Using Standing Waves

1) Measure frequency of microwaves.

AS we move the probe towards the source, we encounter weak signal, then strong signal. This is because we have set up a standing wave, so we have nodes and antinodes. When we are near R, the nodes and antinodes are much more distinct than towards S. This is because as the waves travel, they lose energy so the amplitudes get smaller. At R, the waves have travelled similar distances, so the nodes and antinodes are distinct. At S, they have travelled different distances, so the amplitudes of the two waves are different causing less distinct nodes and antinodes.

We find a node, then record the distance. Then go 5 nodes along and record the distance. divide the difference by 2 to find λ.

c=λf

3*10^8 = (0.104 / 3) f

f = 8.65*10^9 /s
= 8.65*10^9 Hz


2) Measuring Speed of Sound.

This is very similar, but using a signal generator and microphone instead of a microwave source and probe, and a CRO instead of an amp.

λ= 2* node to node.
f=whatever pitch is set on the generator.

c=λf