Definition
The scattering of light in all directions when sunlight passes through Earth's atmosphere and collides with air molecules much smaller than its wavelengths. Because shorter blue wavelengths scatter far more intensely than longer red wavelengths, it paints the daytime sky blue and turns sunsets fiery red.
Why the Daytime Sky Is a Blue Canvas
Sunlight looks white, but it is actually a blend of all the colors of the rainbow, from red to violet. Each color travels as a wave with its own unique wavelength. Red light ripples in long, gentle waves, while blue light ripples in short, tight waves.
When sunlight enters Earth's atmosphere, it collides with tiny gas molecules like nitrogen and oxygen. Long red waves glide straight past these tiny particles with barely any disturbance. Short blue waves, however, crash into them and scatter intensely. The incoming light sets electrons in the molecules vibrating, which then reradiate that light in every possible direction.
This scattered blue light floods the entire sky and reaches our eyes from every angle. When you look up on a clear day, you are actually seeing blue light scattered across the entire sky.
Why Are Sunsets Deep Red?
As evening approaches, the Sun sinks low toward the horizon. At midday, sunlight strikes straight down through a thin slice of atmosphere, but at dusk, it cuts through at a sharp, slanted angle. Because of this, sunlight must travel through a much longer path of air before reaching your eyes.
During this marathon journey through thick atmosphere, the easily scattered blue light scatters away long before it gets to you. Very little blue light survives the trek.
By contrast, longer red and orange wavelengths scatter far less, easily surviving the long journey directly to your eyes. That is why the once-blue sky turns into a breathtaking sunset of vivid reds and golds.
A Closer Look at the Science
British physicist Lord Rayleigh discovered that the intensity of scattered light is inversely proportional to the fourth power of its wavelength. If a wavelength is cut in half, its scattering power surges by sixteen times. For example, blue light (around 450 nm) scatters five to six times more intensely than red light (around 700 nm).
Why doesn't the sky look violet, since violet has an even shorter wavelength than blue? Sunlight naturally contains less violet light to begin with, and human eyes are far more sensitive to blue than violet. Our brains interpret the mixed scattered light as sky blue.
Rayleigh scattering happens only when particles are much smaller than the wavelength of light. When particles are close to or larger than the wavelength, Mie scattering takes over, which scatters all colors almost equally. That is why clouds, made of large water droplets, appear brilliantly white instead of blue.
Common misconceptions
The sky looks blue in outer space just like it does during the day.
Without air molecules in space to scatter sunlight, the sky appears pitch black even when the Sun is blazing brightly.
Where you meet it
The daytime sky glowing vibrant blue on a clear, sunny afternoon.
The horizon bursting into brilliant reds and oranges during sunrise and sunset.
In one sentence
A phenomenon where air molecules scatter short blue wavelengths of sunlight far more strongly than long red wavelengths.