Why is the sky yellow and red at dawn or dusk?

This is because of "Rayleigh scattering". At dawn or dusk, the sun is almost in front of us. At this time, sunlight has to travel a relatively long distance in the atmosphere to enter our eyes, so only red and orange light with longer wavelength can still enter our eyes.

Rayleigh scattering law was discovered by British physicist Lord Rayleigh in 1900, hence the name.

In order to meet the requirements of Rayleigh scattering, the diameter of particles must be much smaller than the wavelength of incident waves, and its upper limit is usually about110 (1-300 nm) of the wavelength. At this time, the intensity of scattered light is inversely proportional to the fourth power of incident light wavelength, that is, the shorter the wavelength, the stronger the scattering.

At sunset or sunrise, the sun is almost in front of us. At this time, the propagation distance of sunlight in the atmosphere is relatively long. A lot of blue light with shorter wavelength that we see in direct light is scattered, leaving only red-orange light, which is why the sun is red near sunset, and the clouds are red because of reflecting sunlight, but the sky is still blue.

Extended data:

Influence of Rayleigh scattering on sky color;

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 also changes from blue to cyan (about 8 km), dark cyan (about 1 1 km), deep purple (about 13 km) and dark purple (about 2 1 km)

It can be said that due to Rayleigh scattering, the energy projected by the sun to the surface is weakened.

Baidu encyclopedia-Rayleigh scattering