Why are clouds white?
The f^4 law applies to the scattering of light by objects much smaller that the wavelength of the light, such as air moleculars. Dust particles and water droplets that make up a cloud are comparable in size to, or larger than, the wavelength of light. For surfaces larger than the wavelength, reflection rather than scattering occurs. But the droplets in a cloud form an irregular surface, we do not get images of the kind we are used to seeing in mirrors. Instead we get diffuse reflection and the clouds look white or gray.
为什么会有两层呢？上面像白色的细沙；下面像一朵朵厚厚的棉花