Tyndall Effect

When beam of light strikes tiny water droplets, suspended particles of dust etc. (called as Colloidal Particles), the path of the beam becomes visible. This is known as Tyndall Effect. The colour of scattered light depends upon the size of colloidal particles. Very fine particles scatter mainly blue light while particles of larger size scatter light of longer wavelengths.

Why sky appears blue in colour?

The colour of the sky appears blue due to scattering of light. When the sunlight passes through the atmosphere, fine particles in air will scatter the blue colour more strongly than red.

Why sky appears red in colour due to sunrise and sunset in atmosphere?

Reddening of Sun at rise and set

During sunrise and sunset, light from the Sun near the horizon passes through thicker layers of air and larger distance in the earth’s atmosphere before reaching our eyes. Light from the Sun overhead would travel relatively shorter distance, resulting in white appearance of sun. Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelengths, hence the reddish appearance.