Wednesday, 2 April 2008

A physicist on holiday

Well, a typical physicist on holiday - a week in the Maldives scuba diving and what do I take photos of (apart from fish, turtles, rays, sharks and coral)?

A glory, a rising red moon, the green flash and vertically-pointing satellite dishes. The first image is of a glory or anthelion. This one was taken from a seaplane, looking down onto the clouds. In the centre you can see the shadow of the plane and surrounding it are coloured rings. These arise from the sunlight being scattered back from the water droplets and, like in a rainbow, the refraction of the light through the water splits it into colours.

The second image is of the setting sun. Actually the sun set a moment before. Alright, about eight and a half minutes before if you're being fussy. As the sun drops below the horizon you occasionally see a green flash of light. Here I was lucky enough to catch it on ccd. (Damn that doesn't sound right, but it wasn't captured on film so what doyou say?). The atmosphere scatters the blue end of the spectrum (which is why the setting sun looks redder) but the red light is refracted least. After the sun is actually below the horizon you can still see it because the atmosphere acts like a lens, curving the light of the sun towards us. Red is curved less than blue so the red sun disappears before the blue. But blue is scattered more than red so blue is blocked by the atmosphere. That leaves green as the last light you see. It's meant to mean good luck if seen from Ireland or Scotland - either Celtic legend or wry acceptance of the chances of clear weather and a calm, flat sea...

Here the rising moon appears quite red. No I didn't photoshop it! It has only just cleared the horizon and, as it is fairly full (waning gibbous) it rose not long after sunset. The first reflected light to hit it is the red light of the sun - red because of the path back and forth through our atmosphere scatters so much of the bluer light. As it gets higher, it gets whiter. I've never seen this before - not been around on a clear enough moon rise I guess, sufficiently far from light pollution.

This last picture is of the various satellite dishes on the island. Being just a few degrees north of the equator, the two dishes pointing to geostationary satellites are, of course, nearly vertical. If you walk down pretty much any street you'll see arrays of satellite dishes for TV, all pointing the same way. In the UK it'll be pretty close to south because of the position of the relevant satellite. Here you can tell your latitude by the angle of the dish. I thought it was worth recording, but perhaps I'm easily amused...
If you want to know more about glories, sunrises and a huge range of optical phenomena then you couldn't do better than Marcel Minnaert's "Light and Colour in the Outdoors".


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