photo #5582253

Think of a rainbow, and you’ll most likely picture a multicoloured arc. But rainbows don’t always contain the full spectrum of hues. At the start and end of the day, ‘all the colours of the rainbow’ are in fact not that many. Rainbows appear when sunlight is refracted as it shines through raindrops and reflects back off their far inner surfaces. The bending, or refraction, as light passes in and out of the water has the effect of separating the wavelengths, and for a full spectrum of visible light, these appear as the rainbow colours. But there isn’t a full spectrum reaching the rain shower when the Sun’s close to the horizon. Its low angle in the sky means its light will have travelled through a lot of the low dense atmosphere before reaching the rain shower. The gasses and particles of the atmosphere scatter more effectively shorter wavelengths of visible light, which appear blue, green, and yellow, than the longer red-looking ones. With the blues, greens, and yellows scattered away, only the red-looking light is left to form a mostly monochrome rainbow, or red bow.Double monochrome rainbow spotted at sunset against a Stratocumulus sky over Trial Islands, British Columbia, Canada by James Gardner (Member 43,543). The primary and secondary bows would in fact have been completely concentric, and only appear to diverge due to the distortion of James’s wide-angle lens.