Transverse and longitudinal waves, the wave quantities (amplitude, wavelength, frequency, period), the wave equation, and reflection and refraction.

What this dot point is asking

Edexcel wants you to distinguish transverse and longitudinal waves, define the wave quantities (amplitude, wavelength, frequency and period), use the wave equation, and describe reflection and refraction at boundaries.

Transverse and longitudinal waves

The wave quantities

Every wave can be described by four key quantities, which you should be able to identify on a diagram of a wave:

• Amplitude: the maximum displacement of a point from its rest position (related to the energy the wave carries).
• Wavelength ($\lambda$): the distance of one complete wave (for example crest to crest), in metres.
• Frequency ($f$): the number of complete waves passing a point per second, in hertz ($Hz$).
• Period ($T$): the time for one complete wave to pass, in seconds.

The wave equation

Reflection and refraction

When a wave meets a boundary between two materials, it can be:

• Reflected: the wave bounces back (for example an echo, or light off a mirror). The angle of incidence equals the angle of reflection, both measured from the normal (a line at right angles to the surface).
• Refracted: the wave changes direction as it passes into a new material because its speed changes. Light slows down and bends towards the normal when it enters a denser material such as glass, and speeds up and bends away when it leaves.

The wave equation can be combined with the frequency-period link to solve a wide range of problems. For example, if you know the period you can find the frequency from $f = \dfrac{1}{T}$, then use $v = f\lambda$ to find the wave speed or wavelength. Always check your units: frequency must be in hertz, wavelength in metres and speed in metres per second.

It helps to keep the quantities clear. The amplitude describes how much energy the wave carries (a louder sound or brighter light has a larger amplitude), while the frequency describes the pitch of a sound or the colour of light. Two waves can have the same frequency but different amplitudes, or the same amplitude but different frequencies. Being able to read these quantities off a diagram of a wave, and to use them in the wave equation, is a core skill in this topic.

Try this

Q1. State the wave equation. [1 mark]

• Cue. Wave speed = frequency times wavelength ($v = f\lambda$).

Q2. Give one example of a longitudinal wave. [1 mark]

• Cue. Sound.