Scattering of Light | Human Eye and Colourful World

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.

 

TYNDALL EFFECT        

 

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.

Atmospheric Refraction | Human Eye and Colourful World

Atmospheric Refraction

The refraction of light caused by the earth’s atmosphere is called atmospheric refraction.

The physical conditions of the refracting medium (air) are not stationary.

Some of the air layers are cold and act like a denser medium whereas other layers, of the atmosphere are comparatively warm and act like a rarer medium.

In the atmosphere the air layers have different optical densities.

So when the light rays pass through the air layers of different optical densities, then refraction of light takes place.

For example:

The air just above the fire becomes hotter than the air farther up.

The hotter air is lighter (rarer medium) than the cooler air (denser medium) above it, and has a refractive index slightly less than that of cooler air.

Since the physical conditions of the air are not stationary, therefore when we see the objects through hot and cold air layers then refraction of light takes place due to which the position of the objects fluctuates.

Some of the optical phenomena in nature which take place due to the atmospheric refraction of light are as follows:

 

TWINKLING OF STARS

The twinkling of stars is due to the atmospheric refraction of star’s light.

Since stars are very far away heavenly bodies and therefore are considered single point sources of light.

When the light coming from a star enters the earth’s atmosphere, it undergoes refraction due to the varying optical densities of air.

The continuous changing conditions of the atmosphere, refracts the light coming from the stars by different amounts from one moment to another.

When the atmosphere refracts more star light towards us, the stars appear to be bright and when the atmosphere refracts less star light towards us, the star appears to be dim.

This phenomenon goes on thereby giving rise to twinkling of stars.

 

PLANETS DO NOT TWINKLE

The planets are much closer to the earth and are thus considered as the collection of infinite point sources of energy.

Therefore the dimming effect produced by some of the point sources of light in one part of the planet is nullified by the brighter effect produced by the other point sources of light in the other part of the planet.

As a result, the total variation in the amount of light entering our eye from all the point sources of light will average out to be zero.

Thereby nullifying the twinkling effect. Hence planets do not twinkle.

 

THE APPARENT POSITION OF THE STARS IS HIGHER THAN THEIR ACTUAL POSITION

The apparent position of the stars is higher than their actual position due to the atmospheric refraction.

The upper layers of the atmosphere act like a rarer medium whereas the lower layers which are close to the earth act like a denser medium.

As the star light enters from rarer to denser medium it bends more towards the normal.

Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position, as a result, star appears slightly higher than its actual position, as shown in the figure.

 


ADVANCE SUNRISE AND DELAYED SUNSET

 

The sun is visible to us about 2 minutes before the actual sunrise and 2 minutes after the actual sunset because of atmospheric refraction.

The actual sunrise takes place when the sun is just above the horizon.

When the sun is slightly below the horizon, the sun’s light coming from rarer medium (i.e. from less dense air) to denser medium (i.e. to more dense air) is refracted downwards as it passes through the atmosphere.

Because of this atmospheric refraction, the sun appears to be raised above the horizon whereas it is actually slightly below the horizon (as shown in the figure).

It is again due to the atmospheric refraction that we can see the sun for about two minutes even after the sun has set below the horizon.

Because of this atmospheric phenomenon, the time from sunrise to sunset is lengthened by about 2 + 2 = 4 minutes.