Scattering of Light

The sky appears in different colors in different time of the day. It appears blue during daytime and appears to be red or orange during sunset. The clouds are white. Why is this so? What is responsible for this phenomenon?

Scattering of Light, one of the properties of light, is said to be the reason why we perceive different colors of the sky and in the surroundings. The light from the sun is absorbed and is re-radiated back in different direction – this process is what the scattering of light means. The light is scattered due to the different molecules and particles in the atmosphere. These particles and molecules are called scatterers. Without the atmosphere, the sky would just appear to be black.

There are different types of scattering of light – Rayleigh scattering or selective scattering and Mie scattering.

Rayleigh scattering or selective scattering explains why the sky looks blue during daytime. Sunlight is scattered by the particles in the atmosphere. As the beam of sunlight pass through the atmosphere, the energy goes into the scattered light, thus, its intensity decreases. But, the intensity of the light scattered by the particles increases. Based on Lord Rayleigh’s equation, \(1 \over \lambda^4\), blue lights whose wavelengths are shorter are scattered more strongly than the red lights whose wavelengths are longer. Scattered light is said to have contain 15 times more blue light than red light, thus, giving blue color to daytime sky.

Mie scattering solved by Gustav Mie explains the reason why clouds look white. Clouds contain a large number of particles such as water droplet and ice crystals which also scatter light. Due to this high concentration of particles in the clouds, all lights of different wavelengths are scattered resulting to white color of the clouds.

During sunset, the distance between the earth and the sun is farther, thus sunlight travels farther to reach the earth. At the atmosphere, a fraction of blue light is removed by scattering leaving a yellowish or reddish color of the sky, since white light minus blue light results to yellow or red.