The sun’s appearance can change dramatically throughout the day, often displaying vibrant red and orange hues, especially at sunrise and sunset. This common observation is a result of specific interactions between sunlight and Earth’s atmosphere, which cause light to behave differently depending on the time of day. The scientific principles governing these phenomena provide a clear explanation for this daily spectacle.
Light’s Journey Through the Atmosphere
Sunlight, which appears white to our eyes, is actually a combination of all the colors of the rainbow. Each color within this spectrum has a unique wavelength, which is the distance between successive crests of a light wave. For instance, violet and blue light have shorter wavelengths, typically ranging from about 380 to 495 nanometers, while red light has longer wavelengths, generally between 620 and 750 nanometers.
The Earth is enveloped by an atmosphere composed primarily of gases like nitrogen and oxygen. This gaseous layer also contains tiny particles, and it acts as the medium through which sunlight travels before it reaches our eyes. The way light interacts with these atmospheric components determines the colors we perceive.
The Science of Scattering
When sunlight enters Earth’s atmosphere, it encounters tiny gas molecules and particles. This interaction leads to a process called Rayleigh scattering, named after the British physicist Lord Rayleigh, who described it. Rayleigh scattering occurs when light is scattered by particles that are much smaller than the light’s wavelength.
Shorter wavelengths, such as blue and violet, are scattered much more efficiently and in many directions by atmospheric molecules. In contrast, longer wavelengths, like red and orange, are scattered to a much lesser extent and tend to travel in more direct paths. This differential scattering explains why the sky typically appears blue; the abundant blue light is scattered across the sky.
The Sun’s Position and Its Hue
The sun’s position in the sky influences the amount of atmosphere its light must traverse. At midday, when the sun is high overhead, its light travels through the least amount of atmosphere. Less scattering of blue and violet light occurs, allowing more of the other colors to pass through, which makes the sun appear yellow or white.
As the sun approaches the horizon at sunrise or sunset, its light must travel through a much greater thickness of atmosphere. This extended path means nearly all of the shorter-wavelength blue and violet light is scattered away by atmospheric particles. The light that remains is predominantly the longer-wavelength red and orange light, giving the sun its characteristic reddish hue.