The white light from the Sun is made up of a spectrum of colors. The
longest wavelengths of light are on the red end of the spectrum and
the shortest wavelengths are on the blue/violet end of the spectrum.
When light pass through a non-uniform medium it is forced to deviate
from a straight trajectory, this phenomena is called scattering.
Selective scattering (or Rayleigh scattering) occurs when certain
particles of the medium the light is passing through are more
effective at scattering a particular wavelength of light. In
particular when the Sun's light passes through the Earth's atmosphere
it is scattered mostly by the Oxygen and Nitrogen molecules, these
molecules are small in size and thus more effective at scattering
shorter wavelengths of light (blue and violet). Small amount of the
red, orange and yellow light is affected by the air (longer
The scattered blue light is radiated in different directions. It gets
scattered all around the sky. Whichever direction you look, some of
this scattered blue light reaches you. Since you see the blue light
from everywhere overhead, the sky looks blue.
Actually, according to Rayleigh's Law about scattering the sky would
appear more violet than blue, because the former have a shorter
wavelength than the latter. The explanation to this lies in the way
our vision works:
"We have three types of colour receptors, or cones, in our retina.
They are called red, blue and green because they respond most strongly
to light at those wavelengths. As they are stimulated in different
proportions, our visual system constructs the colours we see.
Response curves for the three types of cone in the human eye
When we look up at the sky, the red cones respond to the small amount
of scattered red light, but also less strongly to orange and yellow
wavelengths. The green cones respond to yellow and the more
strongly-scattered green and green-blue wavelengths. The blue cones
are stimulated by colours near blue wavelengths which are very
strongly scattered. If there were no indigo and violet in the
spectrum, the sky would appear blue with a slight green tinge.
However, the most strongly scattered indigo and violet wavelengths
stimulate the red cones slightly as well as the blue, which is why
these colours appear blue with an added red tinge. The net effect is
that the red and green cones are stimulated about equally by the light
from the sky, while the blue is stimulated more strongly. This
combination accounts for the pale sky blue colour. It may not be a
coincidence that our vision is adjusted to see the sky as a pure hue.
We have evolved to fit in with our environment; and the ability to
separate natural colours most clearly is probably a survival
From "Why is the sky Blue?" by Philip Gibbs:
In the outer space, there is no atmosphere to scatter the sun's light.
Therefore no scattered light reach your eyes; the stars are seen as
dots of light because its light is not deviated from its straight
trajectory, and the sun appears as a white disc of light.
For further reading and explanations see:
"Science Question of the Week - Why is Space black? - March 28, 2003":
"Olbers' paradox - Wikipedia, the free encyclopedia":
"Blue Sky and Rayleigh Scattering":
"Diffuse sky radiation - Wikipedia, the free encyclopedia":
"sky blue" why
"sky blue" why black night
scattering "sky blue"
I hope this helps you. Feel free to request for a clarification if you need it.