As we all know that light travels in a straight line for example the sunlight coming from the sun to the earth is in the form of straight line and not in the zigzag way. And sunlight is nothing but the electromagnetic waves which has visible spectrum also. The visible spectrum involves seven different colours and these different colours of light are due to their different wavelengths. When light is incident on some transparent media it gets reflected sometimes, sometimes it get refracted and sometimes get transmitted also. Also, when white light is passed through the prism it gets splitter into its seven constituent colours which is called as dispersion of light. The best example of reflection, refraction and dispersion of light is we see rainbow after raining in rainy season.
But we also observed that our sky is always blue in colour and not in red colour. The blue colour of the sky is due to the scattering of light. When the light rays get deflected from the small tiny objects, dust particles from the air then light will be totally spread in different directions on the basis of its wavelength and we see that colour which spread more.
To explain the scattering of light in detail with real example means blue colour of sky the scientist Rayleigh gives the law called as Rayleigh’s law of Scattering and which is stated as follows.
Statement:
- According to the Rayleigh’s law of Scattering, the intensity of the light which is scattered is inversely proportional to the fourth power of the wavelength of that light.
- That means more is the shorter wavelength of the light then there will be more intense scattering of light is possible. And longer is the wavelength of the light then lesser will be the scattering of the light.
- If I is the intensity of light scattered and λ be the wavelength of light then according to Rayleigh’s law of Scattering we write,
I α 1/λ4
Explanation:
- We all know that, the wavelength for red, blue and violet light is in the order of λr>λb>λv
- Here, the wavelength for red light is more and hence there will be less scattering will be possible for red light than blue and violet light.
- Let Iband Iv are the intensities of light scattered for blue and violet light. Then according to Rayleigh’s law of Scattering we write as
- Ibα 1/λb4 and Ivα 1/λv4
But λb>λv
- So violet and blue light scattered is more. But our eyes are more sensitive to blue colour than the violet light and hence we see the sky is blue in colour and not in violet colour.
- This is the main reason behind the blue colour of the sky. Beyond the atmosphere there are no particles, ash, dust and hence there will be no scattering is possible. And hence their sky will appear dark in colour only.
Example:
- The blue colour of the sky is only due to the scattering of light which we have seen earlier on the basis of Rayleigh’s law of Scattering.
- Also,we see that, the sun appears red at the time of sunset and sunrise also. As we know that the sun is at horizon or closer to horizon beforesunset and after the sunrise, so at that time sunlight travels the longer distance and then it will reach the observer. And according to Rayleigh’s law of Scattering we know that the light having less wavelength scatters more. So blue and violet light get scattered more than the red colour light. Although, the blue and violet colour is scattered more they will spread away or scattered away from the path and hence not reach to the observer. Only the light reaching to the observer is red light and hence sun appears reddish at the time of sunset or sunrise.
- In this way, Rayleigh’s law of Scattering explains the blue colour of the sky, red colour of sun at sunset and sunrise more effectively.
