When you turn off the lights of your room everything becomes dark. Now if there is a firefly in the room it looks bright but everything around is still dark and looks black. As outer space is mostly empty and dark, the sky should look black just like it does during the night. So why is the sky blue in color during the day?

You need to know a few things to understand why the sky appears blue on earth. So let’s take it one step at a time.

Light wavelength
Diagram explaining wavelength

 

Visible Light

Radiation is energy that can travel through vacuum and doesn’t need a medium. Visible light is a radiation which is visible to the human eye and is responsible for the sense of sight. It has wavelength in the range of about 380nm to about 740nm. Wavelength is equal to the distance over which the wave’s shape repeats itself.

Visible light region on the electromagnetic spectrum
Visible light region on the electromagnetic spectrum

 

The Colors In Visible Light

Sun’s light is made up of the colors of the rainbow. If you place a prism in the path of a sunbeam you will be able to see a multi-coloured band of light just like in the rainbow. All these colors combine to give you white light. These colors all have different wavelengths and energies; the more the wavelength, the lesser the amount of energy. The order from maximum wavelength to minimum wavelength is – red, orange, yellow, green, blue, indigo and violet.

Dispersiom of Light by Prism
A representation of light being dispersed by a prism

 

Rayleigh Scattering

Rayleigh scattering is the scattering of light by particles much smaller than the wavelength of light. As the earth’s atmosphere contains gases whose molecules are much smaller than the wavelength of light, Rayleigh scattering occurs. The scattered sunlight spreads in all direction due to which the sky looks saturated in color. Also all the component colors of visible light don’t scatter equally. Shorter more energetic wavelengths towards the violet end scatter more than longer wavelengths towards the red end. So violet, indigo and blue are scattered more.

Rayleigh Scattering Diagram
A diagram explaining Rayleigh scattering

 

So why is the sky blue and not violet?

The answer lies in our eyes. Our eyes are more sensitive to blue color as compared to violet. It perceives the combination of more scattered colors(violet, indigo and blue) as a combination of blue and white light and hence the sky appears light blue.

Human Eye Response to Color
Response curves for the three types of cone in the human eye

I hope the question is answered. If you still have any doubts please feel free to ask them in the comments section.

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