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“Why is the Sky Blue?”

Simply Science 01

The blue colour of the sky is primarily caused by a phenomenon known as Rayleigh scattering.

Blue sky

Here's a simplified explanation of why the sky appears blue:

Sunlight (white light) is composed of a spectrum of colours. Each colour corresponds to a different wavelength, with blue light having shorter wavelengths compared to other colours like red or yellow.

Our Earth's atmosphere is primarily composed of nitrogen (~80%) and oxygen (~20%) molecules. These molecules are much smaller than the wavelengths of visible light. When sunlight enters the Earth's atmosphere, it encounters these tiny molecules. Due to their small size relative to the wavelength of blue light, the molecules interact with it more effectively. This interaction scatters (Rayleigh Scattering) the blue light in all directions.

Because blue light is scattered more than other colours, it appears to come from all parts of the sky. As a result, when we look up, we see a higher concentration of blue light coming towards our eyes. This abundance of scattered blue light gives the sky its characteristic blue colour.

Our eyes are also more sensitive to blue light than violet or ultraviolet light, which is also scattered but is not as easily visible to us. This further enhances our perception of the sky as blue.

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