Victoria Falls Discontinuous Bows

Images by Radek Grochowski of the Zambezi river plunging 100m on the Zambia - Zimbabwe border. In this helicopter view the bow above the river (bottom right) does not quite match the one from the gorge.  ©Radek Grochowski, shown with permission

The rainbow over the Zambezi river at right is slightly broader and its colours are shifted inwards - the bow has a smaller radius.

Different sized drops are responsible. Drops are thrown up from the tumult in the gorge and those reaching the top are the smallest.

Two main processes shape rainbows, refraction/reflection and diffraction/interference.

Refraction/reflection is independent of drop size. It is enough for a primary bow that the drop is near spherical and that rays are internally reflected once.

Diffraction/interference becomes more important as drops get smaller. We can no longer consider straight line rays but must instead take account of the interaction of clusters of waves. Colours overlap more and they become less saturated. When the drops are small enough the rainbow is no more - We have instead a broad almost colourless fogbow.

Higher order bows?

Rainbows at waterfalls are predictable. Their position from hour to hour is known or can be calculated. With some planning a dark background can be selected for photography.

Ideal for 3rd/4th order bows? Or even the next prize, the 7th order?

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The two rainbows, one from larger drops (left) and (right) one from smaller, are very evident in this image.

A large waterfall is not necessary, a similar effect is visible from a lawn sprinkler.
Airy theory simulations of three rainbows from different mean drop sizes.

As the drop size decreases the bow radius shrinks but the colour separation widens. Colour saturation decreases.

Supernumeraries become more prominent and their spacings increase.