Then tell me, why is the sky blue?

The blue sky we see is the result of selective scattering of visible light in incident solar radiation by air molecules and other particles. Scattering intensity is related to particle size. When the particle diameter is smaller than the wavelength of visible light, the scattering intensity is inversely proportional to the fourth power of the wavelength, and light with different wavelengths is scattered in different proportions, which is also called selective scattering. When visible light radiated by the sun enters the atmosphere, air molecules and particles (dust, water droplets, ice crystals, etc. ) will scatter sunlight around. Of the seven kinds of sunlight, red, orange, yellow, green, blue, indigo and purple, red light has the longest wavelength and violet light has the shortest wavelength. The red light with longer wavelength has the highest transmittance, and most of it can directly penetrate the particulate matter in the atmosphere and shoot to the ground. However, blue, indigo, purple and other short-wavelength colored light are easily scattered by particles in the atmosphere. Take blue light (wavelength 0.425μm) and red light (wavelength 0.650μm) in the incident sunlight as examples. When light passes through the atmosphere, the blue light scattered by air particles is about 5.5 times that of red light. So on a clear day, the sky is blue. However, when there is fog or thin clouds in the sky, because the diameter of water droplets is much larger than the wavelength of visible light, the effect of selective scattering no longer exists, and light with different wavelengths will be evenly scattered, so the sky appears white.

If light with short wavelength scatters more strongly, you must ask why the sky is not purple. One of the reasons is that when the visible light radiated by the sun passes through the atmosphere, air molecules have a strong absorption rate of violet light, so we observe less violet light in the visible light radiated by the sun, but it is not absolutely absent. We can easily observe the purple light in the rainbow after the rain. Another reason is related to our eyes themselves. In our eyes, there are three types of receptors, called red, green and blue cones, which are only sensitive to the corresponding colors. When they are stimulated by external light, the visual system will reconstruct the color of these lights according to the intensity of stimulation of different receptors, that is, the color of the objects we see. In fact, red cones and green cones also reflect blue and purple stimuli. The red cone and green cone are stimulated by sunlight at the same time. At this time, the blue cone is stimulated by blue light, and finally the result of their combination is blue, not purple.