There is no physical “end” to a rainbow. A rainbow is not a fixed object sitting on the landscape; it is an optical effect produced by sunlight refracting and reflecting inside water droplets, and it exists only relative to where you are standing. The arc always forms at roughly 42 degrees from the point directly opposite the sun behind you, which means it moves with you as you walk toward it. What you would actually see if you managed to stand in the rain shower where the rainbow appears to touch down is surprisingly ordinary, and understanding why reveals some genuinely interesting things about how light and water interact.
Why the End Keeps Moving Away
Every rainbow you have ever seen was personal to you. The specific droplets sending colored light to your eyes are different from the droplets sending colored light to a friend standing ten feet away. That is because the geometry is locked to the observer: the center of any rainbow arc sits at the antisolar point, which is the spot exactly opposite the sun from your head. Draw an imaginary line from the sun through your eyes and extend it forward. That line points at the center of the circle the rainbow traces. The bright primary bow sits about 42 degrees outward from that center line.
When you walk toward the spot where the rainbow seems to meet the ground, the antisolar point walks with you, and the 42-degree cone of refracted light shifts along too. The rainbow does not recede because it is far away. It recedes because it is not a place at all. It is a direction, a particular angle of light arriving at your retina from whatever droplets happen to occupy that angle at any given moment.
This is also why two people standing side by side will see the rainbow in very slightly different positions against the background landscape. Their antisolar points are not quite the same, so the cone of colored light is centered on a slightly different axis. The rainbow each person sees is built from a completely different set of water droplets.
What You Would Actually See Standing in the Rain
Photographs occasionally capture what looks like a rainbow ending right on a road, a field, or even a parking lot. These images are real, not tricks. When the rain shower is close and you happen to be at the right angle, the lowest visible arc of the rainbow can appear to intersect the ground just meters away. In those moments, you can sometimes see colored light playing across the wet surface, tinting puddles or grass with a faint wash of spectral color.
But walk into that patch of rain, and the experience is underwhelming. You are now surrounded by the droplets doing the refracting, and the geometry no longer works in your favor. Each droplet is sending its refracted light at 42 degrees from the sun-observer axis, but you are no longer looking at those droplets from the outside at the right angle. Instead, the rainbow has shifted to a new position farther along. You might notice the light around you looks slightly brighter or more colorful than ordinary rain, but there is no concentrated band of color, no glowing stripe on the ground. The pot of gold, as it turns out, was never even a coherent location.
What is genuinely interesting, though, is what happens to the light right around you in a rain shower on a sunny day. Individual droplets can send tiny flashes of spectral color to your eye if you catch them at the right instant and angle. You can sometimes see this effect in the spray from a garden hose, where the droplets are close enough and sparse enough for your eye to pick out individual bursts of red, green, or violet. That is the rainbow, disassembled into its component parts, each droplet contributing one tiny flicker of one color.
The Hidden Full Circle
A rainbow is actually a full circle, not just an arc. The reason you normally see only a half-arc or less is that the ground gets in the way. The center of the circle is at the antisolar point, which is below the horizon whenever the sun is above the horizon. So the lower portion of the circle is blocked by the earth beneath your feet.
From an airplane, a tall building, or a waterfall overlook, you can sometimes see more of the circle, occasionally the complete ring. Pilots and mountain climbers have photographed full 360-degree rainbows, and they are a remarkable sight. The geometry is identical to the arc you see from the ground, just with nothing obstructing the lower half. This also means that at sunrise or sunset, when the sun is right at the horizon, the antisolar point sits exactly on the opposite horizon, and the rainbow rises as a perfect semicircle. As the sun climbs higher, the antisolar point drops lower, and the visible arc shrinks. By the time the sun is higher than about 42 degrees above the horizon, the entire rainbow is below the ground plane and you cannot see it at all. This is why rainbows are most commonly spotted in the morning or late afternoon.
The Dark Space Between the Bows
If you look carefully at a strong rainbow, you will often see a fainter, wider arc above the primary one. This is the secondary rainbow, produced by light bouncing twice inside each droplet before exiting. The color order is reversed: red on the inside, violet on the outside, the mirror image of the primary bow.
Between the two arcs is a region of sky that looks noticeably darker than the sky above or below either rainbow. This band is called Alexander’s dark band, after the ancient Greek philosopher Alexander of Aphrodisias, who first described it around 200 CE. The darkness is not an illusion or a contrast effect. Light at angles between the primary and secondary rainbow cannot exit the droplets efficiently in your direction, so fewer photons arrive at your eyes from that strip of sky. The sky below the primary bow, by contrast, looks slightly brighter than normal because all colors of refracted light overlap there, producing a faint white glow.
At the “end” of a rainbow, this dark band would extend all the way down to the ground if conditions were right. Photographers have captured it as a visible darkening of the sky right where the arc appears to meet the earth, a patch that looks almost ominously dim compared to the bright landscape around it.
Supernumerary Arcs and Fine Structure
Look just inside the primary bow on a particularly vivid rainbow, and you may spot a series of faint, tightly spaced pastel bands, usually pinkish and greenish, that do not correspond to the normal spectral sequence. These are supernumerary arcs, and they are caused by interference between light waves that take slightly different paths through a raindrop but exit at nearly the same angle. Thomas Young first proposed this explanation in the early 1800s, connecting rainbows to the wave nature of light.1Applied Optics. Supernumerary arcs of rainbows: Young’s theory of interference
Supernumerary arcs are most visible when the water droplets are small and fairly uniform in size. Large or mixed-size droplets blur the interference pattern out. This is why you tend to see supernumeraries in fine drizzle or mist rather than heavy downpours. Near the apparent base of a rainbow, supernumerary arcs can be especially striking if the droplets in that part of the shower happen to be consistently sized, giving the bottom of the arc a layered, almost iridescent appearance that the top of the bow may lack.
Fogbows and What Happens With Tiny Droplets
When water droplets shrink below about 50 micrometers in diameter, something interesting happens: the rainbow loses most of its color and becomes a broad, ghostly white arc called a fogbow. The physics is the same refraction and reflection occurring inside each droplet, but the droplets are small enough that diffraction spreads each color’s arc over a wide angle, and the colors overlap into white. Fogbows are sometimes called white rainbows or cloudbows, and they are more common than most people realize, especially in mountain fog or sea mist.
A fogbow is wider and more diffuse than a normal rainbow. Measurements of fogbows taken in the Arctic found that the angular width of the primary bow could vary from about 5 degrees to nearly 20 degrees, depending on the droplet size, far broader than the sharp, narrow band of a typical rainbow formed in rain-sized drops.2Optica Publishing Group (Applied Optics). Imaging polarimetry of the fogbow: polarization characteristics of white rainbows measured in the high Arctic At the “end” of a fogbow, where the arc appears to touch the ground, you would see an even more diffuse glow than at the base of a normal rainbow, a gentle brightening of the fog with perhaps the faintest hint of reddish-orange on the outside edge and blue on the inside.
Cloud iridescence, coronas, and glories are related but distinct phenomena that also arise from small water droplets or ice crystals. A glory, the colorful rings you sometimes see around the shadow of an airplane on a cloud below, is caused by light scattered directly backward by cloud droplets, a completely different geometry from the rainbow’s forward-scattering refraction. Fogbows sometimes appear alongside glories when the sun is low and the fog is the right density, creating layered optical effects that can be bewildering if you do not know what you are looking at.
Polarization and the Light You Cannot See
There is something happening at the end of a rainbow, or anywhere along it, that your eyes cannot detect but a camera filter can. The light in a rainbow is strongly polarized, meaning the light waves are vibrating mostly in one direction rather than randomly. If you hold a polarizing filter (like a lens from polarized sunglasses) up to a rainbow and rotate it, you can make the rainbow nearly vanish or become much brighter depending on the filter’s orientation.
The degree of polarization depends on the droplet size. Larger droplets produce more strongly polarized rainbow light, while smaller droplets produce a weaker polarization signal.3Applied Optics. Polarized rainbow This is actually less polarization than simple geometric optics would predict, because the wave nature of light and diffraction effects inside the droplets partially scramble the polarization. Polarization is one of the reasons rainbow photographs can look so different depending on whether the photographer used a polarizing filter. With the filter oriented to block the rainbow’s dominant polarization, the arc disappears from the photo even though the rain and sky remain, a striking demonstration that the rainbow is not painted on the sky like a stripe but is a specific directional beam of polarized light reaching your position.
Why Rainbow Bases Sometimes Look Different From the Top
People who pay close attention to rainbows often notice that the colors near the base look different from the colors at the top of the arc. The reds might seem more vivid near the ground, or the arc might look narrower or broader at one end. This is not imagination. The droplets producing the base of the rainbow are close to you and at a low angle, while the droplets producing the top of the arc are farther away and higher in the sky. If the rain shower is not uniform, with larger drops near the ground and smaller drops higher up (which is common, since large drops fall faster), then the lower part of the arc will be produced by large drops and the upper part by small ones.
Large drops produce a sharper, more vivid rainbow with well-separated colors. Small drops produce a broader, more washed-out bow with colors that bleed together. So a single rainbow can look crisp and vivid at its base and pastel and soft at its crown. The supernumerary arcs might be visible at the top but not the bottom, or vice versa. The base may show strong reds and greens while the apex trends toward white and pink. This asymmetry is actually a record of the droplet-size distribution along the height of the rain shower, which brings us to a genuinely useful application of rainbow optics.
Using Rainbow Optics to Measure the Invisible
Scientists and engineers have turned rainbow physics into a measurement tool. A technique called rainbow refractometry uses the exact angular position, width, and fine structure of rainbow-like patterns to determine the size and composition of droplets in sprays, fogs, and combustion systems.4Frontiers in Physics. Right partial rainbow refractometry for measuring droplet refractive index and size By shining a laser through a spray and analyzing the resulting rainbow pattern on a detector, researchers can figure out how big the droplets are, what they are made of, and how their refractive index varies, all without physically touching or collecting the droplets.
This has practical applications in fuel injection design, pharmaceutical spray manufacturing, and atmospheric science. If you want to know whether a diesel injector is producing the right droplet size for efficient combustion, you can read it from the rainbow pattern the spray creates under laser illumination. The same supernumerary spacing that Thomas Young explained as a curiosity of natural rainbows becomes a precision diagnostic in the lab. The angular spacing between supernumerary fringes tightens as droplets get larger and widens as they shrink, providing a direct readout of droplet diameter that can be accurate down to a fraction of a micrometer.
Rainbows on Other Worlds
Rainbows on Earth require water droplets and sunlight, but neither of those is unique to our planet. Any transparent liquid droplet suspended in an atmosphere with a light source can produce a rainbow, and the angular size and color pattern will differ depending on the liquid’s refractive index. Water has a refractive index of about 1.33, which produces the familiar 42-degree primary bow. A liquid with a higher refractive index would bend light more and produce a smaller, tighter rainbow. A lower refractive index would produce a larger arc.
On Titan, Saturn’s largest moon, methane rain falls through a dense nitrogen atmosphere. Methane has a lower refractive index than water, so a methane rainbow on Titan would be wider than an Earth rainbow, roughly 49 degrees from the antisolar point instead of 42. The colors would also shift because methane absorbs different wavelengths. On Venus, sulfuric acid droplets in the upper cloud deck could theoretically produce rainbows with a smaller angular radius, since sulfuric acid has a higher refractive index than water. The European Space Agency’s Venus Express orbiter detected a glory pattern in Venus’s clouds, confirming that light-scattering phenomena involving cloud droplets do occur there.
What the “end” of a rainbow looks like on another world depends entirely on the landscape and atmosphere. On Titan, with its hydrocarbon lakes and orange haze, a methane rainbow’s base might intersect a shoreline of liquid ethane. On a hypothetical ocean planet with water-vapor clouds, a rainbow might appear to touch an unbroken sea surface, with no land to interrupt the view and the lower arc visible all the way to the waterline. These are speculative scenarios, but the optics that govern them are the same well-understood physics that produce the rainbow arcs in your backyard sprinkler.