Dear Matthew Alice: A few weeks back I wandered over to the cliffs by the Toney Pines Glider Port to watch the sun set over the ocean. Just as the upper edge of the sun disappeared over the horizon, I witnessed a bright green flash of light. Can you give me a physical and/or physiological explanation for this phenomenon? — Richard Dyck, La Jolla

It’s refraction. And an unclouded horizon line. And extremely clean, still air. And large areas of uniform temperature between you and the sun. All of which suggests that the atmospheric phenomenon called the “green flash” is not an everyday sight. It’s potentially visible at sunrise and sunset, in the desert or over the ocean, wherever you have an unobstructed view of a distant horizon.

The atmospheric envelope around Earth refracts, that is, bends, the rays of the sun in much the same way a prism does. Light is separated into its component rainbow colors, from longer-wavelength red through shorter-wavelength violet. At sunset you see the flash of the green-wavelength rays when the longer- and shorter-wavelength light is refracted or dispersed away from your line of sight. The flash only lasts a second or so before the sun disappears. The green flash could potentially be blue or violet, but the conjunction of atmospheric conditions needed to create this is even rarer, apparently.