Sunday night rainbow was a showstopper

A double rainbow appeared over Island Alpaca Farm in Oak Bluffs as the herd went for a romp. — Photo by Ralph Stewart

A cold front that pushed across New England late Saturday night and through the day Sunday relieved Martha’s Vineyard of a stretch of hot, humid weather.

Intermittent rain showers Sunday were the prelude to the grand finale, a rainbow, at times double rainbow, that appeared just before 6 pm and lasted for a good 45 minutes according to onlookers.

Rob Gatchell, who was fishing at big bridge on the Edgartown-Oak Bluffs line, said the rainbow stopped traffic as people got out of their cars for a better view.

Click on more photos to see the entire lineup.

For the science behind the colors The Times consulted the Department of Atmospheric Sciences atthe University of Illinois at Urbana-Champaign.

A rainbow is an arc of concentric colored bands that develops when sunlight interacts with rain drops.

A rainbow occurs when rain is falling in one portion of the sky and the sun is shining in another. For a rainbow to be seen, the sun must be behind an observer who is facing falling rain.

If the angle between the refracted light and the normal to the drop surface is greater than a critical angle, the light reflects off the back of the drop.

The critical angle for water (which would apply to raindrops) is 48 degrees (relative to the normal). Therefore, if light strikes the back of a raindrop at an angle greater than 48 degrees, it will be reflected back. If the angle is smaller than 48 degrees, the light will simply pass on through.

The reflected light is refracted as it exits the drop. Violet light (bending the most) emerges at an angle of 40 degrees relative to the incoming sunlight while red light (bending the least) exits the drop at an angle of 42 degrees. Other colors of the rainbow leave a raindrop at angles somewhere in between. According to Descartes’ calculations using laws of optics, the three stage refraction-reflection-refraction pattern that light undergoes when passing through a raindrop produces a concentration of outgoing rays along a line that is 42 degrees above the head of an observer’s shadow. This concentration of light rays is the rainbow that we see.

Since only one color of light is observed from each raindrop, an incredible number of raindrops is required to produce the magnificent spectrum of colors that are characteristic of a rainbow.