Weekly Photo Challenge: Purple

Croatia Sunset over Split

If you have ever wondered why the sky is blue this is the reason:

Light travels through space in a straight line for as long as nothing disturbs it and as it moves through the atmosphere it continues on its journey until it collides with a bit of dust or a gas molecule.  Dust particles and water droplets are much larger than the wavelength of visible light and when light hits these large particles, it gets reflected in different directions. Gas molecules however are smaller than the wavelength of visible light and when light hits them, some of it gets absorbed and then the molecule radiates the light in a different direction.  The colour that is radiated is the same colour that was absorbed but the different colours are affected differently because blues are absorbed more easily than reds.   It’s as simple as that!

This process is called Rayleigh scattering and is named after Lord John Rayleigh, an English physicist, who first explained it a hundred and thirty years ago.

So what about sunsets?  Well, as the sun begins to set, the light must travel farther through the atmosphere before it gets to us and more of the light is reflected and scattered.  As less reaches us directly, the sun appears less bright and the colour of the sun appears to change, first to orange and then to red and this is because even more of the short wavelength blues and greens are now scattered and only the longer wavelengths are left in the direct beam that we can see.

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