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Interference of a bunch of incident electromagnetic waves

  1. Apr 26, 2012 #1

    There is lots of text, but it is actually just a presentation of facts and thoughts to explain where I am in all this.

    There are two main questions:

    1) Are the incident waves from a very distant source, say a star very far away, interfering with one another to create one or more resulting waves reaching your eye or any other detector?

    2) Basically the same question as above, but here for a experiment including a discharge tube, a diffraction grating and a spectrometer.

    I have been searching the internet for a good answer to these thoughts, but no source has actually given any meaningful answers, just some loose facts, often composed by captain obvious himself.

    Anyway, I am taking a course in waves. My wonder is, exactly how do waves originating from a very distant point source interfere with one another. These waves will (approximately) travel the exact same path and will overlap more and more the closer they get to the point of observation, which I assume to be an eye in this case. According to the principle of superposition they will interfere to approximately create a single wave whose wavelength varies. But if this is the case, then a prism must be able to break this wave into fragments and "send" the small wavepackets of different wavelength/frequency in different directions in order to create the famous rainbow pattern. In this case there is nothing that guaranties that the wavelengths of the wavepackets even are located within the visible spectrum.

    Furthermore, now let's use a diffraction grating as example. For the basic principles and derivations of the formulas for the interference pattern on the screen, the waves must be in phase. For this condition to be met, there must be just one (resulting) incident wave consisting of wave fronts with an angle of incidence of 0° relative the gratings normal.

    Also imagine that the distant source is not very very far away, rather just very small, say a discharge tube emitting light of only one wavelength for simplicity, used along with an interferometer. The experimental evidence points in the direction of that there is a single wavefront arriving at the diffraction grating causing the sources of the diffracted light to be in phase. If this is the case, then the lightwaves from the discharge tube must interfere and create a single wave, but the wavelength of this wave would not be constant, and hence cause the diffracted light to have different wavelength, which is not the case when referring to the experimental results. As you see, the course litterature presents facts that, on this level, are contradictory and any attempt to come up with explanations results in even more contradictions.

    This may of course be due to the contradictory characteristics of light, so that there are actually no contradictions in what I wrote.

    Any thoughts and relevant facts would be highly appreciated, but please be clear about whether you are presenting facts or speaking from your own mind.

    Thank you

  2. jcsd
  3. Apr 27, 2012 #2


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    Staff Emeritus
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    To my knowledge, EM waves are typically required to be coherent to interfere with each other. Different wavelengths of light don't usually do so. I'm not sure if there are special circumstances though. My knowledge of interference effects is extremely limited, so I don't guarantee that any of the above is correct.
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