When we wake up in the morning, we see the birds are going for their food, farmers are going to the field, children are going to school, etc. Along with all these things we also see the beautiful sky appear in blue that was dark throughout the night. And also we see the Sun rising in the sky having the color red. And the same happens during sunset also. Have you ever thought about why the sky is blue and the sun is red during the sunrise and sunset? You can answer this by knowing the phenomena of scattering of light. Let's see what the scattering of light is !
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The scattering of light is one of the most important physical phenomena in our life. When a beam of light passes through a medium, a part of it appears in directions other than the incident direction. This phenomenon is called scattering of light. The particle of medium should be comparable in size to the wavelength of light. When light rays pass through the medium, they strike the particle, some of the rays get absorbed and some get scattered in all directions.
Scattering of light depends on the following factors:
The Scattering of light is affected by the size of the particle or molecule by which the light is scattering and the wavelength of the light wave which is to be scattered. The light wave which has less wavelength or more frequency will scatter more. For example blue light has a lesser wavelength so it scatters more. On the other hand if the light wave has more wavelength or lesser frequency it will scatter less. For example red light has more wavelength so it scatters less.
The relationship between scattering and wavelength is given by
Where is the wavelength of light
From the equation it can be concluded scattering of light is higher for the light having shorter wavelength.
There are following types of scattering of light:
The Earth’s atmosphere has a huge amount of dust particles and gas molecules. During daylight, the sun rays make near normal incidence on Earth’s surface and according to Rayleigh’s law of scattering, we know that shorter wavelengths scatter more than any other wavelength. Blue has very less wavelength and hence, it is scattered more. This is the reason the sky appears blue.
During sunrise or sunset, the sun stays near the horizon. Thus, sun rays have to travel a longer path in the atmosphere. During this path, most of the light which has a shorter wavelength gets scattered and doesn’t get a chance to reach us. But, the light which has a longer wavelength (red or orange part of visible spectrum) gets scattered less and hence, reaches us. This is the reason behind the appearance of the sun as red during sunrise and sunset.
Q. Explain why the danger signals are made red.
A. We know the visible light has seven colors. Out of which the red color has the highest wavelength. When the light passes through the atmosphere, it has the tendency to scatter from the molecule present in the medium. As the scattering is inversely proportional to the fourth power of wavelength, and as red color has a greater wavelength than the other color, so it will scatter less and can be seen from the maximum distance. That's why the signals are made red in color.
Q. Why does the sky appear dark to astronauts in space?
A. As we know there is no atmosphere in space. So there is no particle which can scatter the light. Hence no scattering takes place. As astronauts see up in the sky, they see the vacuum in space, no light enters in the astronaut's eye and hence the sky appears dark.
Q. How is the path of light entering from the small opening in the room visible?
A. When the sunlight enters the room from opening, it gets scattered through the air particles. Due to the scattering of light the dust particles suspended in the path shine and the path is visible.
Q. Explain the whitish color of clouds?
A. The cloud consists of water droplets, ice particles and dust particles etc. The size of their particle is greater than the wavelength of all the visible spectrum. So this can be explained by the mie scattering. Here all the wavelengths of incident light scatter almost equally. Therefore the light coming from the cloud consists of all the scattered wavelengths of light, which appears white in color.
Q. What is the elastic scattering of light?
A. When the energy of light remains conserved during scattering, it is called elastic scattering.
Q. What is inelastic scattering of light?
A. When the energy is not conserved during scattering, then the scattering is called inelastic scattering.
Q. What is the basic difference between rayleigh scattering and Mie scattering?
A. If the size of a particle is less than the wavelength of incident light, then it is called rayleigh scattering and if size of particle is greater than the incident wavelength, it is called mie scattering.
Q. What is the color of the sun at noon? And why?
A. It looks whitish in color because at noon the sun is overhead and light ray travels less through the atmosphere and less scattering of light takes place, so all the colors enter the observer’s eye.