Friday, 14 February 2014

ATMOSPHERIC OPTICS - Rainbow science

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°.

Rainbow science

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 last Friday, the sun was about 18° in the sky. The center of the rainbow was 18° under the horizon and the angular distance between the center and the top of the rainbow is about 42° (diffraction for red light)… that means the red band of the primary was 42-18= 42 degrees high. As I’ve said before, the secondary is some 9° away and almost twice as thick, spanning on this particular occasion between 33 and 37 degrees above the horizon.

Three zones of luminosity can be observed: the brightest under the primary, the darkest in between the two rainbows, and the third above the secondary.

Under the right conditions, the primary bow displays several other greenish-purple bands of color below its lowest, purple fringe. They are called “supernumeraries” and you can only see one (barely) on my picture.

Leaving math and optics aside, this was probably the brightest I’ve ever seen… the photo I’ve posted is a .jpeg with minor enhancements: contrast and exposure. White balance is correct and no hue / shades /saturation changes have been made. I’ve actually shot a hundred or so photos in hope of stacking them to maybe see more supernumeraries and a brighter secondary, but after waiting 8 hours for DSS to align and stack them, the results were catastrophic. I may try again, but not much hope of getting anything worth showing.