Have you ever wondered why the sky is blue? Why does the sky become red near the horizon at sunrise and sunset?
Light travels in space just like the waves travel on the sea, rising and falling periodically. The distance between two successive crests or troughs is called the wavelength.
Imagine a boat floating on the sea. When the waves hit it, they lose track of their original path and scatter all around the boat. Smaller the wavelength, larger will be the scattering.
Unlike waves on the surface of the water, we can’t see the crest and troughs of the light waves. So, we can’t perceive their wavelength the way we do it for the water waves. Our eyes perceive the wavelength of the light waves as color. Blue light has a smaller wavelength than the red light.
The white light of the Sun contains the waves of all colors. When the sunlight comes toward Earth and enters its atmosphere, it encounters numerous dust particles. The light waves get scattered all around just like water waves were getting scattered by the boat. Smaller wavelengths scatter more as compared to the larger wavelengths. So, the blue light gets scattered all around making the sky blue. What happens to red light that has the largest wavelength in the visible part of sunlight? It manages to come directly towards us making the Sun and the sky appear red. So, the sky becomes reddish near the horizon during the sunrise and sunset.
(My answer to a question at quora.)