enter image description hereThese are the definitions in my book:

  1. Constructive interference: When waves from two sources meet and the amplitude of the resultant wave is greater than the amplitudes of each of the individual waves, the waves are said to undergo constructive interference.
  2. Amplitude: The maximum distance of any particle from its undisturbed position.
  3. In order for waves to undergo constructive interference, they must be a $\lambda$ ahead of the other.

Consider the wave (1+2) here. The amplitude of the wave (the maximum distance from its rest position) is clearly greater than the amplitude of any of the individual waves. Yet, they are not a $\lambda$ away from each other...


1 Answer 1


The condition you are mentioning is restricted to the case of two waves that have the same wavelength. These two waves do not have the same wavelength. Constructive interference is a more general thing where if two peaks line up, then they add together to make something larger; conversely, destructive interference occurs when the two waves overlap when the peak of one overlaps with the minimum of another. Since these two waves have slightly different wavelengths, sometimes their peaks overlap and sometimes the peak of one overlaps with the minimum of the other. Thus, you get periodic variations in the amplitude in the superposition of the waves. This phenomenon is called "beats" or "beating".

  • $\begingroup$ Let's say you had two waves of the same wavelength and frequency. What would happen if one wave is $\frac{\lambda}{12}$ out of phase from each other? The resultant amplitude would still be larger than any of the individual amplitude. E.g. prnt.sc/475W1dRiqn-c I know these waves aren't exactly drawn perfect but you get the idea - these waves aren't a whole number of $\lambda$s apart yet a bigger amplitude is formed. $\endgroup$
    – photon
    Nov 2, 2022 at 18:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.