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Why is the colour of the clear sky blue?

Class 10 ScienceNCERT chapter: The Human Eye and the Colourful Worldइसे हिंदी में पढ़ें

Exam-ready answer

The clear sky appears blue because the molecules of air and other fine particles in the atmosphere are smaller than the wavelength of visible light. They scatter the shorter blue wavelengths much more strongly than the longer red wavelengths. This scattered blue light reaches our eyes from all parts of the sky, making it look blue.

Explanation

Sunlight is white light made of seven colours. When it enters the atmosphere, it strikes molecules of air and other fine particles. These particles are very small, smaller than the wavelength of visible light. Such small particles scatter light in a way that depends on wavelength: shorter wavelengths (blue) are scattered much more than longer wavelengths (red).

This type of scattering by very fine particles is called Rayleigh scattering. Because blue light has a shorter wavelength than red light, it is scattered about 5 to 6 times more strongly. The scattered blue light spreads in all directions and enters our eyes from every part of the sky. That is why the sky looks blue, not because the sky is actually coloured.

If the atmosphere had no particles, there would be no scattering and the sky would look dark. At sunrise and sunset, sunlight travels a longer path through the atmosphere. Most of the blue light is scattered away, and mainly red and orange light reaches our eyes. That is why the sky near the horizon looks red or orange at those times.

Points to remember

  1. Scattering of light: the spreading of light in different directions by particles present in its path.
  2. Air molecules and fine particles are smaller than the wavelength of visible light, so they scatter shorter wavelengths more.
  3. Blue light (shorter wavelength) is scattered much more than red light (longer wavelength) by air molecules.
  4. The scattered blue light reaches our eyes from all directions, so the clear sky appears blue.
  5. At sunrise and sunset, light travels a longer distance through the atmosphere, so mostly red/orange light reaches us and the sky looks reddish.

Exam tip

Write the answer in three steps: (1) air molecules are smaller than the wavelength of light, (2) blue light is scattered more than red, (3) scattered blue light reaches our eyes. Mention 'shorter wavelength' and 'scattering' clearly to score full marks.

Questions students also ask

Why does the sky look red at sunrise and sunset?
At sunrise and sunset, sunlight passes through a thicker layer of the atmosphere. Most of the blue light is scattered away, and mainly red and orange light reaches our eyes. So the sky near the Sun looks red or orange.
Why is the sky dark for astronauts in space?
In space there is no atmosphere, so there are no air molecules to scatter sunlight. Without scattering, no light reaches the astronaut's eyes from other directions, so the sky appears dark.
What is the difference between scattering and reflection of light?
Reflection is the bouncing back of light from a surface like a mirror. Scattering is the spreading of light in many directions by tiny particles in its path, such as air molecules. Scattering makes the sky blue.

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.