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Why is the Horizon White?

Not a cloud in the sky, and yet the horizon is still white.

Have you noticed how the horizon is white, even on a day when the sky is a perfect blue and there isn’t a cloud in it? If you have, perhaps you have also wondered why?

It is because of an optical phenomenon called ‘scattering‘.

White light is made up of lots of different colours. These are the colours we see in a rainbow and when white light passes through a prism.

When light passes through air, the different colours in the white light behave differently, according to their wavelength. The blue/violet end of the spectrum refracts, ie. bends, the most and the red end refracts the least.

This is why we see reds and oranges in sunsets, the other colours are bent too much on that very long journey through the atmosphere to reach our eyes.

The way light scatters also explains why the sky is blue. The blue wavelengths bounce around the atmosphere and back to our eyes. The reds don’t bounce back and keep on going into space.

Scattering means that the further you look through air, the more colours are mixed in by scattering and the closer to white objects appear.

The most distant hill looks paler/whiter than the closer one. And the most distant thing we can see is the horizon, which always appears white. Or dirty white if there is pollution.

From deep blue to pale to near white

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