Why does the sky look so blue after the rain?

This is because when sunlight passes through the atmosphere, it will be scattered by molecular particles and dust particles in the atmosphere. This kind of scattering obeys Rayleigh scattering law, that is, the intensity of scattered light is inversely proportional to the wavelength of light, and the smaller the size of scattered particles, the more obvious this relationship is. Therefore, when sunlight passes through the atmosphere, blue light with shorter wavelength is scattered more strongly than red light with longer wavelength, and the scattered light is blue. Whenever the rain clears up, the dust particles in the air will decrease. Atmospheric scattering is mainly molecular scattering caused by density fluctuation, and the scale of molecules is much smaller than that of dust. Rayleigh's inverse wavelength effect is more obvious, so the sky will be particularly blue after the rain.