COHERENT LIGHT

Exactly like the forward force of the double-tailed dragonfish, and this time I have made up nothing. The sum of the two pulses, or oscillation trains of the same frequency (color), will depend on the phase of the two: if they start each cycle of oscillation at the same moment (if they are "in phase"), they will reinforce each other and produce double the field and four times the power of a single pulse; if they are exactly opposed to each other ("in antiphase"), they will cancel each other, producing nothing. A general phase difference will produce a result between these two extremes.

This is shown in the figure; the thick continuous line is the resulting oscillation of the two components.

But a single or even several such wave trains (photons) are too weak to be seen by the eye or detected by instruments. What we see as the fight of a candle or the sun is zillions of such pulses from individual atoms, with different frequencies (colors), amplitudes and phases. Even if we pass the light through a filter, rejecting all but a narrow band of colors, the individual wave trains are still randomly phased, so that their total power is n times the power of one wave train. Light whose components add in this way, because its components are uncoordinated and random, is called incoherent.

The reason why this is important is that when "hash" light is focused by lenses or mirrors, each of its components is focused into a slightly different point of a focal volume. That volume is small, so small that when the image of the sun is focused into it, the paper or hay in it will catch fire. But laser light is so pure that when it is focused, it will concentrate almost to a point; the concentration of power at that point is so high that it will melt and vaporize metal almost instantly.