Why Sunsets Are Red
Rayleigh scattering and the color of the sky
Rayleigh scattering. The name sounds complicated, but the principle is simple.
Rayleigh Scattering
When light hits particles smaller than its wavelength (nitrogen and oxygen molecules in the atmosphere), scattering occurs. The intensity of scattering is inversely proportional to the fourth power of the wavelength.
Blue light (~450nm) has a shorter wavelength than red light (~700nm). Do the math and blue light scatters about 5.5 times more strongly.
Daytime: Blue Sky
When the sun is overhead, light passes through a short distance of atmosphere. Blue light scatters in all directions, so the sky looks blue no matter where you look.
Sunset: Red Sky
When the sun is near the horizon, light must travel through much more atmosphere. Blue light gets scattered away almost entirely along the way, leaving only the longer-wavelength reds and oranges to reach the observer.
More dust or moisture in the atmosphere (cities, after fires) intensifies scattering, producing even redder sunsets.
How It Works
Sunlight hits atmospheric molecules (Nβ, Oβ) β Rayleigh scattering occurs
Scattering intensity β 1/Ξ»β΄ β blue scatters ~5.5x more than red
Sunset: light travels longer path β blue exhausted β only red reaches you