OCR GCSE Physics A: Uses and hazards of electromagnetic radiation (J249)

This guide covers statements P5.2g to P5.2j of the OCR GCSE (9-1) Gateway Science Physics A specification (J249, Version 5.0, July 2026, first assessment 2018). They sit in topic P5, Waves in matter, under P5.2 The electromagnetic spectrum. The statement on hidden structures, P5.2i, is tested only in the Higher Tier papers. The other three statements are assessed in both tiers. The nature of the waves, their common speed and the frequency and wavelength relationships are separate statements and are not taught here.

Related pages: the OCR GCSE Physics hub, the printable checklist, the revision notes, the practice questions and the free 10-minute diagnostics.

What this guide covers

Statement	What you must be able to do	Tier
P5.2g	Give examples of practical uses of radio, microwave, infrared, visible, ultraviolet, X-ray and gamma waves	Both
P5.2h	Describe how ultraviolet waves, X-rays and gamma rays can have hazardous effects, notably on human bodily tissues	Both
P5.2i	Explain, in qualitative terms, how differences in velocity, absorption and reflection between different types of waves in solids and liquids are used to detect and explore hidden structures, notably in the body. The specification names infrared, X-rays, gamma rays and ultrasound as an alternative in medical imaging	Higher Tier only
P5.2j	Recall that radio waves can be produced by, or can themselves induce, oscillations in electrical circuits	Both

P5.2g: Practical uses of each wave group

Each group is used where its properties suit the job. Two matter most: how strongly the material absorbs the waves, and how much energy they carry.

Wave group	Practical uses	Why it suits the job
Radio	Radio and television broadcasting, communicating with aircraft and ships	Passes through air and many buildings, so signals reach receivers over long distances
Microwave	Satellite communications, mobile phone signals, cooking food	Passes through the atmosphere to a satellite. Water molecules in food absorb it, so the food heats up
Infrared	Remote controls, electric grills and heaters, thermal imaging cameras, optical fibre links	Emitted by all warm objects and absorbed by surfaces, which raises their temperature
Visible	Vision, photography, lighting, optical fibres	Our eyes detect it, and it reflects well from surfaces
Ultraviolet	Fluorescent lamps, security marking, sterilising water, detecting forged banknotes	Makes some materials glow (fluoresce) and damages microorganisms
X-ray	Imaging bones and teeth, airport baggage scanners	Passes through soft material but is absorbed by dense material
Gamma	Sterilising medical equipment and food, treating cancer, medical tracers	Very penetrating and energetic enough to kill living cells

The specification suggests a demonstration in which microwaves light a bulb in a beaker of water, showing that microwaves heat the water in foods, and using a phone camera to look at the infrared emitter on a remote control.

Worked example 1: choosing a wave

A hospital needs to sterilise a batch of sealed plastic syringes in their packaging without heating them. Which wave group suits the job, and why?

1. The syringes are sealed, so the radiation must pass through the packaging and reach any microorganisms inside.
2. Heating would damage plastic, so the method must not rely on heating.
3. Gamma rays penetrate packaging and kill bacteria and other microorganisms by damaging their cells.

Answer: gamma rays. A good answer names the wave and gives two linked reasons: penetration of the packaging, and killing of microorganisms.

P5.2h: Hazards of ultraviolet, X-rays and gamma rays

Ultraviolet waves, X-rays and gamma rays carry enough energy per photon to damage living cells. X-rays and gamma rays are ionising: they can remove electrons from atoms and molecules, which makes the molecules unstable or breaks them apart. Ultraviolet carries less energy but can still damage cells at the skin and the eye.

The specification asks for hazardous effects “notably on human bodily tissues”. The main ones are:

• Cell death. A high dose can kill many cells at once, causing burns (sunburn from ultraviolet, radiation burns from X-rays or gamma rays).
• Damage to DNA. Radiation can change the genetic material inside a cell. A damaged cell may then divide without control, which is cancer.
• Ultraviolet effects. Skin cancer, premature skin ageing and eye damage.
• X-ray and gamma effects. Cancer, because ionising radiation can reach deep tissue. Very large doses cause radiation sickness.

The size of the risk depends on the dose, meaning the amount of radiation absorbed, and on how long the exposure lasts. A single dental X-ray gives a small dose. Repeated large exposures give a bigger risk.

Reducing the risk

Hazard	Protection
Ultraviolet from the Sun or sunbeds	Sunscreen, clothing, sunglasses, shade, limiting time outdoors at midday
X-rays in a hospital	Lead aprons for patients and staff, radiographers stepping behind a screen, short exposure times, the smallest dose that gives a useful image
Gamma sources	Lead or thick concrete shielding, storing sources in shielded containers, limiting time near them

The specification suggests investigating the balance of risks for staff and patients during radiotherapy. Radiotherapy uses a beam of gamma rays or X-rays to kill cancer cells. The beam is aimed at the tumour from several directions so that the tumour receives a high dose while the healthy tissue along each path receives much less. Staff operate the machine from another room, because they repeat the work every day and would otherwise build up a large total exposure.

Worked example 2: weighing risk and benefit

A radiographer takes an X-ray of a patient’s wrist. Explain why the radiographer leaves the room but the patient does not.

1. X-rays are ionising and can damage cells and DNA, so exposure carries a risk of cancer.
2. The patient receives one short exposure, and the benefit is a diagnosis of the injury. The small risk is judged worth it.
3. The radiographer takes many X-rays every day. Leaving the room, or standing behind a shield, prevents those small doses adding up over months and years.

Answer: the benefit to the patient outweighs the small risk from one exposure, while the radiographer would have a large total dose from repeated exposures with no personal benefit.

P5.2i: Detecting hidden structures (Higher Tier only)

Waves can show what is inside a body or a planet without cutting it open. The idea is the same each time. A wave enters a material, and what comes out, or what bounces back, depends on how the material changes the wave. Three differences carry the information:

• Velocity. The wave travels at different speeds in different materials. For a pulse of ultrasound this changes how long an echo takes to return.
• Absorption. Some materials absorb a lot of the wave and others absorb little, so the amount that gets through varies from place to place.
• Reflection. Part of the wave is reflected where it meets a boundary between two materials. The reflected part tells you a boundary is there.

X-rays

Soft tissue absorbs few X-rays. Bone absorbs many. A detector placed behind the body records where X-rays arrive. Bones leave a shadow, because fewer X-rays reach the detector behind them, and on the image they show as pale areas. Soft organs are poorly shown because they all absorb about the same. A patient may swallow a barium meal, because barium compounds absorb X-rays strongly and outline the gut.

Gamma rays

A patient is given a tracer, a radioactive substance that emits gamma rays and is carried to an organ by the blood. Gamma rays pass out of the body easily because the body absorbs little of them. A gamma camera outside the body detects where they come from. A region with more tracer is an active region. This is detection of a hidden process, not just a shape.

Infrared

Skin absorbs and emits infrared strongly, so it does not pass through the body. A thermal imaging camera detects the infrared emitted by the surface. Warmer areas, such as inflamed tissue, show differently from cooler areas. This explores the skin surface, not deep structures.

Ultrasound

Ultrasound is a sound wave with frequency above the range of human hearing. It is longitudinal and needs a medium, so it is not part of the electromagnetic spectrum. A probe sends short pulses into the body. At each boundary between two tissues, such as muscle and fluid, part of the pulse is reflected and the rest carries on. The probe detects the echoes. The longer an echo takes, the deeper the boundary. A gel between probe and skin removes air, which would otherwise reflect nearly all the ultrasound at the skin. Ultrasound is non-ionising, so it is used to scan unborn babies, where X-rays would be an unnecessary risk.

Worked example 3: depth from an echo

An ultrasound pulse travels through soft tissue at 1500 m/s. The echo from a boundary arrives 80 microseconds (80 × 10⁻⁶ s) after the pulse is sent. The specification asks for qualitative explanations, but this check shows why timing gives depth.

1. The pulse travels to the boundary and back, so the total distance is 1500 × 80 × 10⁻⁶ = 0.12 m.
2. The boundary is half this distance away: 0.12 ÷ 2 = 0.060 m.

Answer: 6.0 cm deep. If the speed in a different tissue were lower, the same echo time would mean a shallower boundary, which is why differences in velocity matter.

The same idea outside the body

Seismic waves from an earthquake travel through the Earth. Some travel through both solids and liquids, and others only through solids. Reflections and speed changes at boundaries are used to work out the layers inside the Earth, including that part of the core is liquid.

Worked example 4: explaining a scan

Explain why an X-ray image of a broken arm shows the break but an ultrasound probe on the same arm would not help.

1. Bone absorbs more X-rays than the surrounding tissue, so bone casts a clear shadow, and a break appears as a gap or line in that shadow.
2. Ultrasound is reflected strongly at the surface of bone, so little reaches beyond it and the break cannot be seen in the same way.

P5.2j: Radio waves and circuits

Radio waves are produced by oscillations in an electrical circuit, and radio waves can also cause oscillations in a circuit. This is how radio communication works.

• Transmitting. An alternating current in a transmitter circuit makes electrons in the aerial move backwards and forwards. The moving charges give out radio waves with the same frequency as the alternating current.
• Receiving. When radio waves reach a receiving aerial, they make the electrons in the metal oscillate. This produces an alternating current in the receiver circuit with the same frequency as the waves. The circuit processes the signal into sound or a picture.

Worked example 5: radio link

Describe how a radio signal from a transmitter produces a sound in a portable radio.

1. An alternating current in the transmitter circuit makes electrons in the transmitting aerial oscillate, which produces radio waves.
2. The waves travel through the air to the radio’s aerial.
3. The waves make electrons in the receiving aerial oscillate, which gives an alternating current in the radio circuit of the same frequency as the waves.
4. The circuit changes the signal into sound through the loudspeaker.

Common errors

• Writing that “radiation” is dangerous without naming the wave. Microwaves, infrared and radio waves are not listed hazards in this statement.
• Saying X-rays “go through bones”. They are mostly absorbed by bone and mostly pass through soft tissue.
• Treating ultrasound as an electromagnetic wave. It is a sound wave.
• Mixing up radio waves in a circuit with sound. The aerial current is an alternating electrical signal.

Use the revision notes next, then try the practice questions.

Official syllabus

OCR, GCSE (9-1) Gateway Science Physics A (J249) specification, Version 5.0, July 2026, first assessment 2018. Section P5.2 The electromagnetic spectrum, statements P5.2g to P5.2j.