Contents
Physics Topics are also essential for space exploration, allowing scientists to study phenomena such as gravitational waves and cosmic rays.
What is Scattering of Light? How does it Occur?
Scattering of light means to throzv light in various random directions. Light is scattered when it falls on various types of suspended particles in its path. Depending on the size of particles, the scattering can be of white sunlight as such or of the coloured lights which make up the white sunlight.
As we will study after a while, the blue colour of the sky and the red colour of the sun at sunrise and at sunset can be explained on the basis of scattering of light caused by the earth’s atmosphere (or air) (see Figure). We will first describe the Tyndall effect.
Tyndall Effect
The scattering of light by particles in its path is called Tyndall effect. When a beam of sunlight enters a dusty room through a window, then its path becomes visible to us. This is because the tiny dust particles present in the air of room scatter the beam of light all around the room. And when this scattered light enters our eyes, we can see the beam of light.
Thus, an example of Tyndall effect is the way a beam of sunlight becomes visible as it passes through dust particles in the air of a room. Tyndall effect can also be observed when sunlight passes through the canopy of a dense forest. Here, tiny water droplets in the mist scatter sunlight (see Figure ).
We have just studied the spectrum of sun’s white light which consists of seven coloured lights. If we look at the spectrum of white sunlight, we will observe that the reds and blues are very predominent in it. Red coloured light has a longer wavelength but the blue coloured light has a shorter wavelength.
In fact, the wavelength of blue light is almost half that of the red light. In 1859, in an attempt to explain the blue colour of the sky, Tyndall discovered that when white light consisting of seven colours is passed through a clear liquid having small suspended particles in it, then the blue colour of white light having shorter wavelength is scattered much more than the red colour having longer wavelength.
Blue light has shorter wavelength, so it is scattered more easily. On the other hand, red light has longer wavelength, so it is not scattered much. In other words, the blue coloured light present in white sunlight is scattered much more easily than the red light. In fact, the blue light present in sunlight is scattered 10 times more than the red light.
The Colour of Scattered Light Depends on the Size of Scattering Particles
The earth’s atmosphere is a heterogeneous mixture of minute particles. These particles include suspended particles of dust, tiny water droplets and molecules of air (the molecules of air means the molecules of gases like nitrogen and oxygen which make up the air).
When light coming from the sun strikes the particles present in the atmosphere (or air), then what happens to the light depends on its wavelength and the size of the particles it hits. This will become clear from the following discussion.
(i) Dust particles and water droplets suspended in the atmosphere are much larger than the wavelength range of visible light. When white light coming from the sun hits these larger particles, it gets reflected or scattered in different directions. The different colours of white light are reflected by the dust and water particles in the same way.
Due to this, the scattered light appears white (because it still contains all the colours of white light). Thus, when white sunlight falls on larger particles (like dust particles and water droplets) present in the atmosphere, it is scattered as such, so the scattered light also appears white.
(ii) The air molecules (nitrogen and oxygen gas molecules) present in the atmosphere are smaller than the wavelength range of visible light. So, when light coming from the sun hits these very small air molecules, it behaves differently.
Since the different colours of white light have different wavelengths, so they are affected differently. The lower wavelength lights (blues) are scattered much more by the air molecules but the higher wavelength lights (reds) are scattered much less.
Thus, when white sunlight falls on the extremely small particles like air molecules present in the atmosphere, it is not scattered as white light. The molecules of air scatter mainly the lower wavelengths of light which have blue shades.
From the above discussion we conclude that the colour of the scattered light depends on the size of the scattering particles in the atmosphere :
(a) The larger particles of dust and water droplets present in the atmosphere scatter the light as such due to which the scattered light also appears white.
(b) The extremely minute particles such as the air molecules present in the atmosphere scatter mainly the blue light present in the white sunlight.
Why the Sky is Blue
The scattering of blue component of the white sunlight by air molecules present in the atmosphere causes the blue colour of the sky. This can be explained as follows.
The sunlight is made up of seven coloured lights mixed together. When sunlight passes through the atmosphere, most of the longer wavelength lights (such as red, orange, yellow, etc.) present in it do not get scattered much by tire air molecules and hence pass straight through.
The shorter wavelength blue light is, however, scattered all around the sky by air molecules in the atmosphere (see Figure). Whichever direction we look, some of this scattered blue light enters our eyes. Since we see the blue light from everywhere overhead, tire sky looks blue.
Thus, the sky appears blue because the molecules in the air (nitrogen and oxygen molecules) scatter blue part of the sunlight much more than they scatter red light (or other shades). Please note that only a little of the blue light present in white sunlight is scattered by the atmosphere which makes the sky appear blue.
Most of the blue light remains behind unscattered due to which the composition of sunlight remains almost unaltered. Because of this the direct sunlight coming through the blue sky still appears to be white.
If the earth had no atmosphere consisting of air, there would have been no scattering of sunlight at all. In that case no light from the sky would have entered our eyes and the sky would have looked dark and black to us.
In outer space, the sky looks dark and black instead of blue. This is because there is no atmosphere containing air in the outer space to scatter sunlight. Since there is no scattered light to reach our eyes in outer space, therefore, the sky looks dark and black there. This is why the astronauts who go to outer space find the sky to be dark and black instead of blue.
We know that the ‘danger’ signal lights are red in colour. This is because the red coloured light having longer wavelength is the least scattered by fog or smoke particles. Due to this the red light can be seen in the same colour even from a distance.
Why the Sun Appears Red at Sunrise and Sunset
The sun and the surrounding sky appear red at sunrise and at sunset because at that time most of the blue colour present in sunlight has been scattered out and away from our line of sight, leaving behind mainly red colour in the direct sunlight beam that reaches our eyes. This can be explained as follows.
At the time of sunrise and sunset when the sun is near the horizon, the sunlight has to travel the greatest distance through the atmosphere to reach us. During this long journey of sunlight, most of the shorter wavelength blue-colour present in it is scattered out and away from our line of sight.
So, the light reaching us directly from the rising sun or setting sun consists mainly of longer wavelength red colour due to which the sun appears red (see Figure). Due to the same reason, the sky surrounding the rising sun and setting sun also appears red. Thus, at sunrise and sunset, the sun itself as well as the surrounding sky appear red.
We will now discuss why the sun appears white when it is overhead in the sky. When the sun is overhead (as at noon), then the light coming from the sun has to travel a relatively shorter distance through the atmosphere to reach us.
During this shorter journey of sunlight, only a little of the blue colour of the white light is scattered (most of the blue light remains in it). Since the light coming from the overhead sun has almost all its component colours in the right proportion, therefore, the sun in the sky overhead appears white to us (see Figure).
Experiment to Study the Scattering of Light
We will now perform an experiment to understand how the scattering of light leads to the blue colour of the sky, and the red appearance of the sun at sunrise and sunset. In this experiment we will prepare a colloidal solution containing tiny particles of sulphur required for scattering the light by the action of sulphuric acid on sodium thiosulphate solution.
Set up the apparatus as shown in Figure. S is a strong source of white light. We will consider this source of light to be the sun. The source of light S is placed at the focus of a convex lens (converging lens) Lj so as to produce a parallel beam of light rays.
A transparent glass tank T is filled with about 2 litres of clear water. A cardboard disc D having a circular hole C at its centre is kept on the other side of the water tank. Another convex lens L2 is kept behind the cardboard disc to focus the light rays (coming from the glass tank and passing through the circular hole of cardboard disc) to form an image on the screen R. We will now describe how the experiment is actually performed.
Switch on the source of light S. We will find that a beam of light passes through water in the glass tank and forms a circular patch of white light on the screen R. We can, however, not see the path of the beam of light inside the water of the tank (because there are no suspended particles in water to scatter the beam of light).
Let us now dissolve about 200 grams of sodium thiosulphate (called ‘hypo’) in water of the glass tank. Then add 1 to 2 mL of concentrated sulphuric acid to the water. We will see that fine microscopic particles of sulphur begin to form in water and a colloidal solution is obtained. As the sulphur particles begin to form in water, we will see the blue light coming from the sides of the glass tank (see Figure).
This is due to the scattering of short wavelength blue light (present in the beam of white light) by the minute colloidal sulphur particles. This is how the sky looks blue (when the blue light present in sunlight is scattered by the molecules of air in the atmosphere).
If we look at the screen on the front side of the glass tank containing colloidal solution of sulphur, we will see a red patch on the screen. This is because mainly the red colour of the beam of white light reaches the screen after passing through the colloidal sulphur solution in the glass tank (the blue colour being scattered away and hence eliminated on the way).
This is how the sun looks red at sunrise and sunset when mainly the red colour of sunlight reaches our eyes after the elimination of blue colour through scattering along the way.
Scattering of Light
When light wave is incident on a particle of small size in com-parison to the wavelength of the incident light, the particle absorbs energy from the incident light without changing its state and radiates this energy as waveform in different directions. Actually the particle here acts as secondary source. When sunlight passes through the earth’s atmosphere, light is absorbed by fine dust particles and air molecules in the atmosphere which radiate light in different directions.
This phenomenon is called scattering of light. It is illustrated in Fig. A dust particle or air molecule when struck by a light wave is set into vibration. Immediately afterwards it radiates the absorbed light in all directions. Since the number of dust particles and air molecules in the atmosphere is very large, the scattered light propagates in all directions.
Rayleigh’s law of scattering: The intensity of scattered light (I) varies inversely as the fourth power of its wavelength, i.e., I ∝ \(\frac{1}{\lambda^4}\).
According to this law, light of shorter wavelengths violet, blue, etc. are scattered much more than the light of the longer wave-lengths red, orange etc.
According to Rayleigh scattering, the intensity (I) of scattered light is proportional to the sixth power of diameter (d) of sus-pended dust particle in atmosphere.
The Blue of the sky
The phenomenon is due to the scattering of sunlight by the sus-pended dust particles, air molecules and gas molecules in the atmosphere. When rays of the sun are scattered, the intensity of the blue and violet colour is high, following Rayleigh’s inverse fourth power law. Again our eyes are more sensitive to blue light compared to violet light. So the sky appears blue.
In the absence of the atmosphere, light rays would not be scat-tered. So the sky would appear black even during the day. Moon has no atmosphere. Consequently the moon’s sky appears black.
Redness of the Rising and the Setting Sun
During sunrise and sunset the sun appears red. This is also due to the scattering of light. When the sun is directly overhead, sun rays have to traverse less distance through the earth’s atmosphere than when the sun is situated near the horizon [Fig.].
As the wavelengths of other colours are less than that of red, they suffer more scattering and spread over a larger expanse before reaching the observer. Red colour suffers least scattering and so this colour reaches to us more in comparison to other colours. So the sun appears red at sunrise and sunset.
The is also why red signals are used to indicate dangers. The red light goes large distances without being scattered much.