Grades 3–5 · Earth & Space
Why is the sky blue but a sunset red?
Sunlight looks nearly white because it contains a mixture of colors. These colors have different wavelengths, or distances between wave crests. As sunlight passes through air, tiny gas molecules redirect some of it in different directions. This redirection is called scattering.
In the diagramAir redirects blue light more strongly than redTwo light paths · molecules and paths are enlargedHigh Sun: short pathLow Sun: long pathBlue reaches eyesfrom across the skyMore blue leavesthe direct light pathLonger air paths remove more blue from the direct beam.Scattering redirects light; it does not turn blue into red.
Air molecules scatter short wavelengths, including blue, much more strongly than long red wavelengths. When we look away from the Sun into a clear daytime sky, scattered light reaches our eyes from many directions. That light contains a strong share of blue. Violet is scattered strongly too, but the Sun’s light and our eyes’ different sensitivity to colors help make the sky look blue.
When the Sun is low, its light takes a longer route through the atmosphere before reaching us. Along this route, much of the blue is scattered out of the direct beam. The remaining sunlight has a larger share of red and orange. Air has redirected different colors by different amounts; it has not changed blue light into red light.
About this explanation
Starting knowledge: Recognize sunlight, air, the horizon and the colors of visible light.
Sources
- NOAA / National Weather Service — Why Is the Sky Blue?
- NASA Goddard Space Flight Center — Rayleigh Scattering