Rays leaving raindrops after three reflections produce a tertiary
rainbow. Unlike the primary and secondary bows which are opposite
the sun and centered on the antisolar point, the tertiary appears
sunwards and centered on the sun. With a nominal radius of 42.5º
it is similar in size to, but very much broader than, the primary
The tertiary bow's total brightness is 24% of the primary bow and we might think
that it ought to be easily visible. There are severe difficulties: (1)
its light is swamped by that of rays that pass through raindrops
without any internal reflection. These generate an intense
glare around the sun, the
zero order glow that masks the tertiary, (2)
the tertiary is much broader than the primary
and secondary, thus its luminosity per unit solid angle of sky
is correspondingly less, (3) glare and scattered light from the nearby sun interferes.
Großmann's 3rd order rainbow. Star field calibration and computation of the sun position and angular properties of the image verified that the coloured arc was indeed the long sought tertiary rainbow. Images ©Michael Großmann, shown with permission
First ever image of natural higher order bow (3rd order)
There are one or two reports of visual sightings, the most significant by Pedgley* but no photographs had ever been reported. All that changed on the evening of May 15, 2011 when Michael Großmann at Kämpfelbach in SW
Germany saw a rain shower approaching from the north. Primary and secondary bows were already visible and the rain then intensified at his camera position. Rain was falling sunward and a dark cloud reduced the intensity of the near-sun sky. He blocked the sun with a tree and thought he could see a faint shimmering trace of a bow for 30s. His image after processing showed a definite rainbow and subsequent very critical and careful analysis have confirmed the sighting (Photographic evidence for the third-order rainbow, M Grossmann, E Schmidt, & A Haussmann, Applied Optics, Vol. 50, Issue 28, pp. F134-F141 (2011)). Michael Großmann has made a special study of rainbows and his success comes from painstaking observation and camera preparation and most important of all, knowing just where to look and what to look for.
The 4th order bow (four internal reflections) is close to the 3rd
order but with reversed colours. Its brightness is only 15% of that
of the primary. That too has now been photographed by Michael Theusner (Photographic observation of a natural fourth-order rainbow, M Theusner, Applied Optics, Vol. 50, Issue 28, pp. F129-F133 (2011)) a month after the first tertiary sighting.