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

These notes condense statements P5.2g to P5.2j of the OCR GCSE (9-1) Gateway Science Physics A specification (J249, Version 5.0, July 2026), topic P5 Waves in matter. P5.2i is tested only in the Higher Tier papers and is labelled below. For full explanations and worked examples, read the study guide first.

Other links: OCR GCSE Physics hub, printable checklist, practice questions and the free 10-minute diagnostics.

Uses of each wave group (P5.2g)

Order from long wavelength and low frequency to short wavelength and high frequency: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.

Group	Uses to quote	One-line reason
Radio	Broadcasting radio and television, communicating with ships and aircraft	Travels long distances through air
Microwave	Satellite links, mobile phone signals, cooking	Passes through the atmosphere. Water in food absorbs it
Infrared	Remote controls, grills and heaters, thermal imaging	Warm objects emit it, surfaces absorb it
Visible	Seeing, photography, lighting, optical fibres	Detected by eyes, reflects from surfaces
Ultraviolet	Fluorescent lamps, security marking, sterilising water, checking banknotes	Makes materials glow, damages microorganisms
X-ray	Imaging bones and teeth, baggage scanners	Passes through soft material, absorbed by dense material
Gamma	Sterilising equipment and food, cancer treatment, tracers	Penetrates strongly and kills cells

Memory hooks:

• Communication: radio, microwave, and infrared or visible in optical fibres.
• Heating: microwave and infrared.
• Imaging: X-ray, gamma, and infrared for temperature.
• Killing microorganisms or cells: ultraviolet and gamma.

Specification demonstrations to remember: microwaves lighting a bulb in a beaker of water, which shows that they heat water in foods, and a phone camera looking at the infrared emitter on a remote control.

Hazards (P5.2h)

Wave	Hazard to body tissue	Typical protection
Ultraviolet	Sunburn, premature skin ageing, skin cancer, eye damage	Sunscreen, clothing, sunglasses, limiting time in strong sunlight
X-rays	Cell damage, DNA damage, cancer	Lead aprons, screens, short exposure, lowest useful dose
Gamma rays	Cell death, DNA damage, cancer, radiation sickness at high doses	Lead or thick concrete shielding, distance, limited time

Key points:

• X-rays and gamma rays are ionising: they can remove electrons from atoms and molecules and so damage cells.
• Damage to DNA can start a cancer. Large doses can kill cells and cause burns.
• The risk grows with the dose and with the time of exposure.
• Radiotherapy aims gamma or X-ray beams at a tumour from several directions, which gives the tumour a high dose and spreads a lower dose across healthy tissue.
• Staff step out of the room because their repeated exposures add up. A patient has one examination with a benefit.

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

Three differences between materials carry the information:

1. Velocity: the wave travels at different speeds in different materials.
2. Absorption: some materials take in more of the wave.
3. Reflection: part of a wave bounces back at a boundary between materials.

Method	What it relies on	What it shows	Note
X-ray	Bone absorbs more than soft tissue	Bone as a shadow, so fractures	Ionising. Barium meal outlines the gut
Gamma tracer	Gamma rays leave the body and are detected outside	Where the tracer collects, so how an organ works	Tracer is swallowed or injected
Infrared	Skin emits and absorbs infrared	Surface temperature patterns	Does not look inside the body
Ultrasound	Partial reflection at boundaries between tissues	Echoes show boundaries and depth	Sound wave, longitudinal, non-ionising, uses gel

Seismic waves use the same idea on the Earth. Reflections and changes of speed at boundaries show the layers, and the fact that some waves pass through only solids shows that part of the core is liquid.

Depth from an echo. The pulse goes there and back, so depth = (speed × time) ÷ 2. Example: speed 1500 m/s and an echo time of 120 microseconds (120 × 10⁻⁶ s) gives 1500 × 120 × 10⁻⁶ = 0.18 m for the round trip, so the boundary is 0.090 m, which is 9.0 cm, deep. The specification asks for qualitative explanations, so treat this as a way of seeing why timing matters.

Choosing between methods (use these phrases):

• Ultrasound is non-ionising, so it is suited to scanning an unborn baby.
• X-rays show bone well but soft tissue poorly.
• Gamma tracers show function rather than structure.
• Thermal imaging shows the surface only.

Radio waves and circuits (P5.2j)

• A transmitter has an alternating current. It makes electrons in the aerial oscillate, and the aerial gives out radio waves of the same frequency.
• A receiver works in reverse. Radio waves make electrons in the receiving aerial oscillate, which produces an alternating current of the same frequency in the receiver circuit.
• The circuit then turns that signal into sound or a picture.

Must-know distinctions

Pair	Difference
X-ray imaging and gamma tracer imaging	X-rays are sent through the body from outside. A tracer inside the body is the source
Imaging and radiotherapy	Imaging uses a small dose to make a picture. Radiotherapy uses a large focused dose to kill cells
Ultrasound and X-rays	Ultrasound is sound, X-rays are electromagnetic. Ultrasound is reflected at boundaries
Transmitting and receiving	Current to waves, then waves to current

Method in steps: answering a “suggest why this wave” question

1. Name the wave group, not just “radiation”.
2. State the property that matters: absorbed, reflected, transmitted, emitted or energetic.
3. Link that property to the job, such as heating, imaging, killing microorganisms or sending a signal.
4. If the question mentions people, add the risk and one way to reduce it.

Example: why is ultraviolet used in a water steriliser? Ultraviolet damages the cells of microorganisms, which kills them, and it does not need the water to be heated or treated with chemicals. The risk to people is skin and eye damage, so the lamp sits inside a closed unit.

Method in steps: explaining an image (Higher Tier only)

1. Say what the wave does in the body: absorbed, reflected or emitted.
2. Say which materials differ, such as bone and soft tissue, or two soft tissues.
3. Say how the difference reaches the detector, such as a shadow, an echo or a bright patch.
4. Add a safety or suitability point if asked, such as ionising or non-ionising.

Quick self-test

1. Name two uses of infrared.
2. Why do microwaves heat food but still work for satellite links?
3. Name three hazards of ultraviolet to the body.
4. Why are X-rays and gamma rays called ionising?
5. Give two ways a radiographer reduces exposure.
6. Why is lead used in protective aprons?
7. (Higher Tier only) Why does bone show up on an X-ray image?
8. (Higher Tier only) Why is gel applied to the skin for an ultrasound scan?
9. (Higher Tier only) A pulse at 1500 m/s returns after 160 microseconds. How deep is the boundary?
10. Describe what happens in a receiving aerial.
11. (Higher Tier only) Why is a gamma tracer better than infrared for finding how a deep organ works?
12. State one difference between imaging with X-rays and treating cancer with gamma rays.

Answers

1. Any two from remote controls, grills or heaters, thermal imaging cameras, optical fibre links.
2. Water in food absorbs microwaves, which heats it. In a satellite link the microwaves pass through the atmosphere and are detected by a receiver.
3. Sunburn, skin cancer, premature skin ageing, eye damage (any three).
4. They can remove electrons from atoms and molecules.
5. Lead apron or screen, standing behind a barrier or leaving the room, short exposure, smallest useful dose (any two).
6. Lead absorbs X-rays and gamma rays strongly, so little reaches the wearer.
7. Bone absorbs more X-rays than soft tissue, so fewer reach the detector behind it.
8. It removes air between the probe and skin. Air would reflect most of the ultrasound at the skin.
9. Round trip = 1500 × 160 × 10⁻⁶ = 0.24 m. Depth = 0.24 ÷ 2 = 0.12 m, which is 12 cm.
10. Radio waves make electrons in the aerial oscillate, which gives an alternating current of the same frequency as the waves.
11. Gamma rays pass out of the body and can be detected, while infrared is absorbed by skin and tissue before it reaches the surface.
12. Imaging uses a small dose to make a picture, but treatment uses a larger, focused dose to kill cancer cells.

Where marks are usually lost

• Writing “radiation” or “rays” when the question needs the name of the wave group.
• Giving a use without a reason, when the question says “explain” or “why”.
• Saying UV, X-rays and gamma rays “burn” you, without mentioning damage to cells or DNA and the risk of cancer.
• Saying the hazard is the same for every wave, instead of naming which wave causes which effect.
• Writing that bones “block” X-rays completely, rather than absorbing more than soft tissue.
• Calling ultrasound an electromagnetic wave.
• Forgetting the there-and-back journey in the echo calculation, so the depth is double the right value.
• Leaving out the unit conversion from microseconds to seconds.
• Mixing up transmitting and receiving, or forgetting that the frequency of the current and the wave match.

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.