REFLECTION AND DIFFUSION OF LIGHT
 
Reflection of light
Light travels in a straight line unless it encounters an obstacle.  When light hits an object, it is partially reflected by the surface of the object.
Reflection of light
 
 
Diffusion of light
Light travelling in the atmosphere is diffused by dust particles and gaseous molecules in the air (oxygen, nitrogen, carbon dioxide, water vapour, etc.).  Diffusion increases as the wavelength of light decreases.
Diffusion of light
 
Thus, blue light gets more diffused than yellow light, which gets more diffused than red light.  This phenomenon repeats itself as light travels over long distances in the atmosphere.  The result is that light progressively loses its blue component and retains a greater proportion of red.  This explains why the sky appears blue during the day and red at sunset.  This also explains why blue light is more disruptive for star gazing.

At night, yellow light is much better perceived by the human eye than blue light, and so a weak yellow light can assure good nocturnal vision.  Due to the moderate diffusion of yellow light, weak bulbs of this colour represent the ideal compromise between being able to see and performing astronomical observations.
 
Observatories are illuminated with red light.  Since the eye is not very sensitive to this wavelength, it is unlikely to produce glare.  In addition, since red light is not strongly diffused by the atmosphere, it does not blur the telescopic images.
Source : Guillaume Poulin
 
 
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