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From Wikipedia on how a Salisbury Screen works:

1. When the radar wave strikes the front surface of the dielectric, it is split into two waves.

2. One wave is reflected from the glossy surface screen. The second wave passes into the dielectric layer, is reflected from the metal surface, and passes back out of the dielectric into the air.

3. The extra distance the second wave travels causes it to be 180° out of phase with the first wave by the time it emerges from the dielectric surface.

4. When the second wave reaches the surface, the two waves combine and cancel each other out due to the phenomenon of interference. Therefore there is no wave energy reflected back to the radar receiver.

The Wikipedia article focuses on use to absorb radar, but the same concept can be applied to other wavelengths of light.

The energy of a photon is found with the equation $E=hc/\lambda$

My question is: When the waves cancel each other out, is the energy converted to heat, or is it expressed some other way?

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Read Feynman's excellent Quantum Electrodynamics.

We usually observe light to be reflected in a straight path by a mirror. In reality there is a equal probability for every point of the mirror and every angle of reflection - it's just that usually all but the shortest paths cancel each other out by interference.

A Salisbury screen could be seen as a special mirror reflecting radiation away from its source. The energy missing there goes into constructive interference in the surrounding.

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