A microwave transmitter emits waves towards a metal plate. The waves strike the plate and are reflected back along their original path. [Figure showing incident and reflected waves, forming a stationary wave. Points P, Q, R, S, T are marked along the line. T is closest to the plate, P is furthest. Points are minima.] A microwave detector is moved along the line PT. Points P, Q, R, S and T are the positions where minima of intensity are observed. These points are found to be 15 mm apart. What is the frequency of the microwaves?

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