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Blue Mountains

Some of the sunlight that reaches our eye is scattered light, that in contrast to direct sunlight has been scattered by air molecules. The scattering of sunlight depends on its wavelength. For very small particles the so-called Rayleigh-scattering applies. Blue light is scattered much more strongly than light from longer wavelengths. This is why the clear sky appears blue as long as the sun is high in the sky, blue light reaches the observer from everywhere. Whithout the scattering of the sunlight, shadows would appear completely dark.
Scattered light affects the appearance of distant objects. Distant mountains often appear bluish in contrast to objects in the surroundings of the observer because the ligh has to travel over a longer distance and more scattering takes place. Further away the blue gets whitish because multiple scattering takes place and not enough scattered light reaches the eye. Apart from this, larger particles block some of the light, Mie-scattering takes place that does not depend as much on wavelength.

The image below, taken on the Isle of Skye in August 2008, nicely shows the blue colour of a distant mountain. Nikon D80, unprocessed compressed image.


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