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Name: Eric D.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 4/22/2003


Question:
I understand the effects of constructive & destructive interference relative to light waves (e.g. Young's double-slit). My question is, when two light waves of the same wavelength destructively interfere, where does the wave's energy go? Is the energy density at the point of interference zero?


Replies:
Energy is always conserved (when people say we should conserve energy to save the environment, they really mean we should reduce entropy. But that is another question -- feel free to ask).

Energy, however, can be redistributed. In Young's double slit experiment, the light intensity (power) goes like cos^2 of the angle from the optic axis. At the maximum (cos^2 = 1), the intensity is four times the intensity from one slit, which is twice the sum of the intensities if the two slits were independent. Since the average of cos^2 is 1/2, the average intensity is exactly what it would be if there were no interference.

So the energy density (intensity) is zero where the destructive interference is complete and exactly large enough elsewhere so the average comes out to conserve energy.

This is, of course, true for all kinds of interference. Energy is always conserved.

Dick Plano



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