A rainbow is an optical phenomenon created when sunlight interacts with tiny water droplets suspended in the atmosphere. The result is a multicolored circular arc that appears opposite the sun. Finding the best view requires understanding the precise science and geometry involved, which helps locate ideal atmospheric conditions and physical locations.
Understanding the Essential Geometry
The formation of a rainbow relies on a specific alignment of three elements: the sun, the water droplets, and the observer. To see a primary rainbow, the sun must always be behind the viewer, shining its light onto a field of raindrops in front. The center of the rainbow’s arc is always the antisolar point, which is the exact point in the sky opposite the sun, where the shadow of the observer’s head would fall.
The light path involves sunlight entering a spherical raindrop, refracting, reflecting off the back inner surface, and then refracting again upon exiting. This process separates white light into its component colors. The most intense light is returned to the observer at a consistent angle of approximately 42 degrees from the antisolar point.
This fixed angle explains why rainbows are most often visible during the morning or late afternoon hours. If the sun is higher than 42 degrees above the horizon, the entire rainbow arc will be below the horizon from a ground-level perspective. Therefore, the lower the sun is in the sky, the greater the portion of the semicircular arc that can be seen.
Red light, which has the longest visible wavelength, is deviated at about 42 degrees, forming the outermost band of the bow. Violet light is deviated at a slightly smaller angle of roughly 40 degrees, forming the innermost band. The consistent geometry means that every observer sees a unique rainbow formed by different drops.
Optimizing Your Vantage Point
Although a rainbow is actually a full circle, the ground typically cuts off the lower half, resulting in a semicircular arc. Viewing the complete 360-degree circular shape requires a high vantage point, allowing the observer to look down onto the rain-filled air. This can be achieved from an airplane window or a tall mountain summit.
For ground-level viewing, select a location with a wide, unobstructed view opposite the sun. Open fields, coastlines, or large bodies of water allow the full visible arc to appear without being blocked by trees or buildings. The dark contrast of a retreating storm cloud behind the rainbow also enhances the colors.
A reliable method for finding a rainbow, regardless of recent rainfall, is to seek out sources of continuous airborne water droplets. Large waterfalls create a constant spray that acts as a perpetual field of prisms. Similarly, the mist created by ocean waves crashing onto a rocky coastline can consistently produce localized rainbows, provided the sun is shining from the correct direction.
One can also artificially create a rainbow by turning a garden hose on a mist setting with the sun directly behind them. This demonstrates the localized nature of the phenomenon, where the water source is mere feet away. Such controlled conditions allow observers to fine-tune their position and experiment with seeing the brightest possible colors.
Seeking Out Unique Rainbow Phenomena
Beyond the primary bow, unique atmospheric conditions can produce several related phenomena. A double rainbow, or secondary bow, forms when light undergoes two internal reflections within the raindrop. This extra reflection reverses the colors of the secondary arc, placing red on the inside and violet on the outside. The bow appears fainter and wider than the primary one.
Between the primary and secondary arcs, a noticeably darker strip of sky often appears, known as Alexander’s Dark Band. This absence of light occurs because the physics of the double reflection prevents sunlight from being returned to the observer from drops in that specific angular region.
For a rare nighttime display, a moonbow, or lunar rainbow, is formed by moonlight rather than direct sunlight. The moon must be nearly full, at least 80 to 90 percent illuminated, to provide sufficient light. Because the human eye loses color sensitivity in low light, moonbows typically appear as a ghostly white arc.
Another variant is the fog bow, sometimes called a white rainbow, which forms in fog or mist rather than rain. Fog droplets are significantly smaller than raindrops, often less than 0.05 millimeters in diameter. This minute size causes the light’s different wavelengths to overlap due to diffraction, resulting in an arc that is mostly white, sometimes with faint reddish outer and bluish inner edges.