The secret of sky appearing blue lies in the light rays coming from sun. Usually the light from sun which looks to be white is actually made of no. of colors. White light contains colors Red, Orange ,Yellow , Green , Blue , Violet. Each of these colors have different wavelengths with Red having longest wavelength and blue , violet have much shorter wavelength.
When the light from sun enters the earth’s atmosphere it gets deflected by the particles present in the air. These particles are gaesous moleclues of nitrogen and oxygen. As the light from sun hits these gaseous molecules it is scattered depending on the wavelength of each color. Shorter the wavelength more is the scattering. Since the blue color has the shorter wavelength it is scattered more as compared to other colors. As our eyes sees more blue which is scattered in atmosphere , the sky appears to be blue.
The sky doesn’t look violet even though the violet color has the shortest wavelength is because our light is more sensitive to blue color and also the amount of blue is sunlight is more than that of violet.
This scattering of light is based on the concept of Rayleigh scattering which states that scattering of light depends on the wavelength. Shorter wavelengths like blue are scattered more and to greater degree as compared to longer wavelengths like red and yellow. The blue light has the wavelength of 450nm while the red has 700nm.
Why Blue sky fades during evening?
During the sunset as the sun gets lower in the sky, the sunlight has to pass through more atmosphere before it reaches us. From the horizon the light gets scattered more , as a result of this increased amount of scattering the effect of blue decreases which makes the sky look more reddish during sunset.