Why is the sky blue?
Over at Dean's World there's an argument over why the sky is blue.
The short answer is "Rayleigh Scattering."
The long answer is behind the cut.
picture stolen from Wikipedia
Here's what Rayleigh Scattering does. When light comes in at some angle, it deflects it. Things to note:
Okay, great. So how does this cause the sky to be blue? Well, there are two factors at work: first, the scattering takes light that is not directly along a line between your eye and the sun and knocks the photon into a new direction. This direction is sometimes towards your eye, so that you can see the light. Second, the scattering is much stronger at lower wavelengths.
The formula for the intensity of Rayleigh scattering is something like: I=A/L^4, where A is a bunch of crap that includes information about distances and angles, and L is the wavelength. Thus Rayleigh scattering is much stronger for light with shorter wavelengths, because the intensity drops off extremely quickly.
Example: If we have light of 300 nm wavelength (blue) and light of 800 nm (red), then ignoring the 'nano' in the math since I'm going to take a ratio later anyway (if it bothers you, multiply the following two numbers by 10^-36), we find that 1/L^4 for blue is 1.23457E-10 and for red it is 2.44141E-12. Thus our 300nm light is about 50 times more likely to Rayleigh scatter under the same conditions as the 800nm light.
Here's an illustration of what happens:
Crappy image drawn by me in Paintshop Pro
The net effect is, a lot more blue gets knocked down to the ground simply because scattering of radiation from a dipole has a much higher chance of occurring for low wavelength light than high wavelength light. The only time the sky is red, or orange, or yellow, is 1) when the clouds reflect the light, 2) the sun is low in the sky and has to pass through much more atmosphere which provides for many more chances of Rayleigh scattering to occur, 3) you're looking directly at the sun, and 4) someone sprayed a lot of gasoline up there and lit it on fire, take shelter.
Thus despite oxygen (and nitrogen) having no real color (take a mile long column of it and look directly through it at a light source, you'll see little to no color emerge), the sky still has a color.
If Rayleigh scattering did not occur, the sky would always be illuminated except for the portion immediately near the sun, and the days would be as black as the nights.
The short answer is "Rayleigh Scattering."
The long answer is behind the cut.
picture stolen from Wikipedia
Here's what Rayleigh Scattering does. When light comes in at some angle, it deflects it. Things to note:
- It scatters light from dipoles, which means most any molecule or atom can do this.
- The molecule has to be smaller than the light.
- The light retains the same energy (the scattering is elastic). If the energy changed, we would be talking about Raman scattering. Not to be confused with Ramen Scattering.
Okay, great. So how does this cause the sky to be blue? Well, there are two factors at work: first, the scattering takes light that is not directly along a line between your eye and the sun and knocks the photon into a new direction. This direction is sometimes towards your eye, so that you can see the light. Second, the scattering is much stronger at lower wavelengths.
The formula for the intensity of Rayleigh scattering is something like: I=A/L^4, where A is a bunch of crap that includes information about distances and angles, and L is the wavelength. Thus Rayleigh scattering is much stronger for light with shorter wavelengths, because the intensity drops off extremely quickly.
Example: If we have light of 300 nm wavelength (blue) and light of 800 nm (red), then ignoring the 'nano' in the math since I'm going to take a ratio later anyway (if it bothers you, multiply the following two numbers by 10^-36), we find that 1/L^4 for blue is 1.23457E-10 and for red it is 2.44141E-12. Thus our 300nm light is about 50 times more likely to Rayleigh scatter under the same conditions as the 800nm light.
Here's an illustration of what happens:
Crappy image drawn by me in Paintshop Pro
The net effect is, a lot more blue gets knocked down to the ground simply because scattering of radiation from a dipole has a much higher chance of occurring for low wavelength light than high wavelength light. The only time the sky is red, or orange, or yellow, is 1) when the clouds reflect the light, 2) the sun is low in the sky and has to pass through much more atmosphere which provides for many more chances of Rayleigh scattering to occur, 3) you're looking directly at the sun, and 4) someone sprayed a lot of gasoline up there and lit it on fire, take shelter.
Thus despite oxygen (and nitrogen) having no real color (take a mile long column of it and look directly through it at a light source, you'll see little to no color emerge), the sky still has a color.
If Rayleigh scattering did not occur, the sky would always be illuminated except for the portion immediately near the sun, and the days would be as black as the nights.