Scattering of Light :Scattering of Light by Different Particles

Key Concepts

• Scattering of light The blue sky

• The white clouds The red sunrise and sunset

• The white 12 noon

• The red danger signals

Introduction: 

In this session we will get to know the reason behind phenomena like the blue sky, the white clouds, the red sunrise and sunset, the white 12 noon, and the red danger signals. All this happens due to a single phenomenon, the scattering of light. 

Explanation: 

Scattering of Light: 

The earth’s atmosphere is a heterogeneous mixture of minute particles including tiny water droplets, smoke, suspended dust particles, molecules of air etc. Such particles scatter light in the earth’s atmosphere. When sunlight enters the earth’s atmosphere, it is first absorbed by the particles in the air and then re-radiated in several directions, spreading it all over. This phenomenon is called the scattering of light. 

This phenomenon of scattering of light by various particles in the atmosphere such as dust, water droplets, air molecules etc. is called Tyndall effect. 

Scattering of Light by Different Particles: 

The scattering of light by a specific particle depends upon its size compared to the wavelength of the visible light (400 to 700 nm). The size of the air molecules such as nitrogen and oxygen is much smaller than the wavelength of the visible light. However, the size of the dust and water particles is much larger than the wavelength of visible light.  

Scattering of light by water and dust particles: 

When sunlight falls on a water molecule or a dust particle, it scatters all the colors of light equally. This means that the scattered light appears white in color.

Scattering of light by air molecules: 

When white light falls on an air molecule, it scatters different colors of light by different amounts. It scatters the colors with smaller wavelengths such as blue color the most whereas it scatters the colors with larger wavelengths such as red color the least. This phenomenon of unequal scattering of various colors of light if called preferential scattering. 

Applications of Scattering of Light: 

Visible path of the light: 

In the picture below, sunlight falls on the ground through a canopy of a dense forest. The sunlight is scattered here by tiny water droplets in the mist. As a result, the particles suspended shine and the path of the light rays can be seen. 

The blue sky: 

The atmosphere mostly consists of air molecules, which preferentially scatter the white sunlight that falls on them. This means that the air molecules scatter blue light the most and red light the least. In fact, the blue light is scattered around 10 times more than the red light. This scattered blue light when enters the eyes of the observer, makes the sky appear blue. 

The white clouds: 

The clouds mostly consist of water droplets and dust particles, which scatter all the colors of white light equally. This means that the light scattered by the clouds is white in color. This scattered white light when enters the eyes of the observer, makes the cloud appear white in color. 

The red sunrise and sunset: 

The sun is near the horizon during sunrise and sunset, from where the sunrays need to travel a longer distance through the denser part of the atmosphere to reach the observer. When white light from the sun travels towards the observer on the Earth, most of the shorter wavelength colors get scattered away.  

However, only the red color is able to reach the eyes of the observer, as it scatters the least because of its larger wavelength. This is the reason why the sun appears to be red during sunrise and sunset. A small amount of other colors also reach the observer’s eyes, but they are negligible compared to the amount of red light. 

The white 12 noon: 

When the sun is overhead, it has to travel comparatively smaller distance. In this case, even though the smaller wavelength colors are scattered, they reach the observer’s eyes along with the colors of longer wavelengths, which are less scattered. Thus, the sun appears white during noon, as all the colors enter the observer’s eyes. 

Red danger signals: 

Danger signals are usually made in red color for the same reason. These danger signals will be visible to an observer even from a large distance as the red light scatters the least. It will be visible even in the presence of smoke, fog, or smog. 

Questions and answers: 

1. How will the sunrise and sunset look if there were no atmosphere? 

Answer: 

The sunrise and sunset, in both the cases the sun would look white in color as there is no atmosphere to scatter the light. 

2. What will be the color of the clouds if there were no atmosphere? 

Answer:  

The color of the clouds will remain the same i.e., white even if there is no atmosphere. This is because the clouds will still have dust and water particles which scatter all the colors of the light equally giving rise to a white color. 

Summary

• The sunlight is white in color. It is a combination of several colors, predominantly, violet, indigo, blue, green, yellow, orange and red.
• The Earth’s atmosphere is a heterogeneous mixture of minute particles including tiny water droplets, smoke, suspended dust particles, molecules of air etc., which scatter light .
• When sunlight enters the Earth’s atmosphere, it is first absorbed by the particles in the air and then re-radiated in several directions, spreading it all over.
• When sunlight falls on a water molecule or a dust particle, it scatters all the colors of light equally.
• When white light falls on an air molecule, it scatters the colors with smaller wavelengths such as blue color the most whereas it scatters the colors with larger wavelengths such as red color the least. This phenomenon of unequal scattering of various colors of light if called preferential scattering,
• The scattering of light leads to several other magnificent phenomena in nature as follows:

a. The blue colored sky

b. The red sunrise and sunset

c. The white clouds

d. The danger signals are made in red color