Have you ever thought about why the sky looks blue? Why the sun appears red at the time of the sunrise and sunset? Why clouds appear white? This is because of the phenomenon of the scattering of light.

 

Scattering of light

 

Scattering of light is the phenomenon of the change in the direction of light on striking with an obstacle like a dust particle, water droplet, etc. (or any colloidal particle). Now in order to understand, we can observe,  that the path of a beam of light passing through a true solution is not visible but its path becomes visible through a colloidal solution, where the size of particles is relatively large.

 

The earth�s atmosphere is a heterogeneous mixture of minute particles including smoke, tiny water droplets, suspended dust particles. When a beam of light strikes such fine particles, the path of the beam becomes visible. This effect of light is known as the Tyndall effect.

 

The color of the scattered light depends on the size of the scattering particles. Very fine particles scatter the shorter wavelength like blue light. While the particles of larger size scatter light of longer wavelength like a red light.

 

We can understand this better with the help of an activity

 

1. Take a strong source of while light, two convex lenses a glass tank, water, cardboard with a hole in the center, a screen, solid sodium thiosulphate (hypo), concentrated sulphuric acid.
2. And arrange the setup as shown here.
3. First, take water in the tank.
4. When light is made to fall at the focus of the convex lens L1, it provides a parallel beam of light. This beam passes through the transparent clear water in the beaker.
5. A sharp image of this hole is obtained on the screen, through convex lens L2.
6. Now dissolve 200 g of sodium thiosulphate (hypo) in about 2 L of clean water taken in the tank. Add about 1 to 2 mL of concentrated sulphuric acid to the water.  

 

We observe that:

 

1. Fine microscope sulphur precipitates in water in a couple of minutes.
2. Light gets scattered from the minute colloidal sulphur particles and blue color will be observed from the three sides of the tank.
3. From the fourth side of the tank facing the circular hole, we find first the orange red color and then bright crimson red color on the screen Y.

 

It shows that very fine particles scatter mainly blue light of smaller wave length and the transmitted light contains longer wave lengths. Now from the above experiment, we can easily make out why the color of the sky is blue. As the molecules of air and other fine particles in the atmosphere have size smaller than the wave length of visible light.

 

When sunlight passes through the atmosphere, the fine particles in the air scatter the blue color more strongly than red. The scattered blue light enters our eyes that is why a clear sky looks blue.

 

If the earth had no atmosphere, no color of sunlight would be scattered and the sky would have looked dark during day time as it does at night and very interesting to note that the sky appears dark to passengers flying at very high altitudes, as at such heights the scattering is not prominent.

 

Why danger signals are red?

 

We know light, red has the largest wavelength. When sunlight passes through the earth�s atmosphere, the size of the particles responsible for scattering is much less than the wavelength of the light.  Hence, the red color is scattered the least and can be seen from the distance. That is why danger signals are red.

 

Why the clouds look white in color?

 

The clouds are at a much lower height. They are seen due to the scattering of light from lower parts of the earth�s atmosphere which contains large dust particles, water droplets, ice particles, etc.

 

As the size of particles responsible for scattering is much bigger than the wavelength of the light so all wavelengths are scattered equally and all equally scattered colors merge together to give us the sensation of white. Hence clouds generally appear white.

 

Color of the sun at sunrise & sunset

 

At the time of sunrise and sunset, the Sun is closer to the horizon. The sunlight near the horizon passes through denser layers of the air and covers a larger distance before reaching our eyes. Therefore, most of the blue light gets scattered away. The light that reaches our eyes is of longer wavelengths, mainly orange and red.

 

Out of all the colors of sunlight, the color scattered least, i.e., red color reaches our eyes. Hence the sun appears red, both at the time of sunrise and sunset. At noontime when the sun is nearly overhead, the sunlight passes through the rarer layers of the air and covers a shorter distance before reaching our eyes.

 

Only a small amount of blue light gets scattered and most of the light of other colors reaches us.  As a result, the sun appears white. So now you must have understood why the clear sky appears blue, why the color of the sun at sunrise and sunset, why the cloud appears white, why the danger signals are red.

 

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