By Linda Conlin, Pro to Pro Managing Editor

After hours of rain, a huge arc of color appeared across the sky – a rainbow, reward for enduring a dreary day. An optical illusion, I know, but the optics are fascinating. To learn more, I found a most informative article from National Geographic. 

As experts on the properties of light, we know that rainbows result from the refraction and reflection of light in water droplets. Light entering the droplet at an angle of 42 degrees is refracted. It is then reflected by the back of the droplet. As this reflected light leaves the droplet, it is refracted again, at multiple angles, separating the wavelengths of light into their corresponding colors. The length of the arc of a rainbow depends on the refractive index of the water droplets through which light passes. The higher the refractive index, the shorter the arc. For example, seawater has a higher refractive index than rainwater, so sea spray produces a rainbow with a shorter arc. And rainbows are actually circular, with the center of the circle called the antisolar point. Because we can see only what appears above the horizon, we perceive it as an arc. What’s more, people viewing rainbows will each have a different horizon and antisolar point, and so will see different rainbows. 

We know, too, that rainbows appear as the ROY G BIV spectrum of visible light:  Red, orange, yellow, green, blue, indigo, violet. But remember that rainbows are refracted sunlight, and so include the invisible spectrum of infrared, ultraviolet and beyond. And the colors of a primary rainbow always appear in the same order, by wavelength. Red has the longest wavelength and will always be on the outside of the arc, progressing through the other colors to violet with the shortest wavelength on the inside. However, when we see a ‘double rainbow,’ the secondary arc is the result of light reflected twice inside the droplets, and so the colors are reversed.

In addition to double rainbows, there are many other types of rainbows, identified by the number of times light is reflected and the angles of reflection, referred to as ‘order.’ National Geographic reports that scientists have identified 200 orders of rainbows! Some rainbows, such as tertiary (third order), are seen looking at the sun, as opposed to away from it. Supernumerary rainbows are lighter and narrower and appear below the primary rainbow. They are the result of constructive and destructive interference of reflected light rays. (Remember AR coatings?) 

The next time it rains, enjoy the rainbow along with the fact that you know much more about it than pretty colors. Learn even more about the science of light with our CE, The A-B-Cs of UV and Sunwear- Part 1, at