Halos and Flashes

It’s a balmy 3° on a December night in Ithaca, New York. Wearing shorts, you walk outside and look up to see the bright, shining moon. But circling around it is a mysterious halo. What is it?

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It’s not a camera lens flare. This circle of light actually demonstrates several neat features of light.

The moonlight is actually being refracted by ice crystals in the upper atmosphere. Hexagonal crystals with 120° angled sides refract light upon entering and exiting. If you follow Snell’s Law, you can calculate that the light is deflected by 22°. What’s more, you can sometimes see a reddish fringe on the inside of the halo, and a bluish one on the outside. This is caused by dispersion: the index of refraction of ice is different for different wavelengths of light. In this way the moon halo is similar to its cousin, the rainbow.

Now daytime comes. You are on the West Coast of somewhere. The sun begins to set. The sky turns a familiar reddish orange. You watch the sun go down over the ocean, but just as it disappears below the horizon, you see a quick green flash in its place for about two seconds. What is the green flash?

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Start a little earlier. The sky goes red near sunset because of atmospheric scattering. White light, which includes red, green, blue and everything in-between, is deprived of blue and violet light because Rayleigh scattering is much stronger at those short wavelengths. (Now you should be able to figure out why the sky is blue in the day!) Near sunset, red, yellow, and some green remain. The sun actually goes lower than the horizon in reality, but due to refraction, the reddish light bends and the sun appears higher than it is. This is the “red” image of the sun. The “green” image of the sun may be slightly higher, but it isn’t noticeable because they are almost completely superimposed. But then the “red” image falls under the horizon. The “green” image is also nearly down, but for a brief moment, its top is still visible, although no longer bleached out by the red light. You see a green flash!

BRIAN STERN – PHD STUDENT, CORNELL UNIVERSITY

Brian is currently researching ways to improve integrated photonics systems and developing new on-chip technologies.

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