Microwave Interference: Reflection from a Moving Metal Plate

A direct microwave and a reflected microwave interfere at the receiver R. Move the metal plate M towards X to see the signal pass through a series of maxima and minima.

Top-down schematic

Waves arriving at R (live)

Signal at R
0.50

Signal strength vs plate position

Live data

Direct path length (T → R)—
Reflected path length (T → M → R)—
Path difference—
Path difference (in wavelengths)—
Phase difference—
Normalised signal strength—
Interference type—
Wavelength λ = 0.03 m (microwaves, 3 cm)

What is happening?

The receiver R picks up two microwaves at the same time: one travelling directly from T, and one reflected from the metal plate M. As M moves, the reflected path length changes, so the path difference between the two waves changes, and so does the phase difference when they arrive at R. This is why the signal passes through a series of maxima and minima rather than simply getting stronger or weaker.

Exam-style answer (4 marks)

As the metal plate is moved, the signal received by R goes through a series of maxima and minima. This happens because the direct and reflected microwaves interfere at the receiver. Moving the plate changes the reflected path length and therefore path difference changes between the two waves. This means the phase difference changes, so the waves alternately arrive in phase, giving constructive interference (a maximum), and in antiphase, giving destructive interference (a minimum).