Question

Why does the sky appear blue during the day?

Hint:

Shorter wavelengths of light are scattered more strongly by air molecules.

Answer 1

Step 1. Sunlight reaching the atmosphere is a mixture of all visible colours, from red to violet, along with invisible radiation.
Step 2. When this light strikes the tiny gas molecules of the atmosphere, such as nitrogen and oxygen, it gets scattered in many directions. This process is called Rayleigh scattering, and it applies when the scattering particles are much smaller than the wavelength of light.
Step 3. Rayleigh scattering is much stronger for shorter wavelengths than for longer ones, since the amount of scattering is proportional to \(1/\lambda^4\), where \(\lambda\) is the wavelength of light.
Step 4. Blue and violet light have shorter wavelengths than red or yellow light, so they are scattered far more strongly across the sky. Our eyes are more sensitive to blue than to violet, and some violet is absorbed higher in the atmosphere, so the overall effect we perceive is a blue sky.
Answer. The sky looks blue because of the scattering of sunlight by air molecules, an effect known as Rayleigh scattering, which favours shorter wavelengths like blue light.