Waves carry energy, not matter

A wave is a disturbance that transfers energy from one place to another without transferring matter. The particles (or fields) only vibrate about a fixed point — they don't travel with the wave.

• A cork on the sea bobs up and down as waves pass — it doesn't get carried to the shore.
• Sound passes across a room, but the air itself stays put — only the energy moves.

Watch out — top misconception: waves do not carry the medium along. They transfer energy only; each particle just oscillates and returns to where it started.

What does a wave move?

Transverse and longitudinal

CCEA splits all waves into two types by how the vibrations line up with the direction of energy transfer:

• Transverse — the vibrations are at right angles (perpendicular) to the direction the wave travels. Examples: water waves, waves on a stretched rope/slinky moved side to side, and all light/electromagnetic waves.
• Longitudinal — the vibrations are parallel to (along) the direction the wave travels. Examples: sound waves and ultrasound.

Slinky demo: shake a slinky side to side for a transverse wave; push it back and forth along its length for a longitudinal wave.

Transverse or longitudinal?

Tap the correct wave type for each example.

Amplitude, wavelength, frequency

• Amplitude — the maximum displacement of a point from its undisturbed (rest) position. Bigger amplitude = more energy. Unit: metres (m).
• Wavelength (λ) — the distance for the wave pattern to repeat once, e.g. crest to crest. Unit: metres (m).
• Frequency (f) — the number of complete waves passing a point each second. Unit: hertz (Hz).

Easy slip: amplitude is measured from the middle (rest) line to the crest — not from trough all the way up to crest (that's twice the amplitude).

Reading a wave

Period: how long one wave takes

The period (T) is the time taken for one complete wave to pass a point. It is the reciprocal of frequency:

So a high frequency means a short period, and vice-versa. If 5 waves pass each second (f = 5 Hz), each one takes T = 1 ÷ 5 = 0.2 s.

Your turn — period

What actually travels in a wave?

How can you time a ripple too fast to follow?

Wave speed: v = f λ

For any wave, the wave speed is the frequency multiplied by the wavelength:

Rearranged: f = v ÷ λ and λ = v ÷ f.

Misconception: a wave's speed is fixed by the medium it travels through — not by its frequency. Raise the frequency in the same medium and the wavelength simply gets shorter so that v = f λ stays the same.

Your turn — wave speed

Your turn — find the wavelength

Measuring wave speed: v = d ÷ t

Speed can also be found from how far the wave travels in a given time:

This is how wave speed is measured in practice:

• Water (ripple tank): count how many waves pass a point in a measured time to get f; freeze the image to measure λ; then use v = f λ.
• Along a string/slinky: time a pulse over a measured distance and use v = d ÷ t.

Your turn — speed from distance & time

How can you time a ripple too fast to follow?

How can an echo measure a distance?

Sound is a longitudinal wave

Sound is a longitudinal wave made of compressions and rarefactions travelling through a medium. Because the particles must pass the vibration on, sound needs a medium — solid, liquid or gas — and cannot travel through a vacuum.

• The frequency of a sound sets its pitch — higher frequency = higher pitch.
• The amplitude of a sound sets its loudness (volume) — bigger amplitude = louder.
• Humans can hear roughly 20 Hz to 20 000 Hz (20 kHz). Sound above 20 000 Hz is ultrasound — used in medicine (e.g. to measure a foetus's head diameter) and in industry (to detect defects in metals).
• The speed of sound differs in different media — fastest in solids, slowest in gases (about 330–340 m/s in air).

Misconception: sound is not "carried by the air moving across the room". The air particles just oscillate back and forth, passing the energy on. In space (no particles) there is no sound at all.

Sound in space

Reflected sound: echoes & sonar

An echo is sound that has reflected off a surface and is heard again. If you know the speed of sound and time the echo, you can find a distance with d = v × t.

Don't forget the "there and back": the timed echo covers double the distance to the object, so depth = ½ × v × t. Radar works the same way to detect aircraft and ships, but uses electromagnetic waves (radio waves or microwaves), which travel at the speed of light.

Your turn — echo timing

Your turn — speed of sound

How can an echo measure a distance?

Waves at barriers and boundaries

In a ripple tank, straight (plane) water waves are drawn as wavefronts: lines joining the crests. The arrow shows the direction the waves travel.

• Reflection at a plane barrier: angle of incidence = angle of reflection. Wavelength, frequency and speed do not change.
• Refraction at a boundary: going into shallower water the waves slow down, so the wavelength gets shorter (the frequency stays the same). Meeting the boundary at an angle, they change direction.

Same as light: light also reflects with i = r, and refracts because it changes speed at a boundary. Water waves are a model you can watch.

The electromagnetic spectrum

Radio waves, light and X-rays are all electromagnetic (EM) waves: transverse waves that all travel at the same speed in a vacuum. They differ in wavelength and frequency. In order of decreasing wavelength:

• Radio waves: TV, radio and communications. CCEA states that large doses are believed to cause cancer and leukaemia; claims of harm from overhead power cables have not been reliably proven.
• Microwaves: cooking, mobile phones, satellites, radar. Cause internal heating of body tissues.
• Infrared: heaters, remote controls, thermal imaging. Felt as heat; can cause skin burns.
• Visible light: seeing, photography, optical fibres. Intense light can damage eyes.
• Ultraviolet: security marking, sterilising water. Certain wavelengths damage skin cells and can lead to skin cancer.
• X-rays: images of bones, airport baggage checks. Damage cells, which may lead to cancer.
• Gamma rays: killing bacteria on food, treating cancer. Damage cells, which may lead to cancer.

The rule: the higher the frequency of the radiation, the more damage it is likely to cause to the body.

Same family, same speed

How do we know the outer core is liquid?

Seismic waves & the Earth

Earthquakes send out seismic waves through the Earth. Two types are worth knowing (this goes beyond the CCEA specification):

• P-waves (primary) — longitudinal, the faster waves, and they travel through both solids and liquids.
• S-waves (secondary) — transverse, slower, and they travel through solids only (not liquids).

Reading the Earth

Transverse or longitudinal?

Tap a wave, then tap the box it belongs in.

How do we know the outer core is liquid?

Match the term to its meaning

Tap a term on the left, then its matching description on the right.

The facts & equations to know

You've covered the core of the CCEA Waves sub-topic — wave types, properties, the equations, sound, echoes & sonar, reflection & refraction, the electromagnetic spectrum, and seismic waves. Press Finish to see your score.