To reach the earth's surface, the sun's light must pass through these objects. But part of the light does not reach the surface, and if it comes, then indirectly.

Why? Because of the light scattered by the object in the atmosphere.

Visible light is an electromagnetic wave fairly narrow range of the length (approximately 390-700 nanometers, where 1 nanometer is one-billionth of a meter). This electromagnetic spectrum ranges from very long radio waves to the extremely short gamma rays.

Within the visible part of the spectrum of red light has a large number of wavelengths than blue - 650 nm against 450 nm.

When the light is diffused in the atmosphere, the size and angle scattering depend on the wavelength and the size of the scattering object.

If the size of the scattering object is significant in comparison with the wavelength of the scattered light, its shape is also important.

Molecules are the least scattering objects, they are 1000 times smaller than the wavelength of visible light. For molecules such as nitrogen molecule, which is 78% of the atmosphere, the dependence of the intensity of scattering on wavelength is inversely proportional to the fourth power of the wavelength.

That is, comparing the blue and red, we take the ratio of the wavelength (650/450) and erected it in the fourth degree, to calculate how much more probable is the scattering of blue than red (obtained 4.3 times).

Blue light passing from the sun through the Earth's atmosphere scatters toward your eyes.

Thus, the sky looks blue because the scattering from molecules more likely to blue light than red.

In any direction blue light so much that he completely overwhelms the light of the stars, which, however, continue to Shine and day.

If you get close to the cosmos close enough, you will see that the sky there is black. This is because there are no significant objects which could scatter sunlight before your eyes.

Conversely, if you look directly at the sun through a clean atmosphere (without smoke or dust), a significant amount of blue light eliminated with line-of-sight, which gives the sun is yellow.

This kind of dispersion - when diffuse object much smaller than the wavelength of light is called Rayleigh scattering, after the British physicist of the 19th century, Lord Rayleigh.

At sunset the path of the sunlight through the atmosphere to your eyes is particularly long. In this case, so many blue light is lost (i.e. eliminated)that the sun looks orange or even red.

Space does not look blue, because there is not enough molecules to scatter blue light towards the beholder.

Water droplets or ice crystals that make up the clouds, quite large compared to the wavelength of visible light (at least 20 times more). In this case, the dispersion of light in a strong and almost independent of the wavelength, anyway, in the visible range. Clouds are white or different shades of neutral gray color, if they are in the shadow of the other clouds.

This shot of the setting sun shows a painting in red light, caused by the scattering effect. We can't see in this picture that the sun itself looks red. However, the scattered clouds of sunlight red. Particle clouds are very small dependence of the wavelength on the scattering object (visible light). Thus, the color of the clouds in the picture are caused by falling on sunlight.

If you take into account the dependence of the intensity of scattering on the wavelength of the particle is medium in size, you will find that wave in a narrow range this dependence changes in the opposite direction. That is in this range, which could cover the visible spectrum, the intensity of scattering stronger for longer waves.

Particles of this size can be in the visible spectrum. In this case, when sunlight passes through the air in which there are such particles, the red light is scattered, and the sun (or moon) will appear bluish.

This is a rare phenomenon occurs in the case when emissions of volcanic ash or forest fires fill the atmosphere of particles of the appropriate size.

Unlike the Earth, Mars, the atmosphere is quite dusty, and the sky on Mars orange color, sometimes with blue sunsets. This is because the particles of dust on Mars is much larger than the size of the molecules of carbon dioxide, which consists of the atmosphere of the red planet.

The fact that the atmosphere on Mars is very rarefied, means that the intensity of rasseleniya because of the dust is there a relatively greater importance than on Earth.

At sunset eliminated so many blue light that the sun looks orange or even red.

Someday, lying in the grass and admiring the white clouds floating on a beautiful blue sky, think of a physical phenomenon that determines what you see.