Rayleigh Scattering: Why the Sky Is Blue and Sunsets Are Red

Rayleigh scattering describes how light interacts with particles much smaller than its wavelength — primarily gas molecules in the atmosphere. Scattering intensity scales with the inverse fourth power of wavelength (λ⁻⁴), meaning shorter (blue) wavelengths scatter roughly 5–10× more than longer (red) ones. ## Why the Sky Is Blue Sunlight enters the atmosphere and blue wavelengths scatter in all directions toward the observer, making the sky appear blue regardless of viewing direction. The sun itself appears slightly yellowed because some blue light has been removed from the direct beam. ## Why Sunsets Are Red At sunrise and sunset, light travels a much longer atmospheric path. Blue light scatters away entirely over that distance, and longer red and orange wavelengths dominate what reaches the observer. ## Beyond the Sky The same λ⁻⁴ dependence explains why The "Blue Blood" Myth: Why Veins Look Blue but Blood Never Is: tissue preferentially scatters shorter wavelengths back to the observer, while longer red wavelengths penetrate deeper and are absorbed by hemoglobin. Rayleigh scattering also sets a fundamental lower bound on signal attenuation in glass optical fibers, limiting long-distance telecommunications performance even in perfectly manufactured fiber.