SCATTERING OF LIGHT
The interplay of light with objects around us gives rise to several spectacular phenomena in nature. The blue colour of the sky, colour of water in deep sea, the reddening of the sun at sunrise and the sunset are some of the wonderful phenomena we are familiar with. The path of a beam of light passing through a true solution is not visible. However, its path becomes visible through a colloidal solution where the size of the particles is relatively larger.
The earth’s atmosphere is a heterogeneous mixture of minute particles. These particles include smoke, tiny water droplets, suspended particles of dust and molecules of air. When a beam of light strikes such fine particles, the path of the beam becomes visible. The light reaches us, after being reflected diffusely by these particles. The phenomenon of scattering of light by the colloidal particles gives rise to Tyndall effect. This phenomenon is seen when a fine beam of sunlight enters a smoke-filled room through a small hole. Thus, scattering of light makes the particles visible. Tyndall effect can also be observed when sunlight passes through a canopy of a dense forest. Here, tiny water droplets in the mist scatter light. The colour of the scattered light depends on the size of the scattering particles. Very fine particles scatter mainly blue light while particles of larger size scatter light of longer wavelengths. If the size of the scattering particles is large enough, then, the scattered light may even appear white.
Why is the colour of the clear sky blue
The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end. The red light has a wavelength about 1.8 times greater than blue light. Thus, when sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters our eyes. If the earth had no atmosphere, there would not have been any scattering. Then, the sky would have looked dark. The sky appears dark to passengers flying at very high altitudes, as scattering is not prominent at such heights. You might have observed that ‘danger’ signal lights are red in colour. Do you know why? The red is least scattered by fog or smoke. Therefore, it can be seen in the same colour at a distance.
Colour of the sun at sunrise and sunset
The sky and the Sun at sunset or sunrise appears red. 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. This gives rise to the reddish appearance of the Sun.
Mirage or Inferior mirage
Definition : It is an optical illusion, seen in deserts at summer noon, due to which an inverted image of a distant tree is seen formed in hot sand below it, as if formed in water. Actually there is no water anywhere.
Explanation : It is due to total internal reflection. At summer noon, in desert, sand becomes hot. The air in its contact becomes very hot and hence rarer. As we move up, air becomes less and less hot, hence less and less rarer. The air can be divided into layers of different optical density (fig.).
For rays (like number 1 ray), incident at small angle at upper most layer, the angle of incidence on lowest layer may not be more than critical angle. These rays are all absorbed by sand.
The ray no. 2 starting from tree top and making a bigger angle since beginning, reaches the lowest layer at bigger angle. The angle of incidence may become just more than the critical angle. The ray is totally reflected upward and outward. All rays on the right of ray no. 2 will start with still bigger angle and will have angle of incidence becoming more than critical angle from upper and upper layers. They are also totally reflected.
Looming or Superior mirage
Definition : It is an optical illusion, seen, at sea–shore in winter evening, due to which an image of a ship is seen formed in air in sea–sky. The actual ship is nowhere visible.
Explanation : It is due to total internal reflection. In cold evening, over sea–bed sea water becomes too cold. Air layer in its contact is cold and denser. As we go up, air layers become less and less colder and hence rarer. (Fig.)
Rays from invisible ship going upward go from denser to rarer air layers. They are totally reflected downwards and received by an observer at sea–shore. The observer sees an image (virtual) of the ship hanging in the sky.