Tuesday, November 3, 2009

Golden Gate: Green Flash!

I was photographing the sunset behind the Golden Gate Bridge tonight and saw the green flash for the first time in my life!

I love astronomy and meteorology and so was really excited to see it. The flash was only visible for 2-3 seconds and I managed to snap one image during that time.

From Wikipedia on the green flash phenomenon:

Green flashes and green rays are optical phenomena that occur shortly after sunset or before sunrise, when a green spot is visible, usually for no more than a second or two, above the sun, or a green ray shoots up from the sunset point. Green flashes can be observed from any altitude. They are usually seen at an unobstructed horizon, such as over the ocean, but are possible over cloud-tops and mountain-tops as well.

The reason for a green flash lies in refraction of light (as in a prism) in the atmosphere: light moves more slowly in the lower, denser air than in the thinner air above, so sunlight rays follow paths that curve slightly, in the same direction as the curvature of the Earth. Higher frequency light (green/blue) curves more than lower frequency light (red/orange), so green/blue rays from the upper limb of the setting sun remain visible after the red rays are obstructed by the curvature of the earth.

Green flashes are enhanced by mirage, which increase the density gradient in the atmosphere, and therefore increase refraction. A green flash is more likely to be seen in clear air, when more of the light from the setting sun reaches the observer without being scattered. We might expect to see a blue flash, but the blue is preferentially scattered out of our line of sight and remaining light ends up looking green.

Canon 5D, Canon 100-400L, f/10, 1/50, ISO 50.


The WoodLand School said...

How incredibly exciting! And to think you were able to capture a photo of it ... wow!

Joy said...

phenomenal capture, Della... what a moment!!