I'm not able to quite understand this:

Consider a superposition of 2 opposite mechanical waves: when they interfere destructively, we say that the particles in the region of superposition have 0 intensity. I know it can be explained by the fact that the amplitude is 0 at that instant, but don't the particles have energy, in this case, kinetic which changes as time passes and waves start moving apart. Shouldn't the intensity not be 0 considering this fact?


1 Answer 1


I'm a high school student and acknowledge that my understanding of this concept may not be as in-depth as that of more experienced physicists, and I welcome any corrections or additional insights on this topic.

When two opposite mechanical waves interfere destructively, it means that the crest of one wave overlaps with the trough of the other wave. In this case, the amplitudes of the waves add up to zero at the point of superposition, which means that the displacement of particles from their equilibrium position is zero. When the particles have zero displacement, they have zero velocity and hence zero kinetic energy. Therefore, at the point of superposition, the particles have zero energy and zero intensity.

It is true that as the waves move apart, the particles will gain kinetic energy and the intensity will increase. However, at the point of superposition, where the waves are cancelling each other out, the particles have zero kinetic energy and zero intensity. This is because the energy associated with the wave has been transferred to another region of space, where the waves interfere constructively, and the particles there have a higher energy and intensity.

  • $\begingroup$ I'm not sure about the part of energy redistribution : in the region with constructive interference , isn't the energy the sum of 2 local pulses( a mechanical wave can be thought to be made up of many pulses) encompassing that region ? $\endgroup$
    – Nilaay
    Mar 30 at 5:04
  • $\begingroup$ You are correct that in the region with constructive interference, the energy is the sum of the energy of the two individual waves that are interfering. In the case of destructive interference, the energy associated with the wave is not actually destroyed or lost. Instead, it is redistributed to other parts of the medium where the waves interfere constructively. This is because the total energy of the wave is conserved, which means that energy cannot be created or destroyed, only transferred from one form to another. $\endgroup$ Mar 30 at 5:27
  • $\begingroup$ When two waves interfere destructively, the energy of one wave is used to cancel out the energy of the other wave at the point of interference. This results in a lower amplitude, and hence a lower energy density and intensity at that point. However, the energy that is being canceled out is not actually lost, but rather it is being transferred to other parts of the medium where the waves interfere constructively. $\endgroup$ Mar 30 at 5:27

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