The striking blue color of the sky and the exotic reddish orange color of a sunset has always made us wonder at the beauty of nature. It serves as inspiration for all those who appreciate this beauty.
This colorful display might also make you wonder, what process is responsible for this exotic display of colors? When we observe the earth from space, we see it covered with a blue layer of atmosphere, which actually is its lower atmosphere. This blue can be seen fading into blackness of space.
So why is our sky blue? The colors of sky we see at different times of the day are due to the scattering of light waves by the particles present in our atmosphere. You might assume, that you hardly see anything in a clear sky that could interact with the light waves, but our atmosphere is a gaseous sea consisting of many particles, mainly Oxygen and Nitrogen in a gaseous form. The sunlight bounces off these air molecules, and gives our sky its blue color.
The visible light emitted by the sun consists of violet, blue, green, yellow, orange and red. This light takes 8 minutes to reach the earth surface; some of it is reflected back into space by air molecules while some of it reaches the earth surface after interacting with the air molecules, and occasionally some is reflected back by clouds.
Then among all these colors why is our atmosphere blue? The scattering of light waves depends upon the wavelengths of colors that make up the light. Violet and blue colors have the shortest wave lengths, while red and orange have the longest. Air molecules can scatter only the waves which have a wavelength nearly identical to the size of these molecules. Thus blue and violet are easily scattered by air molecules as they have shorter wave lengths; however, orange and red are scattered much less or passed without any interaction, due to their longer wavelengths. So the scattering of blue light by air molecules is what gives our sky the blue color.
When we look at the blue sky, we are actually looking at the scattering of short wave lengths by the air molecules.
Then why is the sunset red? As suggested earlier, the orange and red colors hardly get scattered by the air molecules, so the red and orange color we see is what is left after the blue is scattered by the air molecules. At sunset when the sun reaches the horizon line, the path of light reaching us is elongated, and light now has to move through a much thicker layer of the atmosphere. You can see it in the diagram below.
The distance covered through the atmosphere by the sunrays from the sunset, is much greater than that of sunrays from the midday.
Due to the elongated path more of the blue, yellow and violet light is scattered due to much more interaction with the air molecules, and what is left of scattered light is the exotic orange and blue. This is how the sunset gets its orange and red color.