Rayleigh Scattering
Why is the sky blue? Why are sunsets red? White light entering the atmosphere scatters off molecules, and short wavelengths scatter far more than long ones. Watch a beam of sunlight traverse an atmosphere you control and see the physics of color unfold.
About this lab
Rayleigh scattering describes how electromagnetic radiation interacts with particles much smaller than its wavelength. Lord Rayleigh showed in 1871 that the intensity of scattered light is inversely proportional to the fourth power of wavelength: I ~ 1/lambda^4. This means violet light (380 nm) scatters roughly 9.4 times more intensely than red light (700 nm). As sunlight enters Earth's atmosphere, nitrogen and oxygen molecules scatter shorter wavelengths in all directions, painting the overhead sky blue.
The sky appears blue rather than violet because sunlight contains less violet to begin with, and our eyes are more sensitive to blue. At sunrise and sunset, sunlight traverses a much longer atmospheric path. Nearly all blue and green light is scattered away before reaching the observer, leaving the warm reds and oranges that color the horizon. The same physics explains why distant mountains look blue-grey: short-wavelength light scattered by the intervening air column reaches your eye, washing out the mountain's true color.
In this simulation, a beam of white light enters from the left and passes through an atmosphere you control. At each scattering event, a photon is deflected from the beam with probability proportional to 1/lambda^4. The transmitted beam progressively reddens as optical depth increases. Scattered rays are drawn with their wavelength color. Increasing atmospheric depth or particle density simulates the transition from a noon sky to a sunset.