Which of the following conditions is not necessary to observe an interference pattern between two sets of waves?
a) the waves must have the same wavelength.
b) the waves must have a constant phase difference.
c) the waves must have the same amplitude.
d) the waves must have the same frequency.

To achieve an interference pattern, we must have two coherent wave sources. For coherence, the waves must have the same frequency and either have no phase difference or a constant phase difference.

For two waves traveling in the same medium, having the same frequency implies having the same wavelength by v = fλ where v = velocity, f = frequency, and λ = wavelength.

An interference pattern requires all of the above conditions except for having the same amplitude.

Choice C