Waves Study Guide

The speed of a wave can be determined by measuring its frequency and wavelength, then multiplying them together (wave speed = frequency × wavelength). For sound in air, a common method involves timing how long a sound travels a known distance, or by using an oscilloscope connected to two microphones to measure the time difference for a sound to travel between them. The distance divided by the time gives the speed. Alternatively, resonance in a closed tube can be used to find the wavelength, from which the speed can be calculated if the frequency is known.

For ripples on water, a ripple tank is typically used. A vibrator creates periodic waves, and a stroboscope can be used to 'freeze' the ripples, allowing for the measurement of their wavelength. By knowing the frequency of the vibrator (often marked on it or easily measured) and the measured wavelength, the speed of the ripples can be calculated. The stroboscope's frequency is adjusted until the ripples appear stationary, at which point the flash frequency matches the ripple frequency or a multiple thereof. The distance covered by several wavelengths is measured and then divided by the number of wavelengths to find an average wavelength. It’s important to minimise reflections for accurate measurements in both scenarios.

When a wave pulse is sent from a source, travels to a reflector, and returns, you can calculate the depth or distance to the reflector using the wave speed and the total time measured. The core formula is: depth (or distance to reflector) = (wave speed × round-trip time) / 2. This accounts for the wave traveling to the reflector and back (so, twice the actual one-way distance). This principle allows accurate measurement in applications such as sonar and seismic surveys, where knowing precise wave speeds and measuring round-trip times lets you deduce the depth or distance to a target or boundary.