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While rainbows are a familiar sight, the rare red rainbow—an optical illusion that displays only shades of red—stands out as a truly remarkable celestial event. Though many people study the sky, only one or two of us will see this monochrome arch in a lifetime.
In a classic 1881 issue of Nature, physicist and electrical engineer Silvanus P. Thompson described a vivid red rainbow he observed over Lake Lucerne, Switzerland. He noted that it appeared at sunset, when the sky itself was tinged in deep reds. Unlike the full spectrum rainbow, the red rainbow revealed only red and orange hues.
Like all rainbows, a red rainbow is an optical illusion created when sunlight enters and exits water droplets at an angle of roughly 42°. The phenomenon can manifest over rain, sea spray, waterfalls, or fog.
Sunlight is a mixture of all visible wavelengths. When it passes through a droplet, each wavelength refracts and reflects at a slightly different angle, producing the familiar rainbow spectrum. A red rainbow, however, emerges only when the sun is near the horizon—during sunrise or sunset. At these times, sunlight must travel a longer path through the atmosphere before reaching the droplets.
Shorter wavelengths (yellow to violet) scatter in the air, while the longest wavelengths—red—survive the journey. The result is a rainbow composed solely of red, and occasionally orange, bands.
