Saturday, 7 June 2014


A rain shower shortly before 9pm on June 4th produced this particularly large rainbow @ 39° above horizon

A rainbow appears when drops of rain split the Sun’s white light in Red / Orange / Yellow / Green / Blue / Indigo / Violet (order from the top down). There are always two rainbows: the primary and the much fainter secondary (43% less light spread over 1.8x the angular distance of the primary). The secondary is turned around, so that the rainbows’ two red bands face each other. The distance between them is 9°.

Very large rainbow on June 4th, 2014
(Sun at 2.91° above horizon)

I’ll spare you the math, but the diffraction angle for violet is around 40° and that of red is 42° - this tells us that the thickness of a rainbow is roughly 2 degrees (2.36° to be more precise). The center of the rainbow is the antisolar point – the point exactly opposite the Sun. It is always below the horizon and goes as far below as the Sun is high in the sky. This means that larger rainbows will appear when the sun is low, after sunrise or in the evening. When this particular photo was taken, the sun was about 3° in the sky, making this one of the largest rainbows possible.

Three zones of luminosity can be observed: the brightest under the primary (to the right in the image below), the darkest in between the two rainbows (Alexander's band), and the third above the secondary (to the left). Light is scattered in such a way in between the primary and the secondary so that it doesn't reach the observer. There are other rainbows there - but seen by other people.

Alexander's band - Rainbows seen by others

Under the right conditions, the primary bow displays several other greenish-purple bands of color below its lowest, purple fringe. They are called “supernumeraries” (not present in my picture)

Detail of the primary rainbow - Alexander's band is to the right

Detail of the secondary rainbow - Alexander's band is to the left
The secondary bow has 43% less light spread over 1.8x the angular distance of the primary 
making it considerably fainter