Is the sky blue because of the sunny weather?

In a homogeneous medium, light travels in a straight line. When light propagates in the air, it will inevitably encounter gas molecules and other particles in the air. These particles have physical effects on light absorption, reflection and scattering, and it is these physical effects that make the clear sky appear blue.

The correct explanation of why the sky is blue begins with 1859. Scientist Tedor first discovered that blue light is much stronger than red light, which is the "Tedor effect". A few years later, scientist Rayleigh studied this phenomenon in more detail. He found that the scattering intensity is inversely proportional to the fourth power of wavelength. Later, more scientists called this phenomenon "Rayleigh scattering". Rayleigh scattering can be easily verified by the following small experiment: use a water cup filled with water, then drop a few drops of milk in the water cup, and use a flashlight as a light source to illuminate from one side of the water cup. From the other side of the glass, you can see red light, but from the direction perpendicular to the light, you can see blue (the effect is more obvious in the dark).

At that time, both Tedor and Rayleigh believed that the blue sky was caused by small dust particles and water droplets in the air, similar to the suspended particles of milk in water. Even today, many people still think so. Actually, it's not. If the sky is made entirely of small dust particles and water droplets, the color of the sky will change with humidity. In fact, the color of the sky will not change with humidity unless it rains or clouds are gathering. Later, scientists speculated that nitrogen and oxygen molecules in the air were enough to explain the "Tedor effect" in the sky. This guess was finally confirmed by Einstein, who made a detailed calculation of this scattering effect, and the calculation results were consistent with the experiments.

The blue sky we see is the result of selective scattering of incident sunlight 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 sunlight 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 sunlight passes through the atmosphere, air molecules have a strong absorption rate of purple light, so we observe less purple light in sunlight, but it is not absolute. 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.