A microwave transmitter emitting waves of frequency f f\,f is pointed at a vertical metal reflector plate, creating a stationary wave pattern. A receiver probe is moved along a linear track between the transmitter and the plate to detect the signal intensity.
As the receiver is moved, the student records the position of the first minimum (node) in intensity. The receiver is moved further, and the position of the fifth minimum is recorded. The distance between the first and fifth minima is DDD.
Which expression gives the speed c c\,c of the microwaves?
c=fD2c = \frac{f D}{2}c=2fD
c=fDc = f Dc=fD
c=2fDc = 2f Dc=2fD
c=4fDc = 4f Dc=4fD