A rainbow can look as if it lands in a field, behind a house or beside the road. The temptation is to imagine a colored arch occupying that exact place. But a rainbow is an optical effect formed by light reaching an observer from many water droplets. Its apparent location changes with the observer.
That makes the elusive end less disappointing and more interesting. You are not failing to catch up with a distant object. Moving changes the arrangement of light paths that creates the rainbow you see.
A raindrop changes the route of sunlight
Sunlight contains a range of colors. When it enters a water droplet, changes direction, reflects within the droplet and emerges, its colors are separated because they bend by different amounts. The National Weather Service describes the familiar rainbow as sunlight spread into its spectrum and directed toward the observer by droplets.
A visible arc involves a great many droplets. One droplet is not drawing the whole rainbow for you. Instead, light of particular colors reaches your eyes from droplets in suitable directions. Across the rain shower, those directions form the pattern that your eyes recognize as an arch.
The color bands blend continuously. Naming seven colors is a familiar convention, not evidence that nature has divided the visible spectrum into seven sharply bounded strips. A photograph may also emphasize or mute particular colors depending on exposure and the display on which it is viewed.
Your position is part of the arrangement
For an ordinary sunlight rainbow, the Sun is behind the observer and droplets are in front. Turning toward the Sun is therefore generally the wrong way to search for the familiar bow. The height of the Sun affects how much of the arc is visible above the horizon.
The full geometry is circular, although the ground often hides much of it. From an elevated viewpoint, more of that geometry can sometimes become visible. An ordinary rainbow seen from a garden is usually an arc because your view is bounded by the ground and by where illuminated droplets are present.
If you walk sideways, a different collection of droplets can send the relevant light toward your new position. The apparent end shifts with this change. A person standing elsewhere is seeing a closely related pattern, but the exact light paths reaching their eyes are not identical to yours.
Why a second bow can appear
Sometimes a fainter, larger arc appears outside the main rainbow. An additional internal reflection within droplets produces the secondary bow. Its colors appear in the opposite order to those of the primary bow, giving you a useful feature to look for when conditions allow both to be seen.
The outer bow is often less conspicuous, so it is easy to overlook while concentrating on the brighter one. The sky between the bows can also appear darker. These details come from how light is distributed, not from a strip of cloud that has been painted between two independent arches.
Not every colored arc is the same phenomenon. Halos around the Sun or Moon, for example, involve different circumstances and often ice crystals. There is no need to identify every atmospheric effect from memory; recording the location, weather and position relative to the light source gives you better clues to investigate later.
Enjoy the view without chasing the weather
Look from a safe location when sunlight illuminates rain away from you. A rainbow can occur around changing weather, so do not let it distract you from lightning, traffic, wet footing or a park closure. Never look directly at the Sun to locate the other side of the geometry.
You might notice a small bow in naturally occurring spray, but there is no need to approach a waterfall edge or create slippery conditions to see one. The useful observation is positional: where is the light, where are the droplets, and where are you standing?
Once those three elements are part of the picture, the moving end becomes understandable. The rainbow is real light reaching your eyes, but it is not a painted structure waiting in a field. It is a brief arrangement that includes you, which is why the view changes when you do.
Sources
- National Weather Service: How do rainbows form?
Water droplets refract, reflect and disperse sunlight toward an observer.