Why is it still bright at night sometimes?

Because of the scattering principle of the sun.

The first thing to realize is that the sun is an extremely strong light source-it is much brighter than the moon. Secondly, nitrogen atoms and oxygen atoms in the atmosphere will have a certain influence on the sunlight passing through them.

There is a physical phenomenon called Rayleigh scattering, which makes light scatter when it passes through particles with a diameter equivalent to one tenth of the wavelength (color) of light. Sunlight is composed of different colors of light, but due to the existence of some elements in the atmosphere, the scattering degree of blue is much higher than other colors.

Therefore, when you look at the sky on a clear day, you will see the sun like a bright disk. The blue of the whole sky is the color of all the atoms in the atmosphere that scatter blue light (because the scattering degree of red light, yellow light, green light and other colors is very low, the sky you see is blue).

Extended data:

Because the intensity of Rayleigh scattering is inversely proportional to the fourth power of wavelength, in the solar spectrum, blue-violet light with shorter wavelength is more obvious than red light with longer wavelength, and blue light energy is the largest in short wave. Therefore, when the rain clears or the air is crisp in autumn (there are few coarse particles in the air, mainly molecular scattering), under the strong scattering of atmospheric molecules, blue light scatters to the sky, and the sky is blue.

In addition, due to the sharp drop of atmospheric density with height, the scattering effect of atmospheric molecules is correspondingly weakened, and the color of the sky changes from blue to cyan (about 8 km) and dark cyan (about 1 1 km) with height.

Deep purple (about 13 km), deep purple (about 2 1 km), further up, the air is very thin, the scattering effect of atmospheric molecules is extremely weak, and the sky is annihilated by darkness. It can be said that due to Rayleigh scattering, the energy projected by the sun to the surface is weakened.

Baidu encyclopedia-Rayleigh scattering