3.1 General properties of waves

A pulse sent along a rope can make a distant end move. A marked point on the rope moves briefly about its original position; it does not travel to the distant end. The pulse has carried energy through the rope without carrying that marked piece of rope with it.

Watch one marked point as well as the overall pattern. The point begins moving only when the disturbance reaches it, and the next points then respond. This delayed sequence is evidence that the wave is propagating.

Measure amplitude from the equilibrium line to a crest or trough, not from crest to trough. Measure wavelength between matching points on consecutive cycles, not between a crest and the nearest trough.

Use $v=f\lambda$, $f=v/\lambda$ or $\lambda=v/f$. Convert kilohertz to hertz and centimetres to metres before substituting.

Electromagnetic radiation, water waves and seismic S-waves (secondary waves) can be modelled as transverse. For a rope pulse travelling horizontally, a marked point may vibrate vertically; those two directions are perpendicular.

The vibration produces alternating compressions, where particles are closer together, and rarefactions, where particles are farther apart. These regions travel even though individual particles only oscillate about their equilibrium positions.

Sound waves and seismic P-waves (primary waves) can be modelled as longitudinal.

The incident wave does not become a new kind of wave. These names describe what its propagation does at a surface, speed-changing boundary or gap.

Keep the wave generator steady while comparing the wavefront pattern before and after each obstacle or depth boundary. Direction of travel is perpendicular to the wavefronts, and wavelength is read from their spacing.

For a fixed gap, increasing wavelength increases diffraction. For a fixed wavelength, decreasing gap width increases diffraction. Both changes increase the ratio $\lambda/\text{gap width}$.

Passing through the gap does not by itself change frequency, speed or wavelength. The spacing of the outgoing wavefronts stays the same as the incident spacing when the medium is unchanged; only their spread changes.

Low-frequency sound has a longer wavelength than high-frequency sound when both travel at the same speed. It therefore diffracts more around a building or hill, so the low-frequency sound can be heard more clearly out of the direct line of sight.