Why the sky is blue:
Light from the sun must pass through the Earth's atmosphere before reaching us at the surface. Like a flashlight shining through the air, most of the light passes through the atmosphere uninterupted and reflects off the Earth's surface where ever it strikes. But, unlike the flashlight analogy, in the atmosphere not all light passes through uninterupted. Tiny particles (mostly oxygen and nitrogen) in the upper atmosphere are about the same size as the shorter wavelengths of light. These tiny particles begin to vibrate as a result of the light waves passing by. This causes the electrons that make up the tiny particles to vibrate at exactly the same frequency (and wavelength) as the light wave that strikes them. This is very similar to the example we saw with sound where we could get a second identical tuning fork to start vibrating by placing it close to, but not touching, the original one. When the tiny particles begin to vibrate they give off photons at the same frequency as the light that hit them. So, if a blue wavelength strikes an oxygen atom, it will vibrate the oxygen's electrons at the same frequency as blue light. When this happens, photons are given off at a frequency equal to blue light. The newly given off photons are emitted in all directions. We then "see" these photons as blue light. In this same way, all eight colors of the rainbow get scattered (each color a different amount) in the upper atmosphere.
Why do we see only blue? The high frequency colors (violet, indigo and blue) get scattered the most because most of the particles in the upper atmosphere are about the same size as their wavelengths. All the other colors get scattered less and less as you move from blue to red because their wavelengths gets increasingly longer and there aren't nearly as many particles that size. For example, red is scattered only a tenth as much as violet.
Why don't we see violet, then, instead of blue, if it is scattered the most? Although violet is scattered the most (10 times more than red), our eyes are a lot more sensitive to the blue light coming from the sky, so we see the sky more as a blue color than a violet one.
In the picture below, taken from outer space, we see the moon rising through the blue atmosphere that surrounds the whitish appearing Earth. The blackness above the blue is outer space. This photo was taken from space shuttle Discovery in 1999.
The blue of the sky varies in different places under different conditions. Where there are a lot of particles of dust and other particles larger than oxygen and nitrogen molecules, the lower frequencies of light are scattered almost as much as the higher frequencies. This makes the sky less blue, and it takes on a whitish appearance (when all frequencies are present we see white). After a heavy rainstorm, when the particles have been washed away, the sky becomes a deeper blue again.
In space where there are no molecules (no atmosphere), as in the picture below, taken from Apollo 11 orbiting the moon in 1969, the "sky" is black. The brown at the bottom of this picture is the moon and the planet rising is Earth.
A simplified explanation of why the sky is blueWhy are clouds white?
Clouds are made up of clusters of tiny water molecules in a variety of sizes. The different sizes result in all frequencies being scattered more or less equally. (lower frequencies from larger droplets and higher frequencies from smaller droplets) When all the different frequencies are scattered equally the light that reaches our eye is essentially made up of all frequencies and therefore we see white.
Why are sunsets red?
At the beginning and end of the day light must travel through much more atmosphere than during the day because of the angle of the sun. As a result, almost all the light energy in frequencies higher than red gets scattered out before it reaches you. Since most of the remaining light energy is traveling at the frequency that our eyes sense as ''red", then the sky appears red. If you watch carefully the sun appears yellowish then orangish and then reddish as it sets. (and the reverse rising) This observation supports this theory because red has the longest wavelength, then orange, then yellow. Remember R O Y G B I V ?
