By Me on Wednesday, May 8, 2002 - 05:07 am:
From the World Wide Web:
Rainbows
A few things to look out for when next you see a rainbow:
As the centre of a rainbow is the antisolar point, it is an arc scribed around the shadow or your head. Since the radius of a rainbow is 42 degrees, it will never be visible if the sun is more than 42 degrees above the horizon.
Look out for a rainbow when there is a spectacular red sunset -- the rainbow reveals the spectrum of the present illumination, so it will be predominantly red too.
The explanation for the formation of rainbows was first provided by Descartes.
Look for the secondary rainbow, with a radius of about 51 degrees. It is much fainter, and the colours are reversed.
Look for the supernumary bow of alternating green and pink on the inner edge of the main bow. This is caused by diffraction within the raindrop, rather than refraction, the cause of the main bow, and is therefore very sensitive to raindrop size. The supernumary bow will be best with the largest drop size (1-2mm), and will become more yellow and vanish altogether as droplet size reduces to around 0.1mm. At less then 0.05mm drop diameter, all colour vanishes from the bow, resulting in a white 'mist-bow'.
The sky between the two rainbows, and the surrounding sky is darker than normal since the light has been 'diverted' into the bow itself. The dark band between to two bows is called 'Alexander's Dark Band' (as opposed to his ragtime one).
The light from the rainbow is polarised. Try looking at one through a polarising filter or polaroid sunglasses; part of it will be missing, and the missing part will move as you rotate the filter.
There is, of course, an infra-red component to the rainbow. You could try photographing it with infra-red film.
Each of your eyes sees a separate rainbow, since it is looking from a slightly different viewpoint. Try making your own rainbow with a garden hose, and then shut each eye alternately, to see it move.
By the Ghoul on Tuesday, July 2, 2002 - 10:14 am:
Those are some cool photos!