Physics Question #2196

JP, a 22 year old male from Tempe asks on August 6, 2004,

If you had a seamless sealed box which was lined with a 100% reflective material, and somehow generated a light-source that would leave no trace for the light to be absorbed by, for instance a theoretical "clean" firecracker, placed in the box just before it was sealed, which consumes itself upon detonation, leaving only the light from the explosion. would you be able to open the box, in a lightless room, at any point after the explosion and see the captured light escape and dissipate? or would the light particles, which also act as waves cancel each other out?

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The answer

Donald J. Barry answered on August 7, 2004

Conservation of energy is a fundamental building block of physics. If electromagnetic radiation is not absorbed, it will continue to propagate, and on opening the box, the same radiation would escape.

The idea of "cancellation" of electromagnetic waves is ingrained into us at an elementary level of science education, but it's almost never taught with a full flavor of the complexities involved with this concept. In general, circumstances which are normally described as "cancellation" are better described by the idea of "alteration of propagation". Noise "cancelling" headphones, for instance, sample an incoming (practically planar) wavefront and issue an expanding spherical wavefront of opposite phase and similar amplitude. This mutes the propagation in a narrow cone from the cancelling speaker along the direction of the original planar wave, but adds ADDITIONAL energy to some other directions, where cancellation does not take place.

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