By Jimmy Liu
When asked where the color green occurs in nature, one would probably immediately turn to the earth. The color green, however, is not limited to what springs from the ground. If you were to look towards the horizon at the setting sun under the right atmospheric conditions, you might just catch a glimpse of what is known as the green ray, an optical phenomenon first popularized by Jules Verne’s 1882 novel of the same name: at the moment of sunset, a distinct green flash shoots out from the sun’s upper disk, lasting no more than seconds. There are people who dedicate years, decades, and even their entire lives, in search of this fabled phenomenon, but what exactly makes the green ray so rare? What happens when the lucky stars align to allow for a successful observation?
To start, the observer needs an unobstructed view of the horizon as the sun is setting. This allows one to gaze as far down the sky as possible. To see why, imagine two concentric circles. The smaller circle represents Earth’s surface, while the larger one represents the outer boundary of the atmosphere. Standing on the surface of the Earth and peering up at the sky, our vision can be represented as a line segment connecting a point on the inner circle to a point on the outer circle. To view the greatest depth of the sky then, would mean to maximize the length of this line segment, which is tangent to the inner circle. For our vision to follow a line tangent to Earth and thus maximize the “amount of sky” we can see through, there have to be no obstacles between us and the setting sun—we have to see the horizon.
How does this help with observing the green ray? The answer is refraction, the bending of light as it travels through different mediums. Though our atmosphere is made up of roughly the same mixture of air throughout, its density and temperature vary as we increase our altitude, allowing light to refract. This refraction is normally unnoticeable, but the story changes when we look towards the horizon. Because the depth of sky we see is maximized, the refracted light is able to travel a much longer distance before it hits our pupils, increasing the effect of refraction.
Atmospheric refraction alone is not enough for the green ray to be observed, however. Most sightings are accompanied and amplified by mirages, which occur when the temperature profile changes rapidly with altitude and the warm air is directly above sea level. Mirages result in multiple images, which form the setting sun’s jagged edges and are necessary for viewing the green flash. While refraction and mirage by themselves are already rare optical phenomena that depend on specific atmospheric conditions, a green ray sighting needs both to occur simultaneously. For the green ray enthusiast, this makes the range of conditions that need to be met largely external and extremely rare, adding value to the phenomenon’s sighting.
When we see white, unrefracted light, we are actually seeing the combination of all the frequencies of visible light, hence a combination of all the different colors. When white light is refracted, however, the different frequencies are bent to different degrees. Low-frequency light is bent less, and high-frequency light more—Pink Floyd’s album cover illustrates this idea well. As the radiant sun sets, its light experiences this same “peeling” of different colors, staggering the times at which different colors disappear from the horizon. The red sun sets first, followed by orange, yellow, green, blue, indigo, and violet.
From refraction alone, we should see violet rays, not green—something else is at play. The higher frequency a wave is, the more easily it gets scattered by particulates; so while the violet sun is always last to set, it gets scattered before we can see it, as are the indigo and blue, leaving us with a green ray. On a hazy day, green light gets scattered too, resulting in a yellow ray, and on a particularly clear day, one can see a blue ray.
By describing the right conditions, I have perhaps led you to believe that the green ray is a common occurrence now that you know where to look. Unfortunately, the truth is far from it. If just one condition is not met, the green ray will not happen. You, like most people, will likely never see the green ray in person. But perhaps its rarity is a gift in disguise: the sense of wonder a green ray instills will always stay with those who do witness it.