The Earth is covered by an atmosphere above it. The atmosphere is composed of various types of gases. The main gases present in the atmosphere are Nitrogen, Oxygen, Carbon Dioxide, Argon, Water Vapour, etc. Since all these atoms are present in the atmosphere, the sunlight coming from Sun will cause an effect on these molecules.
As we know in Physics, light can act as both particle and a wave. The light photon, when it comes near an electron of the molecule, causes an effect on the molecule known as scattering. This particular scattering that gives color to the sky is known as Rayleigh Scattering.
Rayleigh Scattering was discovered by the British physicist Lord Rayleigh in the late 1800s. The condition for this scattering to occur is that the size of the particle must be comparable to the wavelength of the light. The effect also suggests that the scattering effect is inversely proportional to the fourth power of wavelength.
Since the sunlight is composed of all wavelengths in the visible range, the color that gets the most scattered will be the one with the least wavelength. In the visible range, the color red has the highest wavelength whereas blue has the lowest. Thus, the blue light gets scattered more.
So then why the sky appears red at sunrise and sunset? At sunrise and sunset, the Sun is on the horizon. Hence, the sunlight must travel through a much larger distance through the atmosphere to reach the observer. During this travel, most of the light of shorter wavelengths is scattered more, hence only the longer wavelengths are seen by the observer. Hence the red color of the sky at sunrise and sunset.
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