Waves carry energy, not matter

A wave is a way of transferring energy from one place to another without transferring matter. The medium (water, air, a slinky) just oscillates on the spot. There are two shapes:

Watch out: the slinky (or water) does not move along with the wave — it vibrates about a fixed point. Evidence: a floating duck bobs up and down but doesn't get carried to shore. It's the wave that travels, not the medium.

The four words to describe a wave

• Amplitude — maximum displacement from rest (metres). Bigger amplitude = more energy.
• Wavelength (λ) — distance for one complete cycle, e.g. crest to crest (metres).
• Frequency (f) — number of complete waves passing a point each second (hertz, Hz).
• Period (T) — time for one complete wave to pass (seconds).

Frequency and period are two views of the same thing:

On the oscilloscope: a sound wave shown as a trace — taller spike = louder (amplitude); spikes closer together = higher pitch (frequency). The trace itself looks transverse, but the sound is still a longitudinal wave.

How fast does a wave travel?

Every wave obeys the wave equation:

You can also find a wave's speed by timing it over a measured distance — this is how you measure the speed of a wave in the required practical (e.g. ripples across a tank, or the speed of sound down a tube):

Watch out: wavelength is in metres. If you're told 5 cm, use 0.05 m, or your answer will be 100× too big.

Your turn — period

Your turn — wave speed

Your turn — rearrange

Waves at a boundary

When a wave meets the boundary between two materials, three things can happen — and usually a mix of all three:

• Reflection — the wave bounces back (echoes, a mirror).
• Transmission — the wave passes through into the new material.
• Absorption — the wave's energy is taken in by the material (it warms up).

Ultrasound (sound above ~20 000 Hz, beyond human hearing) and sonar use partial reflection at boundaries: a pulse is sent in, reflects off a boundary deeper in, and the echo time gives the distance. Uses include foetal scans, detecting flaws in metal, and ships measuring the depth of the sea floor.

HT extension (P5.1f): when sound passes from one medium to another its speed and wavelength change together, but its frequency stays the same.

Modelling & measuring waves

• Ripples on water model a transverse wave — you can see crests and troughs and measure λ directly.
• Sound in air is a longitudinal wave — you can find its speed by timing an echo, or with two microphones a known distance apart.
• Floating objects bob but don't drift along — evidence that the wave, not the water or air, travels.

HT — hearing (P5.1h–i): the ear converts sound waves in air into vibrations of solid parts (eardrum → tiny bones → cochlea). This only works over a limited frequency range — roughly 20 Hz to 20 000 Hz in healthy young humans — and the upper limit drops with age.

Which wave is which?

The electromagnetic spectrum

EM waves are a family of transverse waves that transfer energy from a source to an absorber. In a vacuum they all travel at the same speed — the speed of light, 3 × 10⁸ m/s. They differ only in frequency and wavelength:

Watch out: a common error is thinking these are separate, unrelated things. They are one continuous spectrum — and crucially, in a vacuum all EM waves travel at the same speed. They are not "faster" or "slower" because of their frequency.

How EM waves are made & detected

• Light is an EM wave — visible light is just the thin band our eyes can detect (red → violet). We can only see a tiny slice of the whole spectrum.
• EM waves are generated by changes in atoms and nuclei, and are absorbed by them too — so atoms can emit and soak up radiation across a wide frequency range.
• HT (P5.2j): radio waves can be produced by — and can themselves induce — oscillations in electrical circuits.

Generated & absorbed: heating an object makes it emit infra-red; nuclear changes can release gamma rays. Absorption is the reverse — energy is transferred from the wave into the absorber.

Uses & dangers

• Radio — TV & radio broadcasting, communications.
• Microwave — cooking (heats water in food), satellite & mobile-phone signals.
• Infra-red — heating, cooking, remote controls, thermal imaging, optical fibres.
• Visible — seeing, photography, fibre-optic communication.
• Ultraviolet — security marking, sun-tanning, sterilising water. Danger: skin damage & cancer, eye damage.
• X-rays — medical imaging of bones, airport security. Danger: ionising — can mutate cells & cause cancer.
• Gamma — sterilising equipment & food, killing cancer cells (radiotherapy), medical tracers. Danger: highly ionising — kills/mutates cells.

The pattern: the higher the frequency (UV → X-ray → gamma), the more energy each wave carries and the more hazardous to body tissue it is, because it becomes ionising. HT (P5.2i): differences in how tissues absorb/reflect IR, X-rays, gamma and ultrasound let us image structures hidden inside the body.

Put the spectrum in order

Tap the band that has the longest wavelength of those still left. Build the spectrum radio → gamma.

Spot the right statement

Lenses & ray diagrams

A lens refracts (bends) light because light changes speed when it enters the glass. OCR expects you to use ray diagrams (qualitative — no equations) to compare two lenses:

HT (P5.3a–b): how much a substance absorbs, transmits, refracts or reflects EM waves can vary with wavelength, and refraction effects come from the wave's change in velocity between substances.

Colour, filters & reflection

• An object's colour comes from differential absorption, transmission and reflection: a red apple reflects red light and absorbs the rest.
• A colour filter transmits its own colour and absorbs the others — it does not add colour to the light.
• Specular reflection (smooth surface, like a mirror) gives a clear image; diffuse scattering (rough surface) sends rays in all directions.

Watch out: mixing coloured light is not the same as mixing paint. A filter can only remove colours — never create new ones.

Lenses & colour

Unstable nuclei decay — at random

Some nuclei are unstable. To become more stable they emit radiation. We can't predict when any one nucleus will decay — radioactive decay is a random, spontaneous process. The unstable nuclei can emit:

• Alpha (α) — a helium nucleus: 2 protons + 2 neutrons. Symbol ⁴₂He (or ⁴₂α).
• Beta (β) — a fast electron,  ⁰₋₁e, made when a neutron turns into a proton.
• Gamma (γ) — a high-frequency EM wave; carries away energy but no mass or charge.
• Neutron (n) — some unstable nuclei can also emit a neutron, ¹₀n.

Use the P1 atom model, don't re-teach it: you already know an atom is a tiny positive nucleus of protons + neutrons surrounded by electrons, and that isotopes are atoms of the same element with different neutron numbers. Here we use that to follow what happens to mass and charge.

Penetration, range & ionising power

The three radiations differ in how far they travel and what stops them:

Watch out — the trade-off: alpha is the most ionising (it deposits its energy quickly) but the least penetrating. Gamma is the least ionising but the most penetrating. Penetrating power and ionising power run opposite ways.

Match radiation to its barrier

Tap a radiation on the left, then tap the material that just stops it on the right.

Balancing nuclear equations

In any decay, both the mass number (top) and the charge / atomic number (bottom) must be conserved — the totals on each side are equal.

• Alpha: nucleus loses 2 protons + 2 neutrons → mass number falls by 4, atomic number falls by 2.
• Beta: a neutron becomes a proton + the emitted electron → mass number unchanged, atomic number rises by 1.
• Gamma: only energy leaves — no change to mass number or atomic number.

Check both lines balance: 238 = 234 + 4 ✓ and 92 = 90 + 2 ✓. If they don't add up, your equation is wrong.

Balance an alpha decay

What changes in beta decay?

Half-life

Because decay is random, we describe a source by its half-life: the average time for half the unstable nuclei in a sample to decay (equivalently, the time for the count-rate to halve).

HT (P6.1k): after a whole number n of half-lives, the fraction remaining is (½)ⁿ. So the net decline as a ratio after 3 half-lives is 7:1 (1 part left, 7 parts decayed). You should also be able to read half-lives off a graph.

Count-rate after n half-lives

Background radiation

We are always surrounded by low-level background radiation from natural and human-made sources:

• Natural: radon gas from rocks & soil, cosmic rays from space, food and drink, building materials.
• Human-made: medical X-rays & nuclear medicine, fallout, the nuclear industry (a small fraction of the total).

Watch out — subtract the background: a Geiger–Müller tube always picks up background. To find a source's true activity you measure the background count first and subtract it from your readings to get the corrected count-rate. Always correct before doing half-life sums.

Correct for background

Irradiation, contamination & safety

• Irradiation — being exposed to radiation from an outside source. The object does not become radioactive itself, and stops being irradiated when the source is removed.
• Contamination — getting radioactive atoms onto or inside you. They keep emitting radiation until they decay, so contamination is hard to remove and can be long-lasting.

The hazard depends on the half-life and the type of radiation. Inside the body, alpha is most dangerous (very ionising); outside, gamma and beta are the bigger worry because they penetrate the skin.

Uses: smoke detectors (alpha source — short range makes them suitable & safe), medical tracers to image organs, and radiotherapy to control or destroy cancer tissue.

Watch out: an irradiated apple is not radioactive — irradiation does not make things radioactive. Only contamination transfers radioactive material.

Nuclear fission & fusion

• Fission — a large unstable nucleus splits into smaller nuclei, releasing energy and neutrons. Those neutrons can split more nuclei — a chain reaction. In a reactor, control rods absorb neutrons to keep the chain reaction steady.
• Fusion — small nuclei join to form a larger one, releasing energy. This is how the Sun shines. In fusion, mass is converted into the energy of radiation.

HT note: both fission and fusion release energy because the products are more stable than the starting nuclei. Fusion needs enormous temperatures and pressures, which is why it's so hard to do on Earth.

Irradiation or contamination?

Waves or Radioactivity?

Tap a fact, then tap whether it belongs to P5 Waves or P6 Radioactivity.

The must-knows

That's the whole of OCR Gateway P5 Waves in matter and P6 Radioactivity. 🎯