Wave behaviour

What you'll learn

• How a wave transfers energy without carrying matter along with it.
• What amplitude (maximum displacement), wavelength (distance from one repeat to the next), frequency (cycles per second) and period (time for one cycle) mean.
• How to use the wave equation $v=f\lambda$ to calculate speed, frequency or wavelength.
• How transverse waves (vibrations across the direction of travel) differ from longitudinal waves (vibrations along the direction of travel), and what happens at material boundaries.

1. Waves transfer energy

A wave is a disturbance that travels from one place to another. Waves can transfer energy and information, but the particles of the material usually only vibrate around fixed positions.

2. Describing wave motion

When you draw a wave, you usually show displacement, which means how far a point is from its rest position and in which direction. A vibration is repeated motion about that rest position.

The diagram below shows the main wave measurements and compares the two main wave types.

Frequency and period are linked:

$$f=\frac{1}{T}T=\frac{1}{f}$$

3. The wave speed equation

For OCR Gateway GCSE Physics, you need to recall and apply the wave speed equation:

$$v=f\lambda$$

where:

• $v$ is wave speed in m/s
• $f$ is frequency in Hz
• $\lambda$ is wavelength in m

You can rearrange it when needed:

$$f=\frac{v}{\lambda }\lambda =\frac{v}{f}$$

4. Transverse and longitudinal waves

In a transverse wave, the vibrations are perpendicular to the direction the wave travels. “Perpendicular” means at right angles. Water surface ripples are used as a GCSE model of transverse waves, and electromagnetic waves are also transverse.

In a longitudinal wave, the vibrations are parallel to the direction the wave travels. Sound waves in air are longitudinal: air particles vibrate back and forth, making compressions where particles are closer together and rarefactions where particles are further apart.

5. Measuring wave speed and showing what moves

For water ripples, you can measure wave speed in a ripple tank by timing how long a crest takes to travel a known distance, or by measuring wavelength and frequency and using $v=f\lambda$.

For sound in air, you can use two microphones connected to an oscilloscope or data logger. A sound reaches the first microphone, then the second; the time delay and distance between microphones give the speed.

Evidence that the wave travels, not the material, includes a cork bobbing up and down on water as ripples pass. The cork is not carried all the way across the tank. Similarly, air particles in a sound wave vibrate backwards and forwards, passing the disturbance on to neighbouring particles.

6. Boundaries between materials

This separate Physics content in J249 looks at what happens when waves reach a material interface, which means the boundary between two materials.

At an interface, some wave energy may be reflected, some transmitted into the next material, and some absorbed by the material.

• Reflection: the wave bounces back into the original medium. Echoes are reflected sound waves.
• Transmission: the wave passes into the next medium. Its speed and wavelength may change.
• Absorption: wave energy is transferred to the material, often increasing its internal energy, so the wave’s amplitude is reduced.

Transmission: speed and wavelength are linked

If frequency stays the same, the equation $v=f\lambda$ tells you what must happen:

• if wave speed increases, wavelength increases
• if wave speed decreases, wavelength decreases

Echoes, ultrasound and sonar

Ultrasound is sound with frequency above the upper limit of human hearing, usually above 20,000 Hz. Ultrasound imaging uses reflections from boundaries inside the body. A pulse is sent in, and echoes return from tissue boundaries. The time taken tells the depth of the boundary, and the strength of the reflection helps build the image or trace.

Sonar works in a similar way, using sound waves in water. A boat sends out a sound pulse and detects the echo from the seabed or an object.

7. Hearing and converting vibrations

This next part is Higher Tier only in separate Physics. You need to know simple examples where wave disturbances are converted between sound waves and vibrations in solids.

A loudspeaker cone vibrates, pushing and pulling air to create a longitudinal sound wave. A microphone diaphragm does the reverse: incoming sound waves make a solid part vibrate, which is then converted into an electrical signal. The ear is a biological example of the same idea.

In the ear, sound waves travel along the ear canal and make the eardrum vibrate. The ossicles, three tiny bones in the middle ear, pass on these vibrations. The cochlea helps convert the vibrations into nerve signals, which travel along the auditory nerve to the brain.

Human hearing only works over a limited frequency range, roughly 20 Hz to 20,000 Hz for a young person. The ear’s structures do not respond equally well to every frequency. With ageing, hearing often becomes less sensitive, especially at high frequencies, because parts of the inner ear become less effective at detecting rapid vibrations.