Why is the colour of clear sky blue ?
Why is the colour of clear sky blue ?
The clear sky appears blue because the molecules of air and other tiny particles in the atmosphere have size smaller than the wavelength of visible light. These particles scatter the light of shorter wavelengths (such as blue) much more strongly than the light of longer wavelengths (such as red). Since blue light is scattered the most in all directions by the atmosphere, the clear sky appears blue to an observer on the Earth.
Marking Scheme
- 11 mark: Stating that the molecules of the atmosphere scatter the light of shorter wavelengths (blue) more than the light of longer wavelengths (red), making the sky appear blue.
Hint
Recall that white light consists of seven colours. Among them, which colour has the shortest wavelength? How will the size of atmospheric particles affect light of different wavelengths?
Quick Oral Answer
Q: Why does the sun appear red during sunrise and sunset?\nA: During sunrise and sunset, the sun is near the horizon, so sunlight travels a longer distance through the atmosphere. The shorter wavelengths (blue and violet) are scattered away by the atmospheric particles, and only the longer wavelengths (red and orange) reach our eyes directly. Hence, the sun appears red.
Analysis & Explanation
The phenomenon responsible is known as Rayleigh scattering. The intensity of scattered light is inversely proportional to the fourth power of the wavelength (). Thus, blue light is scattered roughly times more than red light. Although violet light has a slightly shorter wavelength and is scattered even more, our eyes are comparatively less sensitive to violet, and some violet light is absorbed by the upper atmosphere, which is why the sky appears blue rather than violet.
Common Mistakes
- 1Saying the sky is blue because it reflects the colour of the ocean/sea.
- 2Saying only blue light is present in sunlight, instead of explaining wavelength-dependent scattering.
- 3Confusing this with the red colour of the sun at sunrise/sunset.
- 4Mentioning Tyndall effect without specifying that the particle size is smaller than the wavelength of light.
Interesting Facts
The sky on the Moon appears black because the Moon has no atmosphere to scatter sunlight.
At noon, the sun appears white because light travels the shortest path through the atmosphere and very little scattering takes place.
Aerosols and dust particles in the atmosphere sometimes make the sky appear whitish or hazy, as they scatter all wavelengths almost equally.
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Frequently Asked Questions
How many marks does this question carry in CBSE Class 10 Science 2011?
This question carries 1 mark in the CBSE Class 10 Science 2011 examination.
Which chapter does this question come from in Science?
This question is from the chapter "The Human Eye and the Colourful World" in the CBSE Class 10 Science syllabus.
What topic does this question cover in Science?
This question covers the topic "Scattering of Light" from CBSE Class 10 Science.
What type of question is this in the CBSE Class 10 Science 2011 paper?
This is a Very Short Answer question from Section A in the CBSE Class 10 Science 2011 paper.