Why is the sky blue?
Not all colors are the result of the addition or subtraction of light. Some colors, like the blue of the sky, are the result of selective scattering.
Electrons in atoms and molecules scatter (that is absorb and re-emit) light. The tinier the particle, the greater amount of higher-frequency light it will re-emit. This is similar to the way small bells ring with higher notes than larger bells. The nitrogen and oxygen molecules, dust particles, water vapors, etc., that make up most of the atmosphere are like tiny bells that "ring " with high frequencies when energized by sunlight. Like sound from the bells, the re-emitted light is sent in all directions. When light is re-emitted in all directions, we say the light is scattered.
A beam of light falls on atom and increases the vibrational motion of electrons in the atom. The vibrating electrons in turn re-emit light in various directions. Light is scattered.
Of the visible frequencies of sunlight, violet is scattered the most by nitrogen and oxygen in the atmosphere, followed by blue, green, yellow, orange, and red, in that order. Red is scattered only a tenth as much as violet. Although violet light is scattered more than blue, our eyes are not very sensitive to violet light. Therefore the blue scattered light is what predominates in our vision, and we see a blue sky!
On a moonlit night can the sky look blue? Why?
On a moonlit night, the sky to us looks dark or black. For our eyes to sense colour, large numbers of photons ought to strike the retina so as to activate the cones. Thus our eyes perceives the moonlit sky as dark. But a long camera will show that the sky is has a bluish hue-sort of dark blue. The composition of moonlight is same as the sunlight since Moon is shining due to reflection of sunlight itself. The sky looks blue for the same reason as why the sky looks blue during daytime preferential scattering of blue or violet light by air particles.
The blue of the sky varies in different places under different conditions. A principal factor is the water-vapor content of the atmosphere. On clear, dry days the sky is a much deeper blue than on clear days with high humidity. Places where the upper air is exceptionally dry, such as Italy and Greece, have beautifully blue skies that have inspired painters for centuries. Where the atmosphere contains a lot of particles of dust and other particles larger than oxygen and nitrogen molecules, light of the lower frequencies is also scattered strongly. This makes the sky less blue, and it takes on a whitish appearance. After a heavy rainstorm when the particles have been washed away, the sky becomes a deeper blue.
The grayish haze in the skies over large cities is the result of particles emitted by car and truck engines and by factories. Even when idling, a typical automobile engine emits more than 100 billion particles per second. Most are invisible but act as tiny centers to which other particles adhere. These are the primary scatterers of lower-frequency light. The largest of these particles absorb rather than scatter light, and a brownish haze is produced!
Is there a demonstration to show why the sky is blue?
To appreciate how light is scattered by particles in the atmosphere, do the following experiment.
Take a glass and fill it with water. Hold it against a lighted bulb. You will see that the glass and the water are transparent and let the light pass through. Now add a few drops of milk in the water. The water should still appear colourless and not milky. Now hold the glass again in front of a lighted bulb. You will see a bluish tinge to the glass. This is because the milk particles scatter predominantly the blue component of the light rays emitted from the bulb.