atmosphere rises and provides a wider reddish band... and in the afternoon, the atmosphere has been warming up for some 12 hours.... in the morning, it's been cooling off for the other 12 hours and the reddish band is smaller.
