The sky is one of those things most people stop questioning after childhood. But the answer is genuinely fascinating once you dig into it. [1]
What Is Rayleigh Scattering?
When sunlight enters the atmosphere, it collides with gas molecules. Light travels in waves, and different colours have different wavelengths. Blue light has a shorter wavelength than red light, which means it scatters much more easily when it hits a molecule. This effect is called Rayleigh scattering, named after the physicist Lord Rayleigh who described it in the 1870s. [1]
Why Blue and Not Violet?
Violet light actually scatters even more than blue. So why does the sky look blue rather than violet? Two reasons. First, there is less violet in sunlight to begin with. Second, our eyes are simply more sensitive to blue than violet. The combination of these two factors means blue wins out. [2]
Why Sunsets Are Red
At sunset, sunlight travels through a much thicker slice of atmosphere to reach your eyes. By the time it arrives, most of the blue has already scattered away in other directions. What remains is the longer wavelength light — the reds and oranges — which gives sunsets their distinctive colour. [1]
References
- Young, A. (1982). Rayleigh scattering. Physics Today. https://doi.org/10.1063/1.2890003
- Bohren, C. (2006). What Light Through Yonder Window Breaks. Wiley.