Flashes of light below the dripping faucet: an optical signal from capillary oscillations of water drops

TL;DR

This paper reveals that the bright flashes observed below dripping water drops are caused by internal reflections related to capillary oscillations, with optical effects similar to rainbows, and explains their dependence on drop shape.

Contribution

It demonstrates that the flashes are due to internal reflections influenced by capillary oscillations and provides a ray tracing analysis explaining the optical phenomena involved.

Findings

Flashes originate from internal reflections similar to rainbows.

Flashes occur during the oblate phase of drop oscillations.

Optical effects are enhanced during the oblate shape, causing bright flashes.

Abstract

Falling water drops from a dripping faucet, illuminated from above, exhibit a row of bright strips of light, a few centimeters apart at a fixed distance below the faucet. Flash photographs of the drops show that they are oblate in shape when the flashes occur and the bright flashes of light originate from the edge of the drop that is on the opposite of the overhead light source. Here we show that that the spots result from the same internal reflection that gives rise to the rainbow in a cloud of spherical drops . The periodic flashes reflect the capillary oscillations of the liquid drop between alternating prolate and oblate shapes and the dramatic enhancement in the oblate phase result from a combination of several optical effects. Ray tracing analysis shows that the flashes occur when the rainbow angle, which is 42 $\deg$ in spherical drops, but sweeps over a wide range between 35 $\deg$ and 65 $\deg$ for typical ellipsoidal drops and the intensity of the caustic is strongly enhanced in the oblate phase. This phenomenon can be seen in all brightly lit water sprays with millimeter size drops and is responsible for their white color.