Why is the sky blue?

Because many different molecules in the air scatter light. Due to the action of air molecules, violet light and blue light scatter more than red light. The probability of light being scattered by air molecules (the intensity of scattered light) is inversely proportional to the fourth power of light wavelength. Therefore, when the wavelength of light changes to the original 1/2, its probability of being scattered by air (the intensity of scattered light) becomes the fourth power of the original 2, that is, 16 times.

The wavelength of light will be shortened in the order of red, orange, yellow, green, blue and purple, so the probability of their scattering in the atmosphere will increase in this order. Because the wavelength of purple light is about11.8 of the wavelength of red light, the scattered purple light is more than the scattered red light, which is four times of1.8 and about 13 times of the scattered red light.

So the short-wavelength violet light and blue light scattered in the air are far more than the long-wavelength red light. In this way, when we look up at the sky, there will be more purple light and blue light than red light.

But the sky people see is not purple but blue, because the cone cells that perceive color are more sensitive to blue light than to purple light. This is why the sky looks blue.

Extended data

At sunrise, sunset, sunrise and sunset, the light from the sun or the moon must pass through a large amount of atmosphere; The closer they are to the horizon, the more atmospheric light they need to pass through. Blue light will scatter in all directions, while the scattering efficiency of red light is much lower.

This means that the sun or the moon itself will turn red, and the light near the sun and the moon, that is, the light that touches the atmosphere before reaching our eyes and then scatters, will turn red during that time.

During a total solar eclipse, the shadow of the moon prevents sunlight from directly reaching most of the atmosphere near you, and the horizon will turn red, but not elsewhere. Because the light outside the overall path will scatter in all directions, which is why most of the sky is still blue.

But near the horizon, light scattered in all directions is likely to be scattered again before reaching the eyes. Red light is the most likely light wave to pass through everything and eventually surpass blue light which is scattered more effectively.

Baidu encyclopedia-blue sky