The common answer to why the sky is blue—Rayleigh scattering—is technically correct but lacks depth. Understanding requires more than just knowing the term; it demands a predictive model. By exploring how light interacts with Earth's atmosphere, we can explain not just why the sky appears blue, but also predict colors on other planets.

When sunlight enters Earth's atmosphere, most colors pass through unimpeded, but blue photons scatter more due to their higher frequency. This scattering happens because oxygen and nitrogen molecules have electron clouds with resonant frequencies in the ultraviolet range. As visible light approaches these frequencies, scattering increases dramatically—proportional to the fourth power of frequency. This means violet light scatters even more than blue, yet our eyes are less sensitive to violet, making the sky appear predominantly blue.

The phenomenon extends beyond Earth. Any small gaseous molecule would preferentially scatter blue light, creating blue skies on other worlds. The same principles explain why sunsets appear red—when sunlight travels through more atmosphere at low angles, blue photons scatter away completely, leaving only the longer red wavelengths. Understanding these mechanisms transforms a simple observation into a powerful predictive model for atmospheric optics across the universe.