OCR GCSE Physics: Waves and Radioactivity (P5–P6)
OCR GCSE Physics: Waves and Radioactivity (P5–P6)
Topics P5 Waves in matter and P6 Radioactivity of OCR Gateway Science A GCSE Physics (J249) open Paper 2. P5 explains how energy travels as waves — from ripples on a pond to the whole electromagnetic spectrum that carries our light, radio and X-rays. P6 goes inside the atom to the nucleus, explaining radioactive decay, the radiation it emits, and the enormous energies of fission and fusion. Both topics reward precise vocabulary and confident work with a small set of equations. This guide works through them with the definitions and worked examples that earn marks, and it forms part of our complete OCR GCSE Physics revision guide.
P5 — Waves in Matter
Transverse and Longitudinal Waves
A wave transfers energy from place to place without transferring matter — the particles or fields oscillate, but they do not travel with the wave. There are two types:
- In a transverse wave, the oscillations are at right angles to the direction the wave travels. Water waves, and all electromagnetic waves, are transverse.
- In a longitudinal wave, the oscillations are parallel to the direction of travel, producing compressions and rarefactions. Sound is the classic longitudinal wave.
A useful demonstration is a slinky spring: shake it side to side for a transverse wave, push and pull it along its length for a longitudinal one.
Describing Waves
Every wave is described by the same set of quantities:
- Amplitude — the maximum displacement from the rest position; it relates to the energy the wave carries.
- Wavelength (λ) — the distance between two corresponding points on adjacent waves (for example, crest to crest), in metres.
- Frequency (f) — the number of complete waves passing a point each second, in hertz (Hz).
- Period (T) — the time for one complete wave to pass, related to frequency by T=f1.
The Wave Equation
The speed of a wave links its frequency and wavelength through the single most important equation in the topic:
v=fλ
where v is the wave speed in m/s, f the frequency and λ the wavelength. Determining wave speed — in a solid such as a stretched string, and in water in a ripple tank — is one of the required practicals.
Worked example. A water wave has a frequency of 5 Hz and a wavelength of 0.4 m. Its speed is
v=fλ=5×0.4=2 m/s
Wave Behaviour: Reflection and Refraction
When a wave meets a boundary between two materials it can be reflected (bounced back — the basis of echoes and mirrors), refracted (changing direction as it changes speed on entering a new medium), transmitted, or absorbed. Refraction explains why a straw looks bent in a glass of water and why lenses can focus light. On Higher tier, ray diagrams for lenses and the way sound and ultrasound behave extend this section.
The Electromagnetic Spectrum
Electromagnetic (EM) waves are transverse waves that all travel at the same speed in a vacuum — the speed of light — but span an enormous range of wavelengths and frequencies. In order of increasing frequency (and decreasing wavelength), the spectrum runs:
radio waves → microwaves → infrared → visible light → ultraviolet → X-rays → gamma rays.
A memorable order to learn by heart. Each region has characteristic uses and dangers:
| Region | Typical uses | Main dangers |
|---|---|---|
| Radio waves | Television and radio broadcasting | Very low risk |
| Microwaves | Cooking; satellite and mobile communications | Internal heating of tissue |
| Infrared | Heaters, remote controls, thermal imaging | Skin burns |
| Visible light | Vision, photography, fibre-optic communication | Bright light can damage the eye |
| Ultraviolet | Security marking, sterilising, tanning | Skin ageing and skin cancer, eye damage |
| X-rays | Medical imaging of bones, airport security | Ionising — can damage cells and cause cancer |
| Gamma rays | Cancer treatment, sterilising equipment | Ionising — can damage or kill cells |
The higher-frequency waves — ultraviolet, X-rays and gamma rays — are ionising, meaning they carry enough energy to knock electrons from atoms and damage living cells, which is why exposure to them is controlled.
P6 — Radioactivity
The Nuclear Model of the Atom
An atom has a tiny, dense, positively charged nucleus containing protons and neutrons, surrounded by electrons in energy levels. This nuclear model was established by the results of the alpha-scattering experiment, which showed that most of an atom is empty space with the mass and positive charge concentrated at the centre. The number of protons (the atomic number) defines the element; the total number of protons and neutrons is the mass number. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Radioactive Decay and the Three Radiations
Some nuclei are unstable and decay, emitting radiation as they become more stable. Radioactive decay is a random process — you cannot predict when a given nucleus will decay, only the probability. There are three main types of nuclear radiation, and their properties are heavily tested:
| Radiation | What it is | Penetrating power | Ionising power |
|---|---|---|---|
| Alpha (α) | A helium nucleus (2 protons + 2 neutrons) | Stopped by paper or skin; travels a few cm in air | Strongly ionising |
| Beta (β) | A fast-moving electron from the nucleus | Stopped by a few mm of aluminium | Moderately ionising |
| Gamma (γ) | A high-frequency electromagnetic wave | Very penetrating; reduced by thick lead or concrete | Weakly ionising |
An important nuance examiners probe: the danger of each radiation depends on whether the source is inside or outside the body. Outside the body, gamma and beta are more hazardous because they can penetrate to living tissue, while alpha is stopped by the skin. Inside the body, alpha becomes the most dangerous because all its strongly ionising energy is deposited in a small region of tissue.
Nuclear Equations
When a nucleus decays, mass number and charge (atomic number) are conserved — the totals must balance on both sides of the equation. In alpha decay, the nucleus loses 2 protons and 2 neutrons, so the mass number falls by 4 and the atomic number by 2. In beta decay, a neutron changes into a proton and an emitted electron, so the mass number is unchanged but the atomic number increases by 1. Gamma emission carries away energy but changes neither the mass number nor the atomic number. Being able to complete a nuclear equation — filling in a missing mass number or atomic number so both sides balance — is a reliable source of marks.
Half-Life
Because decay is random, we describe how quickly a source decays using its half-life — the average time taken for half the unstable nuclei in a sample to decay, or equivalently for the activity of the source to halve. Half-lives range from fractions of a second to billions of years. If a source has a half-life of 3 hours, then after 3 hours half remains, after 6 hours a quarter remains, after 9 hours an eighth, and so on.
Worked example. A radioactive source has an activity of 800 Bq and a half-life of 2 hours. After 6 hours, three half-lives have passed, so the activity has halved three times:
800→400→200→100 Bq
The activity is 100 Bq.
Uses and Hazards of Radioactivity
Radioactivity has valuable uses — medical tracers and imaging, cancer treatment (radiotherapy), sterilising equipment and food, smoke detectors, and carbon dating — but ionising radiation damages living cells and can cause cancer, so exposure must be minimised through shielding, distance and limiting time. Irradiation (being exposed to a source) and contamination (getting radioactive material on or in you) are distinguished, and the risk assessed accordingly.
Nuclear Fission and Fusion
Two nuclear processes release enormous energy:
- Fission is the splitting of a large, unstable nucleus (such as uranium-235) into two smaller nuclei, releasing energy and further neutrons. Those neutrons can trigger further fissions — a chain reaction — which is controlled in a nuclear reactor to generate electricity.
- Fusion is the joining of two light nuclei (such as isotopes of hydrogen) to form a heavier nucleus, releasing energy. Fusion powers the Sun and stars, but reproducing it on Earth requires extremely high temperatures and pressures, which makes controlled fusion power very difficult to achieve.
How These Topics Connect
P5 and P6 both concern energy on the move — as waves in P5, and as radiation from the nucleus in P6, where gamma rays are themselves part of the electromagnetic spectrum you learned in P5. The ionising, high-frequency end of the EM spectrum links directly to the biological hazards of radiation. Waves reappear in P8 when red-shift provides evidence for the expanding universe, and the nuclear physics of P6 underpins the discussion of nuclear power among the energy resources of P8.
To drill these topics interactively, work through the Waves in Matter course and the Radioactivity course, each taking you from the foundations to exam-level questions with an AI tutor on hand. For calculation and six-mark technique, see the exam technique guide.