Physics
Why the Sky is Blue (Rayleigh Scattering)
Quick fact
Rayleigh scattering is named after John William Strutt, 3rd Baron Rayleigh, who first described it in the 1870s.
Why this is interesting
Have you ever wondered why the sky above you is blue, but the sunset often glows red or orange? The answer lies in how sunlight interacts with the air itself.
Read the full explanation
Understanding Why the Sky is Blue (Rayleigh Scattering)
Sunlight is white light containing all colors, each with a different wavelength. When sunlight enters Earth's atmosphere, it collides with tiny gas molecules—mostly nitrogen and oxygen. These molecules scatter the light in all directions, but they scatter shorter wavelengths (blue and violet) much more effectively than longer wavelengths (red and orange). Because our eyes are more sensitive to blue and the Sun emits less violet, we perceive the sky as blue. This scattering occurs continuously, painting the entire sky blue during the day.
A deeper explanation
Rayleigh scattering occurs when the scattering particles are much smaller than the wavelength of the light. The intensity of scattered light is inversely proportional to the fourth power of the wavelength (I ∝ 1/λ⁴). Blue light (λ≈450 nm) is scattered about 16 times more than red light (λ≈700 nm). This dramatic difference is why the sky is blue. At sunrise and sunset, sunlight travels through a thicker layer of atmosphere, scattering away most blue light and leaving the longer red and orange wavelengths dominant—hence the reddish sunsets. Clouds appear white because water droplets are larger than gas molecules, causing Mie scattering, which scatters all wavelengths equally. Understanding Rayleigh scattering is essential for explaining not only the blue sky but also phenomena like the Tyndall effect and the polarization of skylight.