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The laser beam is 1_____________ by exciting the atoms of a suitable material - ruby is one - until most of the atoms have electrons orbiting in a higher energy level than usual. The excitation is then stopped and all the excited electrons fall back together, to their 2______________ orbits, each one emitting a pulse of light of the same energy. In this way an intense beam of light is generated for a very short time. And every pulse or wave-train in this beam is in step with every other3______________. In this way a beam of light is obtained which is both monochromatic and coherent and easy to focus. This light is reflected by the mirror at one end of the tube. It can only escape at the other 4______________ of the tube.
