OPOD - 6 rainbows

6 Rainbows ~ Michael Gro�mann, the first to photograph the natural 3rd order rainbow and a superb experimenter, has here produced the first six orders of the rainbow.

He made a metal band 300mm in diameter. At its centre a thin (2mm dia) jet of water descends. The water cylinder is lit from the left by a nearly parallel beam from a projector. The beam leaves through a hole at right.

The water cylinder acts as the central slice through a raindrop and sends forth rainbows which show up on the inner rim of the cylinder.

The bright bow at left is the familiar primary bow produced by light deflected back sunwards. The bow's inside is lit white as in nature. To the right of the primary - separated by Alexanders dark band - is the reverse colour secondary bow.

At right - near the 'sun' - the ring is lit by white light, the 'zero order glow' of rays passing through the water slightly deflected but not internally reflected.

Then the fun starts! Michael has aligned the light beam so that higher order bows appear slightly above the lower order ones and are not swamped by the latter. His 3rd order bow of three internal reflections inside a drop or cylinder appears at right with the 4th order bow nearby. They are wider than the primary and secondary. Their width, faintness and the bright sky from the zero order glow combine to make them almost impossible to see.

At left, light has internally reflected five and six times inside the water cylinder to produce faint 5th and 6th order rainbows. Someday someone will surely capture the 5th order as it partially lies inside Alexanders dark band.

Michael's experiment looks easy. It is not, it required careful engineering, optics, alignment and much effort to get a stable water jet.

Top and lower image ©Michael Gro�mann, shown with permission

Above: Silicon and electrons replace Michael's water, glass and metal. A modification of HaloSim where a water sphere has replaced an ice crystal produces a numerical ray tracing simulation of the first six orders of rainbow. The projection is an all-sky view looking directly upwards. The 'sun' is on the horizon at right. The sky is oriented to match Michael's experiment.

The sky lower half containing the primary and secondary bows and the zero order glow is only 7.5% of the brightness of the upper part.

The upper part was numerically filtered so that rays of less than three internal reflections were ignored. Had they been plotted the higher order bows would have been overwhelmed.