Rainbows

Rainbows have been part of human mythology and culture for millennia: from the story of Noah to Viking legends to Native American tribal beliefs to Buddhist traditions. Rainbows have fascinated scientists through the ages as well, including such well known names as Aristotle, Newton and Pascal.

Forming a rainbow requires two items. The first is rain or another source of liquid water drops; the second is a strong light source. While most rainbows we see are caused by sunlight, the moon when it is bright is just as capable in producing bows, called moonbows.

When solar light shines on falling raindrops, it enters the drop and is bent from its straight-line path through a process known as refraction. If the light beam contains multi-coloured wavelengths - unadulterated sunlight contains all the colours of the visible spectrum - they are split by the refraction into bands of pure-colour wavelengths, just as a prism splits "white" light into the spectrum. A portion of these rays, now travelling along slightly different paths than the original beam, is then reflected off the back surface of the raindrop toward the sun. On leaving the drop, a second refraction spreads the colours into a parallel front of beams back toward the sun. Violet light emerges at an angle of 40 degrees relative to the incoming sunlight. Red light exits at an angle of 42 degrees. Other colours emerge from the raindrop at angles between 40 and 42 degrees.

Although we usually see a rainbow as a continuous mix of colours, only one colour actually reaches our eyes from each drop. We see red light from higher-altitude drops producing the outer ring of the bow while their other colour rays are directed outside our view. Violet light from lower-altitude drops produces the inner visible ring, while its longer wavelength rays are directed below our line of sight. It therefore takes millions of falling raindrops to produce the full colour spectrum characteristic of a rainbow arch.

The size of raindrops influences the rainbow we see. The brightest rainbows appear when drop diameters are between 300 and 1000 micrometres. If drop diameters are large - 1000 micrometres or more - red, yellow, and orange colours are bright but blue is weak. When drops are smaller than 300 micrometres, the red colour bands weaken in favour of the blues. Very small water droplets - those less than 30 micrometres in breadth - produce rainbows that are faint and appear almost white. Fog and clouds contain such small droplets, and thus the bows arising from them are known as fogbows or cloudbows rather than rainbows.