← all cardsTopic 11 · Wave Motion · Longitudinal & sound
Longitudinal bookkeeping
λ = C-to-next-C (or R-to-R). C to adjacent R = λ/2, not λ. Pressure ideas exist only for longitudinal waves — never sketch Δp for a transverse wave.
What this actually means
Wavelength is measured compression to next compression, or rarefaction to next rarefaction. Compression to the adjacent rarefaction is only half a wavelength.
Pressure language belongs exclusively to longitudinal waves. A transverse wave has no compressions, so a Δp sketch for one is physics nonsense and marked as such.
The trap
Measuring λ from a compression to the adjacent rarefaction — that is λ/2.
Prove it — watch it be true
- Check the C-to-C marker: exactly λ
- Check the C-to-R marker: exactly λ/2
- Two different rulers — never swap them