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Q & A: interference and conservation

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Most recent answer: 01/19/2010
Q:
In a superpostion of 2 indentical light waves each with intensity of say I, the intensity at a point where they superimpose is dependent on the phase of the wave. If I shine 2 such light waves in a manner such that they intersect at a point in space such that at the intersection point they are in phase, then the intensity of the resultant wave at that point is 4I. Since intensity is directly proportional to energy, then it appears that 3I worth of energy is produced, with no other corresponding energy loss in the system. Can energy be created in such a manner? I believe not, but I cannot find the mistake I have made. Help please! :)
- Shu Chin (age 19)
Singapore
A:
At the point you mention, with constructive interference, the sum of the input intensities is 2I, but the resulting intensity is 4I. That leaves 2I to be accounted for.

There are always neighboring points where the same two inputs interfere destructively. For 2I input, 0I results. When you add this up over the whole pattern, the net resulting energy is exactly what went in. (I haven't proved that here, but it's true.)

Mike W.

(published on 01/19/2010)

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