Demonstrating stationary waves with microwaves
A microwave transmitter faces a metal reflector. The reflected wave has the same type, amplitude, frequency and speed as the incident wave but travels in the opposite direction along the same line, so they superpose to form a stationary wave. Moving a small detector along the line between them gives alternating maxima and minima; the distance between adjacent minima is λ/2, and f = c/λ.
λ = 2 × (distance between adjacent minima) · c = fλ, c = 3.00 × 10⁸ m s⁻¹What this actually means
This is the cleanest of the three required demonstrations because you can see the geometry: one source, one mirror, one probe between them.
Describe it in the mark-scheme order. First say the metal plate reflects the wave, second say the reflected wave has the same amplitude, frequency and speed but opposite direction, third say superposition of the two forms the stationary wave, fourth say the detector locates the nodes and antinodes.
Measure across several minima and divide. Individual minima are broad and easy to misplace, so quoting λ from a single gap gives a poor value.
Because microwaves are electromagnetic, you get the frequency for free from c = fλ. A typical lab microwave has λ around 3 cm, giving f around 10 GHz.
Language matters here. Do not write bright and dark or loud and soft for microwaves. Write maximum and minimum detector reading, or strong and weak signal.
Writing bright and dark fringes for a microwave stationary wave instead of maximum and minimum detector readings.
Prove it — watch it be true
- Use the graphical formation view to confirm that a wave plus its opposite-travelling reflection gives a fixed node pattern.
- Switch on the measurement brackets and confirm adjacent nodes are λ/2 apart, which is exactly what the microwave detector measures.
- Move the probe along the axis and confirm the amplitude readout cycles between zero and maximum, matching the detector's minima and maxima.