Randomly reading Ken Rockwell pages, I came across a link to a photograph of a "green flash after sunset" (scroll down, as there's not actually the tag there). Apparently it is a physical effect of seeing the sun set on a particularly clear day with the right conditions -- at just the right time for a few seconds as the sun sets the colours get spread (prism like, here due to refraction) in such a way that you see the green end of the spectrum and nothing else. (In theory it's also possible at sunrise, but much harder to predict when/where it'll happen; with sun set you can at least tell where it is going down!)

Some pages dedicated to Green Flash include physical explanations of how the phenomena happens. It's by no means every sunset as the conditions need to be just right. But it helps to know where to look, and a flat low well defined horizon (such as a sea horizon) helps a lot. Pictures are difficult due to the tendancy for the green flash to blow out (and hence go white), and the need for a long telephoto to show much more than a few pixels of the sun at this point as it sets. (Some photography hints, and warnings against artifacts; small picture and explanation from an observatory; fairly clear picture; framed by the Golden Gate bridge, with video; green, blue and violet flashes.)

I'm reminded of knife edge diffraction of radio signals (which can even be used for receiving Digital TV; interesting propogation primer). One of the more interesting bits in a tutorial for my previous job was a description of a reliable DMR (Digital Microwave Radio) link made between two points without line of sight on either side of a steep sloped mountain, by using knife edge diffraction across the top of the mountain. (Interestingly the green flash seems to be explained as refraction (bending due to transition of substance, in this case the earth's atmosphere) where as the microwave radio instance is explained as diffraction (bending due to proximity to edge); comparision of refraction and diffraction in electromagnetic waves.)

See also:

• Ken Rockwell on Color, Timing and the relative unimportance of lens sharpness (since for most practical purposes all modern lenses are sharp enough).

• the origins of Photoshop which was originally written "in a basement" and licensed to Adobe.

• Michael Fatali, photographer who takes photographs with vivid colurs (and sells prints) -- they look more like fine art, than conventional photography