SCATTERING OF LIGHT
Scattering of light means to throw light in various random directions.
The scattering involves bouncing off of light by atoms or molecules of the medium through which they are travelling.
The beautiful atmospheric phenomena like blue colour of the sky, the red colour of the sun at sunrise and at sunset can be explained on the basis of scattering of light by the atmosphere.
The scattering of light by the colloidal particles of the medium due to which the path of the light becomes visible is known as Tyndall effect.
This phenomenon is seen when a fine beam of sunlight enters a smoke-filled room through a small hole.
Tyndall effect can also be observed when sunlight passes through a dense forest.
The colour of the scattered light depends upon the size of the particles scattering the light.
Very fine particles scatter mainly blue light of smaller wave length while particles of bigger size scatter light of longer wavelength.
If the size of the particles scattering the light is large enough, the scattered light may even appear white.
EXPERIMENT TO STUDY SCATTERING OF LIGHT
Let us do an experiment to understand the blue colour of sky and the reddish appearance of the sun at the sunrise or sunset due to scattering of light by the atmosphere.
A strong source S of white light is placed at the focus of a Convex Lens L1.
This lens L1 provides a parallel beam of light.
This parallel beam of light is passed through a transparent glass Tank T containing clear water.
The beam of light after coming from the tank is made to pass through a circular hole ‘C‘ made in a Cardboard.
A sharp image of the circular hole is obtained on a screen MN by using a second Convex Lens L2 as shown in the figure.
Now dissolve about 250g of sodium thiosulphate in about one litre of clear water taken in the tank. Add 1 to 2 ml of concentrated sulphuric acid to this water.
In a few minutes, fine microscopic sulphur particles are precipitated in water. As the sulphur particles begin to form, light gets scattered from the sulphur particles and we observe the blue colour from the three sides of the glass tank.
This is due to the scattering of light of short wave length by minute colloidal sulphur particles. When we observe the colour of the transmitted light from the fourth side of the tank facing the circular hole, we see first the orange red colour and then the bright crimson red colour on the screen.
BLUE COLOUR OF THE CLEAR SKY
The scattering of blue component of the white sunlight by the atoms and molecules present in the air of the atmosphere causes the blue colour of the sky.
The sunlight consists of seven coloured lights mixed together.
When sunlight passes through the atmosphere, the shorter wave length of blue light is scattered all around the sky by the tiny particles (atoms and molecules) present in the atmosphere.
Some of the scattered blue light enters in our eyes as a result the sky appears blue.
Whereas the longer wave length light such as yellow, orange and red etc. do not get scattered much and hence they pass straight through.
If the earth had no atmosphere, there would not have been any scattering and the sky would have looked dark.
The sky appears dark to the astronaut flying at very high attitudes because scattering is not prominent at such heights due to the lack of atmosphere.
COLOUR OF THE SUN AT SUNRISE AND SUNSET
At the time of sunrise and sunset, the sun is near the horizon.
The sun rays have to travel much larger part of the atmosphere to reach on earth.
As a result most of the light of smaller wave length i.e. blue coloured light gets scattered away.
Whereas the light of larger wave length i.e. red coloured light is scattered least.
Out of all the colours of sunlight, the red coloured light is scattered the least and reaches the earth.
Hence, the sun appears reddish at the sunrise and sunset.
When the sun is overhead, the sunlight has to travel much smaller portion of earth’s atmosphere.
As a result, a little of the blue and violet colours are scattered out, due to which the sun appear silver shiny (white).
DANGER SIGNALS ARE RED
Danger Sign (Source)
Out of all the colours of visible light, red colour has the largest wavelength.
Therefore red colour is least scattered.
As a result, it can be seen from maximum distance.
That is why danger signals are red.