Two ways a wave can vibrate

A wave transfers energy (and information) from place to place without transferring matter. The particles of the medium just vibrate about a fixed point. There are two types:

• Transverse — the vibrations are at 90° (perpendicular) to the direction the wave travels. Example: ripples on water, waves on a string, all electromagnetic waves.
• Longitudinal — the vibrations are parallel to the direction of travel, making compressions (particles squashed together) and rarefactions (particles spread apart). Example: sound.

Watch out: a wave carries energy, not matter. A cork on a pond just bobs up and down as ripples pass — it doesn't travel across with the wave.

Transverse or longitudinal?

Amplitude, wavelength, frequency, period

Eduqas expects you to define each of these:

• Amplitude — the maximum displacement of a point from its rest position (height of a crest). Bigger amplitude = more energy.
• Wavelength (λ) — the distance between two matching points on adjacent waves (e.g. crest to crest), in metres.
• Frequency (f) — the number of complete waves passing a point each second, measured in hertz (Hz).
• Period (T) — the time for one complete wave to pass, in seconds.

Link: a high-frequency wave has a short period. If 5 waves pass each second (f = 5 Hz), each one takes T = 1 ÷ 5 = 0.2 s.

Your turn — frequency from period

What actually travels in a wave?

Wave speed = frequency × wavelength

For any wave, the speed it travels is linked to its frequency and wavelength:

You can also find speed directly from the distance a wave travels in a time:

Watch out: the speed of a wave depends on the medium it travels through, not on its frequency. In a single medium, if you increase the frequency, the wavelength must get shorter to keep v the same.

Your turn — wave speed

Your turn — rearranging

Measuring the speed of waves

Eduqas's specified practical is SP5.1 Investigation of water waves (ripple tank). Waves on a string give a useful second way to measure wave speed:

• Ripple tank (water waves): a vibrating bar makes straight ripples. Use a stroboscope or video to freeze them, measure the wavelength with a ruler, count the frequency of the dipper, then use v = f λ.
• Waves on a string: a signal generator drives a vibration generator to set up a standing wave. Measure the wavelength from the loops and read the frequency off the generator, then use v = f λ.

How can an echo measure a distance?

Sound and human hearing

Sound is a longitudinal wave. A vibrating object pushes on the particles of a medium, sending compressions and rarefactions outwards. Sound therefore needs a medium — it cannot travel through a vacuum.

Higher tier: the human ear can detect sound over the range:

Watch out: louder sound = bigger amplitude. Higher-pitched sound = higher frequency. Don't mix the two up.

The ear works because the eardrum vibrates when these pressure variations reach it; tiny bones in the middle ear pass the vibrations to the cochlea, which sends signals along the auditory nerve. These parts can only vibrate in step over a limited range of frequencies, which is why hearing is limited to this range.

The range of hearing

Ultrasound

Ultrasound is sound with a frequency above 20 000 Hz — too high for humans to hear. Because it reflects from boundaries between different materials, it is used to explore structures without cutting them open:

• Medical imaging — pulses sent into the body reflect off the boundaries between tissues; the echoes build up an image (e.g. scanning a foetus in the womb). It is non-ionising, so it is safe to use.
• Echo sounding / sonar — a ship sends an ultrasound pulse down to the seabed and times the echo to find the depth, or to detect shoals of fish.

For echo sounding, the pulse travels down and back, so it covers twice the depth. The depth is therefore half of speed × time:

Your turn — echo sounding

How can an echo measure a distance?

How do we know the outer core is liquid?

Reflection and refraction at a boundary

When a wave meets a boundary between two materials, part of it can be reflected, part absorbed, and part transmitted (passes through), where it may be refracted:

• Reflection — the wave bounces off the boundary. The angle of incidence equals the angle of reflection (an echo is reflected sound).
• Refraction — the wave that crosses into the new material changes speed. If it slows down it bends towards the normal; if it speeds up it bends away. Its frequency stays the same, so its wavelength changes.

Watch out: refraction happens because the wave changes speed at the boundary — not just because it "bends". If it hits the boundary head-on (along the normal) it slows but does not change direction.

Why does it bend?

Seismic waves inside the Earth

Earthquakes send out seismic waves that travel through the Earth. Because the two types behave differently, recording them on the surface lets us map what is deep inside — a structure we can never reach directly:

• P-waves (primary) — longitudinal. They are faster and travel through both solids and liquids.
• S-waves (secondary) — transverse. They are slower and travel through solids only — they cannot pass through a liquid.

Watch out: the absence of S-waves in the shadow zone tells us the outer core is liquid (transverse S-waves cannot pass through a liquid). The waves also refract: their speed changes with depth, so their paths curve, and they bend sharply at the boundary of the core.

P-waves vs S-waves

Tap a wave fact on the left, then tap its matching wave type on the right.

Reading the shadow zone

How do we know the outer core is liquid?

The key facts to know

You've covered the key ideas in Eduqas Topic 5 — waves in air, fluids and solids, and waves at material interfaces. Press Finish to see your score.