Supernumerary Rainbows
above Boulder, Colorado, on late afternoon of July 19, 1999.

Copyright 1999 by Mikolaj and Pawel Sawicki.
 

Each picture shows a primary rainbow, a fainter secondary rainbow above it, and several pastel-shaded rainbows inside the primary rainbow.

The primary rainbow results from a single internal reflection of refracted light inside a raindrop, and the secondary rainbow results from a double internal reflection. But the additional rainbows are not explainable by geometric optics, and hence had been termed "supernumerary".

Supernumerary rainbows result from interference of light which undergoes single internal reflection but travels along different paths inside a raindrop. Supernumerary rainbows provide a strong indication of the wave nature of light. In fact, it was a mystery of supernumerary rainbows that prompted Thomas Young to do the famous double-slit experiment in 1801 that confirmed the wave nature of light and led to his explanation of supernumerary rainbows in 1803. For more details, see the paper1 by Jearl E. Walker, and the book2 by R.T. Lee, Jr. and A. Fraser.

While we think we have counted as many as 4 supernumerary rainbows taking the picture, only 3 of them are visible in the developed prints.

References

1.   Jearl D. Walker, “Multiple rainbows from a single drops of water and other liquids”, American Journal of Physics Vol. 44, No. 5, pages 421-433 (May 1976).

2.   Raymond L. Lee, Jr. and Alistair Fraser, The Rainbow Bridge, Chapter 8, The Pennsylvania University Press, 2001.