Wave Interference

To see interference, we need two sources with the same frequency and with a constant phase difference. In the given waves,

and
have a constant phase difference , so interference is possible between them.
For.
and ,
frequency is not equal and there is no constant phase difference. For
,
and ,
frequency is different and there is no constant phase difference.

In the normal adjustment of , Path difference between the waves at central location is always zero. So maxima is obtained at central position. Central point on the screen is bright.