Word: wave-front
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These are the theoretical scenarios. And at first 1987A seemed to be following the rules: it jumped from near invisibility to respectable brightness literally overnight, and while its wave-front speed was high, its spectrum revealed the unmistakable hydrogen-bearing signature of a Type II. But when the International Ultraviolet Explorer satellite reported a rapid drop in ultraviolet light, scientists began to wonder. Says Robert Kirshner, of the Harvard-Smithsonian Center for Astrophysics: "The spectrum we're seeing in the ultraviolet resembles the spectrum of a Type I. That's a puzzle." Admits Texas' Wheeler: "There are some funny features...
...Light moves slower in a denser medium. So when a "wavefront" of light passes at an angle from dense cold air into less dense warm air, the part that reaches the warm air first races ahead of the remainder. This "refraction" has the effect of bending the wave-front toward the cold...
...heated" where the sun enters it vertically. From that point winds of heat-expanded air blow out in all directions, carrying tides of dense ionization, like a jet of water pouring over a round ball. But the Earth-ball is also rotating. Where the expanding waves go in the direction of rotation the wave-front is smooth; where they go against it they are like whitecaps in a tide...
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