What is Scattering of light?
When white light from the sun enters the earth’s atmosphere, the light gets scattered. Here the light spreads in all directions through the dust particles, free water molecules and the molecules of the gases present in the atmosphere. This is called the scattering of light. Or in other words, we can say it is the phenomenon of the change in the direction of light on striking an obstacle like a dust particle, water droplet, etc. To understand this more, we can observe the path of a beam of light passing through a true solution. Here the path is not visible but its path becomes visible through a colloidal solution. Also here the size of particles is relatively large.
Earth’s atmosphere is a heterogeneous mixture of minute particles consisting of smoke, tiny water droplets, and suspended dust particles. The path of the beam becomes visible when a beam of light strikes such fine particles. This effect of light is called the Tyndall effect. The colour of the scattered light depends on several factors, mainly the size of the scattering particles. Very fine particles scatter the shorter wavelength like blue light,while the particles of larger size scatter light of longer wavelength like a red light.
Scattering effect is experiences differently for molecules with a larger size than the wavelength of light, this phenomenon is known as the Mie effect. Because of the largeness of particles, the light appears white. This is why the clouds, which are made of droplets of water are white. The blue colour is present in the higher percentage of the lower wavelengths. When the wavelength of light decreases, the scattering efficiency of the small molecules in the atmosphere decreases. The natural source sun radiates its light and its rays fall into the earth’s envelope and therefore sunlight gets scattered in the atmosphere.
Colour of the sunrise and sunset
At the time of sunrise and sunset, there is a peculiarity that the Sun is closer to the horizon. The sunlight near the horizon passes through dense layers of the air. Also, it covers a larger distance before reaching our eyes. Thus, most of the blue light gets scattered away. The light that comes our eyes is of longer wavelengths, mainly orange and red.
Of all the colours of sunlight, the colour scattered least is red colour. Hence the sun appears red both at the time of sunrise and sunset. At noontime when the sun is nearly overhead, the sunlight passes through the rarer layers of the air and covers a shorter distance before reaching our eyes when a beam of light strikes such fine particles.
Why danger signals are red?
We all know light, red has the largest wavelength. The size of the particles responsible for scattering is much less than the wavelength of the light when sunlight passes through the earth’s atmosphere. This is the reason why the red colour is scattered the least and can be seen from a distance and danger signals are red.
Read more: State of Matter