If you put a light beam on glass some of the photons are reflected but the most of them will pass through the glass. Depending on the thickness of the glass light will be reflected from 0% till 16% divided on on the front or back site of the glass. But unregarded the thickness of the glass it seems to be that no more than 16% is reflected and with increasing thickness the reflection is going up and down like a sine-wave.

But is this sine-wave only seen in glass or also in other partly transparent materials like plastics or other minerals? Or perhaps is there deviation when other elektromagnetic waves are used like radio or rontgen waves?

As I understood the cause of the sine-wave is a subject of quantumelectrodynamics, but there not really given a reason/cause for this sine-wave it is only a probability of reflection given. But is it true that interference is not a cause of the sine-wave?


1 Answer 1


The sinusoidal oscillation in intensity is simply due to interference. The glass sheet is acting as an etalon, and any transparent material like plastic will act in the same way.

Your insistence on addressing the situation using photons is misplaced. Quantum electrodynamics tells us that treating light as photons can be useful in some circumstances while treating it as a wave can be useful in other circumstances. In this case the wave approximation is the one that best captures the physics involved.

As a side note, a beam of light is not simply a hailstorm of photons and attempting to treat it this way is rarely useful. For more on this see What is the relation between electromagnetic wave and photon?.


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