Rainbows Provide Morning Optics Lesson

Driving to work this morning, most NanoSonic staff were treated to visions of a double rainbow because most of us travel from east to west to arrive at NanoSonic’s headquarters in Giles County. This is relevant because, not only was the sun at our backs, but water droplets in the form of rain were at our fronts, in the west. When sunlight shines onto droplets of moisture in the Earth's atmosphere directly across from the sun, it causes the optical/meteorological phenomenon of a spectrum of light in a single arc, arching across the sky – otherwise known as a rainbow.

If you look closely at the color photo below, it becomes apparent that the so-called "primary rainbow" (the closest to earth and brightest rainbow) shows red on the outer (or upper) part of the arc, and violet on the inner section. This rainbow is caused by light being refracted while entering the water droplets, then reflected on the inside back and refracted again when leaving the droplets. The second arc is above and outside the primary arc, and has the order of its colors reversed - red faces inward toward the primary rainbow. This second rainbow is caused by light reflecting twice inside water droplets. We also know that there are triple (tertiary) and quadruple (quaternary) rainbows!

Also, you’ll notice that the rainbows appear as bands of color. In fact, rainbows span a continuous spectrum of colors; the distinct bands are an artifact of human color vision. If you compare the color photo with the black-and-white version below, no bands are visible on the black-and-white - only a smooth gradation of intensity from a maximum, fading to a minimum at the other side of the arc.

If you look at the region between the two rainbows, it appears dark (Alexander’s Dark Band, as it is called). This dark band reveals itself because: while light below the primary rainbow comes from droplet reflection, and light above the upper (secondary) rainbow also comes from droplet reflection, there is no mechanism for the region between double rainbows to show light reflected from water drops. NanoSonic president Rick Claus says it’s because the dark band “sucks the light out.”

As it so happens, Rick is an optics expert who was awarded his Ph.D. by Johns Hopkins University (JHU), home of the first university optics program in the country. Rick’s advisor was C. Harvey Palmer who wrote a book on weird optical phenomena, including rainbows! And Palmer’s advisor was none other than John Strong, one of the world’s foremost optical scientists, also at JHU. After his optics work at JHU, Rick followed the path to Virginia Tech where he taught and founded a fiber optics research center.

So at NanoSonic’s staff meeting this morning, when the conversation turned towards rainbows, Rick offered up the above information instead of stories of leprechauns and pots of gold!