I would like to understand interference using 2 illustrations:

1) Bulbs send out light in all directions, right? If so, at a particular instant of time, shouldn't all the light waves coming out from the bulb interfere and thus gives us light in only direction at that particular instant?

If the above is correct, then we see the bulb emitting light in all directions because at each (infinitesimal) instant, the direction of the resultant wave changes. Is my understanding correct?

2) Below is an image of Bragg's Experiment from Wikipedia -

enter image description here

If the (parallel) light waves getting scattered by the atoms in the solid interfere, then why are the 2 incident light waves shown separately? Shouldn't they interfere and appear as 1 single wave?

Somehow, I can't shake off the feeling that there is something fundamentally wrong about the way I think about light waves and their interference.

Thanks in advance!

  • $\begingroup$ Try to think of the 2 waves as two parts of a single wave. It will be more correct and will come closer to fitting your intuition. $\endgroup$
    – S. McGrew
    Nov 19, 2018 at 18:23
  • $\begingroup$ Or think of the 2 waves as 2 slices through an incoming sinusoidal plane wave--an extended wave that has planes as its surfaces of constant phase. And consider the outgoing wave in the same manner. $\endgroup$
    – user45664
    Nov 19, 2018 at 21:42
  • $\begingroup$ see my answer in your physics.stackexchange.com/q/442004/45664 $\endgroup$
    – user45664
    Nov 19, 2018 at 21:46
  • $\begingroup$ (Think of a plowed cotton field) $\endgroup$
    – user45664
    Nov 19, 2018 at 21:54

2 Answers 2


The light produced by bulb is not coherent, meaning no steady phase relationship. Thus, effect of interference is not observable.

Second part

Actually the disturbance in the field, which is propagating, called wave is spread all over in space. Now for the sake of understanding and explaining wave phenomenon, we use hygens principle or concept of secondary waves. https://en.m.wikipedia.org/wiki/Huygens–Fresnel_principle

This, every point from where the wave passes acts as a secondary source.

Here in this example two out of infinitely possible wave are shown to explain the effect of interference.


Your intuition is right. You have noticed the symmetry between the incoming wave(s) and the outgoing wave(s). The two incoming waves in the figure are parts of a single wave, just as the outgoing waves are parts of a single wave.

I like to illustrate Bragg diffraction differently. Instead of drawing rays with superimposed squiggles, I draw wavefronts.


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