The green and blue wavelengths in the flash excite electrons in the chromium atoms to a higher energy level. Upon returning to their normal state, the electrons emit their characteristic ruby-red light. The mirrors reflect some of this light back and forth inside the ruby crystal, stimulating other excited chromium atoms to produce more red light, until the light pulse builds up to high power and drains the energy stored in the crystal. The laser flash that escapes through the partially reflecting mirror lasts for only about 300 millionths of a second-but very intense. Early lasers could produce peak powers of some ten thousand watts.
Modern lasers can produce pulses that are billions of times more powerful.
Another characteristic of laser light is that it is coherent. That is, the emitted light waves are in phase with one another and are so nearly parallel that they can travel for long distances without spreading. Coherence means that laser light can be focused with great precision. Many different materials can be used as lasers. Some, like the ruby laser, emit short pulses of laser light.