As sun's rays travel through the atmosphere, they undergo scattering, refraction, reflection, diffraction, or any combination of these. This property of being more effective at scattering particular wavelengths of light is named as Rayleigh scattering , which is responsible for the blue appearance of sky. Another form of scattering called Mie Scattering, is responsible for the white appearance of clouds. Mie scattering occurs when the wavelengths of visible light are approximately, equally scattered. Water droplets, in even small clouds, as well as dust and haze particles effectively scatter all wavelengths of visible light in all directions, thus making clouds appear white.
Refracted light can be deceptive.
As starlight enters the atmosphere, it often passes through regions of different air density undergoing repetitive refractions. As a result, the star appear to twinkle or flicker, a condition known as scintillation . Also, since light is bent more on the horizon, the sun and moon both appear to be higher than they really are. In fact, they both rise two minutes earlier and set two minutes later than they would if there were no atmosphere. Twilight is the time at the beginning of the day immediately before sunrise and at the end of the day when sky remains illuminated.