Why is the sky blue?

In fact this phenomenon is caused by refraction. The atmosphere of the Earth is not one clear boundary, but a graduate transition between vacuum and air at 1 atmospheric pressure, so you can count it as many boundaries. As we know, when a ray of light that consist of multiple colours on the electromagnetic spectrum (that comes from the sun) pass through a boundary into a medium where light travels slower, the violet light's side is bent more and the red light's side is bent less(refer to the electromagnetic spectrum) so after many millions of times of refraction, the blue light, which is close to the violet side, from the sun can bend at an angle a lot greater than red light does, so when sun light enter the atmosphere at an angle of almost 90 degree, it can still enter the ground level of atmosphere at almost 0 degree to the ground. So blue light from the sky can hit most places on ground from any angle. When blue light enters our eyes at this angle, it seem to us that the whole sky radiate blue light, but actually it is the colour from the sun. On the other hand, the violet light, which is the highest frequency among the visible light, closest to the violet side, is bent too much when it enter the atmosphere so we don't see a violet sky. Blue light is bent just enough for us to see on ground level. The water vapour and condesed water droplets in air can also help the refraction, therefore the colour of the sky varies with the amount of water in air. Its colour also varies with altitude(height) and angle of sun(time of day). You can take beautiful pictures of the sky in stranger colours(that ranges from orange to purple) because the altitude is higher than ground level.(passage not finished yet)

2012-06-24 23:51:53 補充：
You can see orange sky during the time of sunrise and sunset since the sunlight enter the atmosphere at a different angle and thus passes through more air.

2012-06-25 00:01:09 補充：
Dust, pollen->not blue->not the reason,,Planes/Peaks->not high->no black sky

2012-06-25 00:03:11 補充：
ask me for more

The sky appears blue (instead of black) because light passing through the air hits tiny particles like dust and pollen. These tiny particles are large enough to reflect blue light, but red light (which has a longer wavelength) is not reflected nearly as much. So the result is that the blue light gets scattered by the particles, and you see it when you look up. The other side of this is sunset, when the sun appears reddish-orange low on the horizon. That's because the light rays have to pass through a lot of air (and particles) before they get to your eyes, and all the blue light was scattered away and dispersed. That leaves only the red end of the spectrum to penetrate the atmosphere and reach your eyes. At noon, the sun is coming straight down and only passing through ten miles of atmosphere or so. At sunset, the same light has to pass through many hundreds of miles of air to reach you. You will see more dramatic sunsets (redder and darker) when there's a lot of dust, smoke or pollen in the air. When volcanic eruptions occur, intense sunset colors are seen around the world, sometimes for a couple of years after the event. If the air is truly pure, it doesn't scatter light much at all. If you climb up above the atmosphere, or nearly to the top of it in an airplane, the sky appears black, not blue. If we had particle-free air, the sky would likewise appear black instead of blue. But we don't.