Dear Tom,
Are there specific conditions which predispose to visualization of a double or
supernumerary rainbow?
-Jon Sebastian, Bensenville
Dear Jon,
Area residents were treated to a display of double rainbows (primary and secondary) on
three occasions last spring. A primary rainbow forms when sunlight passes through
raindrops and, after refraction and a single reflection within a raindrop,returns to our eyes
separated into the colors of the spectrum. A wider, fainter secondary rainbow forms
outside the primary when some of the light is reflected twice within the raindrops before
emerging. Supernumerary rainbows sometimes form inside the primary when the
raindrops are all very uniform in size. They result from interference of light, which
undergoes a single internal reflection but travels along different paths inside a raindrop.
