A question commonly asked my many children and which has baffled many adults as well. We all have wondered while lying under the sky watching white clouds slowly drifting by that why the sky appears blue and many times came up with strange answers to the question like due to the reflection of the ocean, or because of the water in the sky. But in order to come up with a reasonable explanation, we first have to understand some concepts of light and sky.
Earth’s atmosphere is a thick layer of various gases protecting the earth from radiations from space and contains millions and millions of tiny solid, gas, water vapor and dust particles scattered throughout the atmosphere. The sky is the lowest level of atmosphere and closest to earth thus it contains much larger quantities of these particles. These dust particles intercept the light rays coming from space and react according to the frequency and wavelength of the light.
Light is an energy wave of vibrating electric and magnetic fields and depending upon the type of light we have a different variation in the wavelength of the light. Sun is the biggest source of light and the white light radiating from this sun is a collection of lights of various colors which we see as in a rainbow. Scientist call this band of rainbow colors VIBGYOR which constitutes violet (blue), indigo, green, and yellow, orange, red. each of these colors has a slightly different wavelength, with red light having the longest while blue light having a much shorter wavelength. So how exactly does this affect our sky and its color?
Scattering of Light
The gas molecules are much smaller in size and are comparable to the wavelength of the light and thus unlike water droplets which reflect the light, gas molecules scatter them. the molecules of mainly nitrogen and oxygen gases which are the main gages present in our atmosphere interfere with the incoming sunlight and the blue color light having the shortest wavelength get most affected by this interference. Because of this, it is scattered in all direction whereas other red, green light is simply passed through or are less scattered. This phenomenon was discovered and studied by an English physicist Lord John Rayleigh in the 1870’s. This process was popularly known as Rayleigh Scattering Process which explains why the sky looks blue which is due to the fact that when the light from the sun goes through air in atmosphere, almost none of the red, oranges or yellow portion of light are affected but the blues are absorbed and scattered by gas and dust particles.
Which leads us to wonder what happens at sunset. Well firstly during sunset we are looking toward the sun and not away, thus we are looking at the red light which gets passed straight through the atmosphere to our eyes. Secondly, the sun is lower in the sky at sunset and so the light is forced to travel through much more atmosphere as it does during the day thus it passes through far more water vapor and dust which completely absorb the short wavelength blue light and as such only red light is seen. So now you know the simple yet tricky answer to the question why the sky appears blue with a little bit more understanding of light and its scattering process.