Light gives our world visual texture and richness. We constantly experience light as it interacts with the different mediums around us: Scattering through our atmosphere, reflected back from a flower, or prismed in water droplets. Humans visually experience a very narrow band of the electromagnetic spectrum called visible light. But even though we see just a part of the electromagnetic spectrum, there are a wide variety of phenomena visible to the human eye. For thousands of years, we’ve been cataloging and categorizing our many experiences with the optical phenomena of our planet.
Make no mistake, these optical phenomena aren’t illusions. They are real in the sense that they are governed by the optical properties of the matter that light interacts with. Usually, the matter involved is dust, water droplets, or the atoms in the atmosphere. Many of these phenomena play on light’s dual nature as particle and wave. Some of these phenomena are incredibly common and others might be seen once in a lifetime. If you’re looking up at the right moment, under the right conditions, you might see one of these tricks of the light.
Refraction: colorful rainbows
Rainbows get a lot of attention in our media and art, but they happen a lot less frequently than you might suppose. A rainbow needs two things to form; sunlight and water vapor opposite the direction of the sun. If you live in a dry region, rainbows can be very rare. The light needs to shine directly onto the large water droplets, so it can’t be obscured by clouds. The sun also can’t be too high in the sky, or a rainbow won’t be able to form. The light is refracted when it enters the droplet, separating into its component wavelengths—or colors, reflects off the inside of the droplet, and then refracts again on its way out.
Rainbows are larger when the sun is close to the horizon and they aren’t visible when the sun gets higher than 45 degrees above the horizon. This is because rainbows form in a 42-degree circle around the antisolar point, an abstract point directly opposite the sun from the viewer's perspective. Rainbows occur when white light enters a droplet of water and is refracted back between a 40.6 and 42-degree angle. The light bends back a little differently, depending on its wavelength, so that red light is refracted at a 42-degree angle and blue light is refracted closer to 40 degrees. This is what causes us to see distinct color bands in a rainbow.
There are different types of rarer rainbow phenomena that can form. One of the most commonly spotted is the "double rainbow" where a secondary bow forms about nine degrees above the primary bow. It's usually about twice the width and half the brightness of the primary bow. During sunrise and sunset, you might see a red bow. This rainbow appears redder because the sunlight has to travel through more atmosphere, in turn, scattering more blue and green light.
The sun isn’t the only light source that can cause a rainbow, the moon can as well. There isn’t any difference in the mechanics of how a moonbow forms, although they are usually dimmer and less colorful than their sunlit counterparts. Moonbows are rarer because you need a full moon, in addition to the other requirements for a rainbow. To the unaided eye, they tend to be very faint, but they can be seen. Cameras, with their long exposure times, are able to capture very vibrant, colorful moonbows.
Water Droplets: Coronas, glories, and iridescence
Water droplets are a key player in optical phenomena because they give the light a medium to play on in the sky. Many of these phenomena involve the beauty of white light as it is refracted through water or diffracted by it, producing the colors commonly associated with a rainbow. Not all of these phenomena are rainbows, though. A corona is probably one of the most common examples you can see.
A corona is an optical phenomenon caused by diffraction, that occurs due to the properties of light waves. They can be seen when thin clouds of water, dust, or pollen partially cover the sun or moon. Coronae will form in several concentric rings around the bright, central light source and will stretch out about 15 degrees in diameter. Coronae get their softer colors from the more subtle mixing that happens through diffraction rather than by separating lightwaves, as in refraction. Though these phenomena bear the same name as the stellar corona, the outermost layer of the sun’s atmosphere, the atmospheric phenomena and solar structure aren’t related.
Cloud iridescence, or 'rainbow cloud' is another diffraction phenomenon, and is usually seen near the sun, but can also appear far away from the sun. These can appear as almost random patches of color and are caused by the same mechanisms that allow coronae to form. Coronae form when the water droplets or ice crystals are uniform in size. As the water becomes less uniform, you can see these more irregular bands of color form. Iridescence is a fragile phenomenon and can change rapidly as the clouds shift in the sky.
They often appear in the late afternoon, on very hot and humid days, and usually form on top of cumulus clouds. As the cumulus cloud boils upwards, it pushes the air layers above it higher and higher, where it expands and cools. Sometimes this moisture will suddenly condense into tiny droplets to form a cap cloud. It is the droplets in the cap cloud which scatter sunlight to form the gorgeous colors.
Glories are another refraction phenomenon that form opposite the sun around the antisolar point. They are normally found near the ground whenever the sun shines on mist or clouds beneath the observer. They are similar to a corona in the sense that they still form in concentric circles, have a bright center, and get more diffuse as they expand. Sometimes, shadows can converge on the glory, giving rise to the Brocken spectre. Glories are “impossible” because simple diffraction theory fails to explain how the water can bend light back 180 degrees to the viewer, a more extreme bend than the theory allows. There is a property of light that allows it to travel over portions of the droplet as a surface wave, rather than being immediately refracted, which gives it the extra room to fully bend 180 degrees back to the observer.
Ice crystals: Halos, dogs, and pillars
Of the refracting phenomena, halos are some of the most common. They are also large, with the most common halos measuring about 22 degrees in diameter, and can stretch out to 50 degrees wide. More rare halos can texture the sky with many delicate arcs created by the intricate geometric play between light and its refraction in ice crystals. Generally, if you’re looking at a circle around the sun or moon and it’s smaller than 20 degrees, it’s probably a corona. If it’s larger, it’s probably a halo. Always take precautions to protect your vision when viewing sun halos, such as blocking the sun with your hand or a building. Never look directly at or near the sun.
A 22-degree halo and sundogs, sometimes called parhelia, are the two most common halos we experience. The 22-degree halo completely encircles the sun at its fullest, but partial 22-degree halos can form depending on the conditions in the sky. Sun dogs usually form 22 degrees away on either side of the sun. They develop due to the refraction of light through ice crystals. The colors usually go from red closest to the sun, out to blue on the outside of the sundog. You can see color when viewing halos around the moon, but it’s much harder for the human eye to see.
Pillars can be mistaken for a light ray, but they are also a type of halo that can form around a source of light, and appear most often at sunrise or sundown. They usually point up, but can also point down. They develop as a result of ice crystals slowly falling through the air, reflecting the sun’s rays off of them. Even though other astronomical bodies, like planets, can technically create halos, they are too faint to produce any we can see. Although, they can be seen forming around venus on very rare occasions.
Displacement phenomena: The green flash
Sometimes, things aren’t exactly as they seem. When the sun is shining on you at an angle, shining through more atmosphere at sunrise or sunset, or because spacetime isn’t flat, objects might appear to be in a different place than they actually are. One such phenomenon that leads people on a lifelong chase is the relatively rare green flash.
A green flash is created when the right conditions of sunlight dispersion and a mirage combine to give us this strange experience. A mirage is critical because the light needs to be refracted through layers of the atmosphere with steep thermal gradients to give us time to perceive it. As the sun sinks below the horizon, the last beams of light are still being scattered through the atmosphere and bent by the mirage. Sometimes, as the light passes through the visible spectrum, a green flash appears on top of the sun.
There are four types of green flashes, each related to a different type of mirage. They are the easiest to see at sunset, with an unobstructed view of a clear horizon. Like with other phenomena, a green flash isn’t limited to the sun. The moon can produce a green flash, but such an event is incredibly rare.