I'm thinking about a laser interferometer like the one used in LIGO. Here's the basic layout (from Wikipedia - Interferometry):

Michelson Interferometer

My understanding is that the half of the light that is reflected by the beam splitter is 180° out of phase with the light that passes straight through. So, if the distances are exactly equal, the reunited beam that hits the detector will be experiencing fully destructive interference, and won't be detected.

My question concerns that part of the light beam. Is it still light after the interference? The sensor can't detect it, and I'm assuming that if you shined it into your eye you couldn't see it.

Is there any way at all to detect the fully canceled beam? Does it still exert radiation pressure? If you set up another beam splitter in place of the detector, could you split it, adjust the phase of one half, and recombine it into a fully un-canceled beam? Or does the light beam effectively end where the destructive interference begins?


2 Answers 2


Try to visualize how a mechanical wave on a rope behaves after it forms a node with a second wave moving the opposite direction. Does it still behave like a wave on a rope? Yes... one of the two waves keeps going left, the other one right. A quarter wavelength from the minimum (node) there is a maximum (antinode).

Light does very much the same thing, except that it does it in three dimensions. The destructive interferences are localized to certain places, they do not destroy the entire wave nor do they take the energy out of it, so for each dark spot or band there has to be a bright one where the interference is constructive. Depending on what we would like to do with our experiment we will chose the location of the detector (or the distances between the mirrors) so that it can see the dark or the bright areas or we decide to observe a larger part of the pattern because the movement of interference stripes is very convenient to detect.


Here's a link to an experiment I did that may help to answer your question. I guess great minds think alike.Experiment in the Dark Fringe Raises Questions


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