Sunset Rainbow

An image from Puerto Rico by Frankie Lucena. There are at least four supernumerary fringes inside the highly reddened main primary bow.

©Frankie Lucena, shown with permission
Air molecules have preferentially scattered away the blue and green light from the sunset rays. Some blues and greens show faintly inside the main bow but for practical purposes it is almost monochromatic.

This constriction of colour helps show up the supernumeraries.

The rainbow’s colours overlap and those of the supernumeraries even more so. This colour blurring is largely absent in the highly filtered red sunset bow and the supernumeraries are sharp and clear.

Try looking at the next rainbow through a colour filter, it could surprise.


Supernumerary formation.

Two classical ray paths contribute to the light at any point on a rainbow.

However, light is a wave phenomenon and the wave crests of the two emerging waves from the ray paths can coincide or be out of phase, depending on the viewing angle and wavelength. When the crests coincide there is a bright arc, a supernumerary bow. The result is a series of fringes - supernumeraries.

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At right is a numerical simulation (Airy) of a rainbow from drops 1mm mean diameter. The colour overlap combined with some spread of drop sizes blurs out the supernumeraries. They fade away quickly.

At left the same drops are illuminated by red light. Supernumeraries appear in their full glory. Notice too the narrowness of the main bow.