Scattering of Light

Question 1 What is meant by scattering of light?

Question 2 What is Tyndall effect? Give few examples?

Question 3 What happens when a beam of sunlight enters a dusty room through a window?

Question 4 Why does the sky appear blue on a clear day?

Question 5 Why does sky appear black to an astronaut instead of blue?

Question 6 Why does Sun appear red at sunrise?

Question 7 Why does Sun appear red at sunset?

Question 8 Out of blue and red light, which one is scattered more and why?

Question 9 What causes the scattering of blue component of sunlight in the atmosphere?

Question 10 State two effects produced by the scattering of light by the atmosphere?

Scattering of Light

Scattering of light means to throw light in various random directions. Light is scattered when it falls on various types of suspended particles in is path.

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 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.

 

Tyndall effect can also be observed when sunlight passes through the canopy of a dense forest.

If we look at the spectrum of white sunlight, we will observe that the reds and blues are very predominant in it. Red coloured light has a longer wavelength but the blue coloured light has a shorter wavelength.

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. Red light has longer wavelength, so it is not scattered much. The blue coloured light present in white sunlight is scattered much more easily than the red light.

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.

(1) 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 direction.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. 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.

(2) The air molecules (nitrogen and oxygen gas molecules) present in the atmosphere are smaller than the wavelength range of visible light: 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. 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.

 

The colour of the scattered light depends on the size of 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.

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.

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 the 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. Whichever direction we look, some of this scattered blue light enters our eyes. Since we see the blue light from everywhere overhead, the sky looks blue.

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). 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.

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.

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.The sky surrounding the rising sun and setting sun also appears red.

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.