Daylight in the upper atmosphere is dominated by ultraviolet scattering, yet the human sky glows blue. The explanation sits at the intersection of atmospheric physics, solar spectra, and biology rather than in a single optical trick.
Incoming sunlight covers a broad spectrum, but its energy is not evenly distributed. Rayleigh scattering in air scales with the inverse fourth power of wavelength, so shorter wavelengths scatter far more. That makes ultraviolet photons the most aggressively redirected by nitrogen and oxygen molecules. However, much of this ultraviolet radiation is absorbed by the ozone layer through strong electronic transitions before it can be redirected into the line of sight near the ground.
The photons that actually reach observers are therefore filtered twice: first by the solar spectrum, which already emits less power in the deepest ultraviolet, and then by atmospheric absorption. Within the remaining band, Rayleigh scattering still favors the shorter visible wavelengths, so blue light is scattered more than green or red. This scattered blue part of the spectrum becomes the dominant diffuse component of skylight that reaches the surface.
Human visual physiology completes the picture. Cone cells have a limited spectral sensitivity range and do not register most ultraviolet wavelengths at all, even when some stray ultraviolet photons penetrate the atmosphere. The spectral response curve of the eye peaks in the visible band, so the effective perceived radiance is skewed toward blue. In terms of sensory entropy, the eye compresses a complex spectrum into a simple color impression, and that compressed signal is what observers call a blue sky.
