General wave properties

A rope pulse is conveniently drawn as an up-and-down shape, whereas sound may be represented by crowded and spread-out particle regions. However, a graph of sound pressure against time can also be sinusoidal. That graph does not make sound in air transverse: its vertical axis represents pressure variation, not a sideways particle position. For a classification question, draw one arrow showing propagation and another showing particle oscillation, then state whether the arrows are parallel or perpendicular. This approach also works when the wave travels vertically or diagonally.

The arriving wave drives the boundary at the source frequency, and the transmitted disturbance follows that timing. If the new medium carries the wave more slowly, consecutive wavefronts travel a smaller distance during each period, so the wavelength decreases. For faster travel it increases. A direction change also depends on the incident angle: a wave arriving along the normal can change speed without bending. Distinguish these three statements in a full answer: frequency unchanged, wavelength adjusted to speed, and direction changed only where the geometry requires it.

In a ripple-tank sketch, a narrow gap can send curved wavefronts into the region beyond it. Keep the outgoing wavefront spacing consistent with the incoming spacing if the water depth and frequency have not changed. Spreading concerns the range of directions in which the disturbance travels, not a sudden reduction in wavelength. If the wavelength increases while the gap stays fixed, diffraction generally becomes more prominent as the two sizes become comparable. The intensity may change too, but that is a different property from the shape of the wavefronts.

A distance snapshot shows where different parts of the medium are at one instant, while a time trace shows how one position changes over successive instants. Measuring several complete cycles and dividing by their number can improve precision. Count the gaps between matching points carefully: five crests enclose four full cycles. The waveform need not begin at zero displacement for this method to work. Choose two clearly corresponding points, state the number of intervals between them, and retain the units shown on the horizontal axis.

A mirror can reflect light while absorbing some energy, and a rope pulse can partly reflect at a change of rope type. These observations explain why equal speed does not imply equal amplitude. Apply the statement about unchanged frequency to a stationary boundary: a moving reflector can shift frequency, which is a different situation. When comparing incident and reflected diagrams in the standard school model, preserve wavefront spacing in the same medium and alter travel direction according to the reflection geometry.

Tie a small marker to a rope and send a pulse along it. The marker moves as the pulse passes but does not follow the pulse all the way to the far end. This demonstrates the difference between the pattern’s progress and the movement of a chosen piece of material. In sound, pressure variations pass through air while molecules move locally around their changing positions. An answer should identify both movements explicitly instead of simply saying “nothing moves”, which would deny the particle motion needed for a mechanical wave.