Rainbows appear after rain because billions of water droplets still hanging in the air act like tiny prisms, bending sunlight and splitting it into its component colors. The process depends on a specific geometry between you, the sun, and those suspended droplets, which is why rainbows show up at particular times and always in the same part of the sky relative to where you’re standing. The optics are straightforward at a glance, but the details get surprisingly rich once you start asking follow-up questions about color order, double bows, and the odd extra bands that sometimes shimmer just inside the main arc.
What Happens Inside a Single Raindrop
When a beam of sunlight hits a raindrop, three things happen in quick succession. First, the light slows down slightly as it crosses from air into water, and that speed change causes it to bend. This bending is called refraction. Second, some of that light bounces off the far inner wall of the droplet, reflecting back toward the front. Third, the light bends again as it exits the droplet and re-enters the air. The net effect is that the light does a U-turn inside the drop, heading back roughly in the direction it came from, but at an angle.
The key detail is that different colors of light bend by slightly different amounts. Red light bends the least, violet bends the most, and every other color falls somewhere in between. So what enters the raindrop as white sunlight exits as a fan of colors, each heading off at a marginally different angle. One raindrop alone sends all those colors out in a cone, and you only see the one color that happens to be aimed at your eyes. A rainbow’s full spectrum comes from millions of droplets at slightly different positions, each contributing its own sliver of color to the arc you see.
Why Timing and Position Matter
Rainbows don’t appear during a downpour, at least not usually. You need the sun shining behind you while rain or mist lingers in front of you. That’s why rainbows are most common toward the end of a storm, when the clouds are breaking up enough to let direct sunlight through while plenty of droplets still hang in the air. Early morning and late afternoon storms are especially good candidates because the sun is lower in the sky, which pushes the rainbow arc higher and makes more of it visible above the horizon.
Your position matters too. Every rainbow is personal in a literal optical sense. The droplets sending red light to your eyes are not the same droplets sending red light to someone standing twenty meters away. You’re both seeing a rainbow, but technically not the same one. This is also why you can never walk to the end of a rainbow. As you move, the geometry shifts and the arc moves with you, always centered on the point directly opposite the sun from your perspective.
The 42-Degree Arc
If you draw an imaginary line from the sun, through your head, and out to the point on the ground directly opposite the sun (your “antisolar point”), the primary rainbow always sits at roughly 42 degrees from that line. This isn’t a coincidence or an approximation. It comes from the physics of refraction inside a spherical water droplet. Light that bounces once inside the drop piles up at a specific minimum deflection angle, and that clustering of rays is what makes the rainbow bright enough to see. For red light, that angle is about 42 degrees; for violet, it’s closer to 40 degrees. The two-degree spread between red and violet is what gives the rainbow its width.
Because the arc is centered on the antisolar point, which is always below the horizon when you’re standing on flat ground, you typically see a semicircle or less. From an airplane or a high mountain, you can occasionally see more of the circle, sometimes even a full 360-degree ring. The geometry hasn’t changed; you’ve just moved to a vantage point where more of the arc clears the ground.
Why Red Is Always on the Outside
The color order of a rainbow is fixed: red on the outer edge, then orange, yellow, green, blue, and violet on the inner edge. This follows directly from the amount each wavelength bends. Red, bending the least, exits the droplet at a wider angle (about 42 degrees from the antisolar point), so it forms the outer band. Violet bends the most and exits at a narrower angle (about 40 degrees), placing it on the inner edge. The other colors slot in between according to their wavelengths.
This order reverses in a secondary (double) rainbow, which is worth exploring because it reveals something interesting about how the light behaves.
Double Rainbows and the Dark Gap
A secondary rainbow forms when light bounces twice inside a raindrop before exiting, instead of once. That extra reflection sends the light out at a wider angle, roughly 51 degrees from the antisolar point, creating a second, fainter arc outside the primary one. Because the light undergoes an additional reflection, the color sequence flips: red appears on the inside of the secondary bow, and violet on the outside. Each reflection also costs some light intensity, which is why the secondary rainbow is always dimmer.
Between the two bows sits a noticeably darker region of sky. This is called Alexander’s dark band, named after Alexander of Aphrodisias, who described it in the third century. The band looks darker because no light from single or double reflections inside raindrops is directed into that zone. Light from single reflections stays inside the primary bow’s angle, and light from double reflections stays outside the secondary bow’s angle, leaving the gap between them relatively unlit. Research on this region has shown that under certain conditions, particularly when raindrops are slightly flattened rather than perfectly round, faint light from rays bouncing five times inside a droplet can creep into that dark band, but the effect is subtle and not visible to the casual observer.
The Faint Extra Bands You Sometimes See
If you look carefully at a bright rainbow, you might notice pale pastel bands, usually pinkish and greenish, hugging the inner edge of the primary arc. These are called supernumerary bows, and they puzzled scientists for a long time because simple ray-tracing through a spherical droplet doesn’t predict them. The explanation turns out to involve interference, the same phenomenon that creates the colorful sheen on a soap bubble.
Thomas Young proposed in the early 1800s that supernumerary arcs arise because two rays of light exiting a raindrop at the same angle can travel slightly different path lengths inside the drop. When those two rays meet, they either reinforce each other (producing a bright band) or partly cancel each other out (producing a dark band), depending on how their waves line up. Young’s basic interference idea turned out to be remarkably accurate once later physicists accounted for a phase shift that light undergoes when it passes through a focal region inside the drop.
Supernumerary bows are easiest to spot when the raindrops are small and uniform in size. Large drops produce broad, overlapping interference patterns that wash each other out. Tiny, evenly sized droplets keep the patterns tight and distinct. This is why supernumerary arcs show up more often in drizzle or mist than in heavy rain with big, varied drops.
Rainbows Beyond the Second Order
If one internal reflection gives you a primary rainbow and two reflections give you a secondary, what about three, four, or more? In principle, light can bounce any number of times inside a raindrop, each time producing a higher-order rainbow at a different angle. In practice, each extra bounce loses more light, and the higher-order bows land in parts of the sky where the sun’s direct glare drowns them out. Third- and fourth-order rainbows, for instance, appear around the sun itself rather than opposite it, making them nearly impossible to see against the bright sky.
For decades, third-order and higher rainbows were considered purely theoretical in nature. That changed in the 2000s and 2010s as digital photography and image-processing techniques allowed researchers to stack multiple exposures and boost contrast to pull faint signals out of bright backgrounds. A striking example came in 2013, when a series of photographs taken near Magdalena, New Mexico, revealed color hues in the correct order and position to be part of a seventh-order rainbow, located roughly 64 degrees from the sun. The detected colors ranged from red to blue-violet and spanned about 12 degrees, matching predictions from detailed optical simulations.
Raindrop Shape Changes Everything
Textbook diagrams almost always show raindrops as perfect spheres, and the basic explanation of rainbows depends on that assumption. Real raindrops, though, aren’t perfectly round. Small drops (under about a millimeter across) are nearly spherical because surface tension dominates, but larger drops get flattened on the bottom by air resistance as they fall, making them slightly oblate, like a hamburger bun. Very large drops oscillate as they fall, wobbling between oblate and prolate shapes.
This matters because the shape of the drop affects the angles at which light exits. Researchers studying the optical patterns created by oblate droplets have found that as the aspect ratio of a drop shifts away from 1.0 (a perfect sphere), the rainbow fringe patterns change in measurable ways. Simulations using vector ray-tracing models show good agreement with laboratory measurements, and the curvature of rainbow fringes can even be used to determine how flattened a droplet is.
For everyday rainbow-watching, the practical effect is that rain with a mix of drop sizes produces a slightly broader, more washed-out bow, while rain with uniform small drops produces a tighter, more vivid one. The flattened shape of larger drops also contributes to the brightness near the top of the arc compared to the sides, because the geometry of an oblate drop concentrates more light along certain exit angles.
Rainbows Without Rain
Rain is the most common source of airborne water droplets, but it’s not the only one. Any collection of suspended water drops in sunlight can produce a rainbow. Garden sprinklers, waterfalls, and even the spray from a boat’s wake all work. Fog can produce a fogbow, which is a broad, pale, mostly white arc. Fogbows look different because fog droplets are so small (typically under 0.05 millimeters) that the color separation is minimal and diffraction effects smear the colors together, leaving a ghostly white band.
You can also see rainbow-like effects from non-water sources. Spray from a car wash, mist in a greenhouse, and even the fine droplets around a fountain all work as long as the drops are roughly the right size and the sun is in the right position behind you. The physics is identical; only the source of the drops changes.
Moonlight can produce a rainbow too, called a moonbow or lunar rainbow. These are rare and faint because moonlight is so much dimmer than sunlight. Moonbows often appear white to the naked eye because the light is too weak to trigger the color receptors in your retina, but long-exposure photographs reveal the full spectrum. Certain waterfalls, like those at Yosemite and Victoria Falls, are famous among photographers for producing moonbows on clear nights around the full moon.
Why You Sometimes See a Rainbow From Only One Spot
People occasionally report seeing a stubby patch of rainbow hanging in the sky without a full arc. These fragments happen when only a small cluster of raindrops exists in the right position. If rain is falling in a narrow column while the rest of the sky is clear, you’ll see only the part of the arc that those drops can produce. The rest of the arc’s geometry is still there, but there’s nothing to light it up.
A related phenomenon is the sun dog, or parhelion, which looks like a bright spot of color at roughly the same height as the sun, off to one side. Sun dogs are not rainbows. They’re caused by ice crystals in high clouds refracting sunlight, not by water droplets. The two get confused because both involve colored patches of sky, but the underlying optics are different. Ice crystals are hexagonal, not spherical, and they produce their effects at different angles, typically around 22 degrees from the sun rather than 42 degrees from the antisolar point.
Common Misconceptions About Rainbows
One persistent myth is that raindrops are teardrop-shaped and that their pointy tops somehow help split the light. As discussed earlier, small drops are nearly spherical, and larger ones are flattened, not pointy. The teardrop shape exists only in cartoons.
Another misunderstanding is that a rainbow contains exactly seven colors. Newton originally divided the spectrum into seven bands partly for numerological reasons, wanting to match the number of notes in a musical scale. In reality, a rainbow is a continuous gradient of wavelengths. There are no sharp boundaries between “orange” and “yellow,” just a smooth transition. You can divide it into as many or as few named color bands as you like. Some cultures historically recognized five or six rainbow colors, and none of them were wrong.
A third misconception involves polarization. Rainbow light is strongly polarized, meaning the light waves coming from the bow vibrate mostly in one plane. If you look at a rainbow through polarized sunglasses and rotate them, you can dramatically dim or brighten the bow. This isn’t a parlor trick; it’s a direct consequence of the angle at which light reflects inside the raindrop. The reflection inside the drop happens close to a specific angle where reflected light becomes almost entirely polarized. Photographers use polarizing filters to either enhance or remove rainbow glare depending on the shot they want.
Why Some Rainbows Look More Vivid Than Others
Not all rainbows are created equal, and the difference usually comes down to drop size and uniformity. Raindrops between about 0.5 and 1 millimeter in diameter tend to produce the most vivid, color-saturated bows. Drops much smaller than that push the rainbow toward a fogbow, and drops much larger introduce so much overlap between colors that the arc looks pale and broad. Uniform drop sizes help keep the color bands crisp, while a wide mix of sizes blurs everything together.
The darkness of the background sky matters too. A rainbow against a dark thundercloud looks far more striking than one against a pale overcast sky, even if the rainbow itself is the same brightness. Your eyes perceive color contrast relative to the surroundings, so a stormy backdrop acts like a dark canvas that makes the colors pop. This is one reason why late-afternoon summer storms, with dark clouds to the east and low sun to the west, produce the most memorable rainbows. The lighting geometry is ideal, the drops are fresh, and the contrast is high.
Atmospheric dust and humidity play supporting roles. Clean air transmits more light to the drops and less scattered light to compete with the bow. Humid air near the ground can add a slight extra glow around the antisolar point inside the arc, a phenomenon called Heiligenschein, where backscattered light brightens the area enclosed by the rainbow, making the sky inside the bow visibly brighter than the sky outside it. The next time you see a good rainbow, compare the brightness of the sky just inside the arc to the sky just outside, and the difference is unmistakable.