If two waves coincide with peaks and troughs matching they are said to be in phase.

If two periodic waves of similar frequency coincide in phase the waves superimpose their wave energy to produce a wave of double the amplitude. This is constructive interference.

If two waves are exactly out of phase they will interfere destructively to produce zero amplitude.

Interference can arise with one source of waves where the waves travel different distances before meeting.

The difference in the distance travelled - the **path difference** must be a full number of wavelengths for **constructive interference**.

For destructive interference the path difference must be an odd number of half wavelengths.

Interference can also occur with two sources of waves provided:

- the frequency of the sources is the same
- there is a constant phase relationship between the sources

Such sources of waves are termed **coherent**.

If two waves are out of phase but not of the same amplitude will there still be destructive interference?

AAnswer