# Condition for constructive interference is that one wave has to be a $\lambda$ ahead of the other

These 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...

• 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. Nov 2, 2022 at 18:15