Transverse and longitudinal waves, the wave quantities (amplitude, wavelength, frequency and period), the wave speed equation, the relationship between frequency and period, and the waves practical.

What this topic is asking

OCR wants you to distinguish transverse and longitudinal waves, define the wave quantities, use the wave speed equation, and carry out the waves practical. This is topic P5.1 of the OCR Gateway Physics A (J249) specification, examined on the Paper 2 or Paper 4 side.

Transverse and longitudinal waves

A good test is the slinky spring: shaking it side to side makes a transverse wave, while pushing and pulling it along its length makes a longitudinal wave with visible compressions.

The wave quantities

A larger amplitude means the wave carries more energy (a louder sound or brighter light). The frequency and period are inverses of each other: $f = \dfrac{1}{T}$, so a high-frequency wave has a short period.

The wave speed equation

Rearranging gives $f = \dfrac{v}{\lambda}$ (to find frequency) and $\lambda = \dfrac{v}{f}$ (to find wavelength). For a wave travelling at constant speed, increasing the frequency decreases the wavelength, and vice versa, because their product is fixed.

The waves practical

In the P5 waves practical you measure the speed of waves. For waves on a string, a vibration generator sets up a standing wave; you measure the wavelength from the pattern and read the frequency from the signal generator, then use $v = f\lambda$. For water waves in a ripple tank, you measure the wavelength and the frequency of the ripples to find the speed. A key skill is timing several waves and dividing, to reduce the uncertainty in measuring a single fast event.

Try this

Q1. A wave has a period of $0.20\,\text{s}$. Calculate its frequency. [2 marks]

• Cue. $f = \dfrac{1}{T} = \dfrac{1}{0.20} = 5.0\,\text{Hz}$.

Q2. State what is meant by the wavelength of a wave. [1 mark]

• Cue. The distance of one complete wave (for example, from one crest to the next).