Thread: Why the first reflective light wave in Fabry-Perot cavity does not cancel with the incoming wave?

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Please look at the diagram (a), and the textbook just mention that the incoming wave A will be interference constructively with wave B, but what about the reflective wave in the middle? why the wave in the middle not cancelling out the incoming wave since there is a 180 degree phase change? I really confused about this.

And another similar confusion is as below:
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In a dielectric wave guide, the wavefronts (or E-field) will interference constructively for incidence light at A and the reflective light at C, but what about the E-field creating from B to C, should that interferes with A and C either?


Please help! Thanks for your answers!

come on guys...pls help

pls pls help me, anyone have idea? or is my question not clear or too noob? I get confused of this for a week already...pls help

Originally Posted by paulzhen

Please look at the diagram (a), and the textbook just mention that the incoming wave A will be interference constructively with wave B, but what about the reflective wave in the middle? why the wave in the middle not cancelling out the incoming wave since there is a 180 degree phase change? I really confused about this.

See this website. When waves interfere they don't actually "cancel" one another. Imagine a tank of water. I make a wave at one end, and you make a wave at the other. My wave heads towards you and your wave heads toward me. We can watch our waves moving towards the middle of the tank, and we can see the crest-trough height. Then we watch as our waves meet. We've arranged things carefully so that they're out of phase, and we notice a flat spot when they meet. But after that, the waves just keep on going. They go through one another.

A wave conveys energy, and normally you can see that energy making things move. We call that kinetic energy. When it doesn't make things move we call it potential energy. There's potential energy in the flat spot in the middle of our tank as the waves meet. In the cavity the "flat spot" fills the entire cavity, and we have a standing wave with no apparent motion and no apparent kinetic energy. But whip away one of the sides and that standing wave isn't standing any more. It zips away at the speed of light from a standing start. The energy that was always there is now revealed.

Originally Posted by paulzhen

In a dielectric wave guide, the wavefronts (or E-field) will interference constructively for incidence light at A and the reflective light at C, but what about the E-field creating from B to C, should that interferes with A and C either?

I'm not sure about this. Shouldn't this be the wavefront has to interfere constructively for incident light at A and the reflective light from B? Provided it makes it past B the situation at C is more of the same.

Hi Farsight, thanks for your answer!

I actually agree with the idea that waves do not actually cancel each other.

However, then why a "standing wave" only happens in certain wave lengths (pls refer to the theory of Perot-Fabry cavity)? I mean how does a non-standing wave interfere "destructively" with itself and the wave energy become "disappear" in the end? Where did the energy go compare to a standing wave?

I must misunderstand something here....

The standing wave in the cavity has to have a matching wavelength so that the peaks and troughs line up with one another. Draw a one-wavelength sine wave on a strip of paper and fold it in half. See what I mean? It's really simple. You can use a shorter wavelength too, that's what the m=1,2,3 is all about. Play around with strips of paper with two or more wavelengths drawn on them, you'll soon get it.

For a non-standing wave, I think the energy leaks out as an evanescent wave. You can see a mention of that in the wave guide and something about it here. But I'm not 100% sure about it, so please do do your own research.