What is Rayleigh scattering?
Rayleigh scattering—also known as “the green flash”—is an optical phenomenon that can be observed just before sunrise or shortly after sunset. It appears as an intense green reflection slightly above the Sun or other bright celestial objects, such as Venus, Jupiter, or the Moon. It occurs due to the refraction of light as it passes through the Earth's atmosphere, which bends red light more than light of higher frequency wavelengths, such as blue or green. Although it is not too difficult to see, few people have ever observed this phenomenon.
A phenomenon popularized by Jules Verne
You have surely enjoyed more than one sunrise or sunset on the beach or in the countryside. Although many people are unaware of it, in those flat places, where the Sun rises or sets over the horizon, a short-lived phenomenon commonly known as the “green flash” takes place. It is actually a physical phenomenon known as Rayleigh scattering, which occurs due to the way the Earth's atmosphere bends the components of sunlight to different degrees. As a result, with a bit of luck and being in the right place, you can see a green flash above the Sun at the moment of sunrise or sunset. The common name became popular thanks to the novel by Jules Verne (“The Green Flash”), whose plot is based on this phenomenon.
The physics behind the green flash
From a physical point of view, it is a phenomenon named in honor of Lord Rayleigh, which basically consists of the scattering of light (or any other electromagnetic radiation) when it passes through a medium made up of particles much smaller than the wavelength of the photons that make up the beam. This scattering occurs when light passes through transparent solids and liquids, but it is more evident when the medium is a gas, such as the atmosphere of our planet. In fact, the reason we see the sky blue and not black is precisely the existence of Rayleigh scattering. If the particle size were larger than the wavelength, all frequencies would be scattered equally and the phenomenon would not occur. That is why we see clouds as white, since the water molecules that form them are much larger than the wavelength of visible light.
Observing the green flash
In summary, the degree of Rayleigh scattering that a light ray suffers when passing through a gas depends on the particle size and its wavelength. The relationship between the scattering coefficient and the intensity of the scattered light is known as Rayleigh's Law. When the Sun is near the horizon, the greatest possible amount of atmosphere lies between the star and the observer, maximizing the described effect and giving rise to the green flash above the Sun. Theoretically, a blue flash could also be seen, but that generally does not occur because light of that frequency is much more affected by atmospheric scattering. Obviously, the green flash not only occurs above the Sun. It can also be observed with a little more difficulty over other bright celestial bodies, such as Venus, Jupiter, or the Moon. It only takes luck or, as NeoTeo readers will surely do, being in the right place at the right time. If you manage to take a good photograph of this phenomenon, you can post it on the forum so we can all appreciate it. Do you dare?
You can read more about Rayleigh scattering, on Wikipedia.
