Green Earth

Why Is The Sky Blue ?

Blue sky is always admired by people . In cloudless day time , we see beautiful blue colour of sky . But have you ever wondered about what makes the sky blue ?

To understand this . we need to consider  that sky is full of molecules such as oxygen molecules , nitrogen molecules  etc . We  know that sun light which appears white to human eyes contains seven colors of rainbow  .  .The white light from the sun is a mixture of all colours of the rainbow.  This was demonstrated by Isaac Newton, who used a prism to separate the different colours and so form a spectrum.   Among them , blue colour has shortest wavelength and red colour has longest wavelength . That means blue colour vibrates faster than red colour . So we get to know three things :-
  1. Our atmosphere contains smaller molecules .
  2. Sunlight is mixture of seven colours of rainbow .
  3. Blue component has shortest wavelength and red component has longest wavelength .

When sunlight enters into atmosphere , it collides and gets scattered by molecules in atmosphere . Blue part is scattered strongly than other component of sunlight . This is usually called Rayleigh scattering—after Lord Rayleigh . He studied deeply found that it is elastic scattering of light by particles that are very small in relation to the wavelength of the light ( about one tenth ) , and in which the intensity of the scattered light varies inversely with the fourth power of the wavelength. The light coming from the Sun and passing through the Earth’s atmosphere suffers from Rayleigh scattering. This physical phenomenon makes the blue part of the light coming from the Sun to be scattered and change its direction due to the interaction of light with the molecules of air. This scattering makes it look like every portion of the atmosphere is emitting blue light.

When we  look arbitrarily  at sky , we only see scattered blue colour ..and hence , we see our sky blue in colour .

But sky doesn’t look blue every time . When we look sky in evening or morning , it appears red . This is because when we view the setting or rising  sun on the horizon, the opposite occurs. We see only the light that has not been scattered into other directions. At sunset, light has to travel the longest through the atmosphere which increases the probability that it will find a particle or air droplets with diameter more than the wavelength of red light and thus red light gets scattered which makes the sky look red.


For more detail ,watch this video.


I am science enthusiast and I love to write blogs for students and youths who love science. I have deep interest in astronomy and cosmology. I believe that science is most effective tool to serve humankind. .