How Are Rainbows Formed?

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She finds an article about the science of rainbows and just as she begins to read it she picks up her cup of coffee only to find that it’s empty. Might as well get another cup before she begins to read the article.

Rainbows are the stuff of myth and legend. They’re impossible to ignore — these sudden arcs of vivid colors streaming across the horizon naturally inspire a sense of awe and wonder. They also awaken our scientific curiosity with questions about the form and function of these fantastic phenomena.

‘Mastering Meteorological Magic

The basic rainbow recipe is simple: water and light. But creating the ideal end result isn’t a question of whether you mix the basic ingredients well enough — it’s a question of weather.

As noted by the Met Office, two environmental conditions are required to reveal a rainbow:

• The sun must be 42 degrees (or less) above the horizon

• Water droplets — typically in the form of rain or fog — must be present

Sunlight entering a water droplet is refracted (bent), with each color in the visible spectrum refracting at a different angle due to differing wavelengths. These multiple rays of light are then partially reflected off the inside edge of the droplet and refracted again when leaving. If the sun is more than 42 degrees above the horizon from the observer’s point of view, the refraction won’t be significant enough to produce a rainbow.

If the sun is below 42 degrees and there’s enough water in the air, refracted light will naturally spread out into a familiar spectrum based on its wavelength. Red light has the longest wavelength at 650 nanometers and appears along the rainbow’s top edge. Violet has the shortest wavelength at 400 nanometers and appears at the bottom.