Uses and hazards of electromagnetic waves
Key idea: Electromagnetic waves carry signals, heat matter or reveal structure through differences in absorption. Absorbed waves can heat tissue, while X-rays and gamma rays can ionise and damage cells.
Offline pages
Saved on this device only. Connect at least once every 30 days to confirm Premium. Sign-out or a change of account removes saved copies.
Loading guide controls…
Guided lessons · coming soon
Guided lesson
Guided steps
- Match each use to the wave’s effect on matter.
- Explain each chosen region’s action and result.
- Compare infrared detection with visible-light photography.
- Explain why ultraviolet makes the banknote pattern glow.
- Explain suitcase contrast and why visible light cannot show it.
- Explain how intense non-ionising radiation can damage tissue.
- Explain what the thermal camera detects and emits.
- Compare ionisation with ultraviolet damage to skin and eyes.
- Explain which sunbed risks the goggles reduce.
- Match each control to the source or exposure route.
- Follow radiation from the treatment lamp to the operator.
- Explain the infrared hazard and choose a suitable control.
- Explain X-ray imaging, tissue damage and the switch-off control.
- Calculate both powers, then compare the final temperatures.
On this page
You stopped at:
ContinueEvery region of the electromagnetic spectrum has a job: radio waves carry broadcasts, microwaves cook food, X-rays show broken bones and gamma rays sterilise food. Each use depends on what that wave does when it meets matter, and the same property can make it harmful: too much exposure can heat tissue or damage living cells.
Uses of each region
Each region is useful because of what it does when it meets matter. Some pass through the atmosphere or through a wall; some are absorbed by water and heat it; some are given off by warm objects; some make certain materials glow; some pass through soft tissue more easily than bone; some are energetic enough to damage living cells. Match the job to that interaction.
| Region | Typical uses | What the wave does that makes it useful |
|---|---|---|
| Radio waves | radio and television communication; astronomy; RFID tags | Broadcast signals carry information to an aerial over long distances. Radio telescopes detect radio waves given off by stars and galaxies. An RFID reader and tag exchange an identifying signal; the information is carried by the wave, not by anything travelling from tag to reader. |
| Microwaves | mobile (cell) phones; microwave ovens; satellite television | Phones carry information in microwave signals. Satellite links use microwaves that pass through the atmosphere. In an oven, water in the food absorbs microwaves, which heats the food; ordinary low-power communication signals do not produce the heating used in a microwave oven. |
| Infrared | remote controllers; intruder alarms; thermal imaging | A remote sends coded pulses of infrared to a receiver on the television. Warm bodies give off infrared: an intruder alarm senses the change in infrared as a warm body moves, and a thermal camera maps differences in the infrared given off, so neither needs visible light. |
| Visible light | photography; optical fibres in medicine and telecommunications | A camera records the visible light arriving from a scene. Optical fibres guide light along their length by internal reflection: medical fibres carry light into the body and an image back out, and communication fibres carry signals. (Many communication fibres actually carry infrared.) |
| Ultraviolet | sunbeds; bank-note authentication; disinfecting water | Ultraviolet causes the tanning of skin on a sunbed, which does not make sunbeds safe. It makes special ink on genuine bank notes glow (fluoresce) with visible light. Ultraviolet damages microorganisms in water so they cannot reproduce. |
| X-rays | medical radiology; security screening; industrial defect detection | X-rays pass through some materials more easily than others. Soft tissue lets more through than bone; a detector behind the body records the pattern, so bones show up. Luggage and metal parts are imaged the same way: a hidden object or a crack changes how much gets through. |
| Gamma rays | sterilising food; detecting and treating cancer | Gamma rays are very penetrating: they pass through packaging and kill microorganisms inside the food. To detect cancer, a suitable substance that gives off gamma rays is put into the body and the rays leaving it are detected outside. To treat cancer, a controlled beam of gamma rays kills cancer cells. Detection and treatment are different uses. |
If a question says “state”, one correct region and use is enough. If it says “explain”, name the region, say what the wave does (passes through, is absorbed, is given off by warm objects, makes something glow, damages cells) and link that to the result.
Worked example 1
Choose and justify a region
Problem
Choose a region of the electromagnetic spectrum for each job. Give the region, what the waves do and the result: an image of a broken bone; receiving satellite television; disinfecting drinking water; sterilising packaged food.
Show full solution
An image of a broken bone
Method
Choose X-rays.Reason
They pass through soft tissue more easily than through bone.Working
Answer
The detector receives less behind the bone, so the bone shows up.
Receiving satellite television
Method
Choose microwaves.Reason
They pass through the atmosphere between the satellite and the dish on the roof.Working
Answer
They carry the television signal to the dish.
Disinfecting drinking water
Method
Choose ultraviolet.Reason
It damages microorganisms in the water.Working
Answer
The microorganisms die or cannot reproduce.
Sterilising packaged food
Method
Choose gamma rays.Reason
They pass through the packaging and damage microorganisms inside it.Working
Answer
The food is sterilised without opening the pack. The same damage can happen to human cells, so the source is shielded and exposure is controlled.
Check each answer
Working
Answer
Each answer names a region, what the waves do and the result. A region alone is not an explanation.
Worked example 2
Why not the other choice?
Problem
A camera must find a warm person in an unlit room without switching on a light. Explain which kind of camera works and why the other does not.
Show full solution
Choose the region
Method
Choose a thermal (infrared) camera.Reason
The warm person gives off infrared even in the dark.Working
Answer
The thermal camera detects the infrared the person gives off.
Rule out the alternative
Method
Compare an ordinary camera.Reason
It records visible light, and the dark room gives it too little.Working
Answer
An ordinary camera does not show the person.
Check the explanation
Working
Answer
Writing only “infrared is used in cameras” names a use but does not explain the choice.
Check your understanding 1
A bank clerk holds a note under an ultraviolet lamp and a pattern glows. Explain what the lamp does, and why a device that detects only warmth could not make the same check.
Show answer
The special ink on a genuine note absorbs ultraviolet and gives out visible light, so the pattern glows. The clerk checks for that glowing pattern. A warmth (infrared) detector measures something different: the note would not show the pattern.
Check your understanding 2
A sealed suitcase must be checked without opening it. Explain why an X-ray scanner works and an ordinary camera does not. Why must different contents let through different amounts of X-rays?
Show answer
X-rays pass through the case and some of its contents to a detector. An ordinary camera uses visible light, which does not pass through the case, so it shows only the outside. Objects that let through fewer X-rays than their surroundings show up on the image; if everything let through the same amount, the image would show no difference between objects.
To practise choosing a region for a job, and linking uses to hazards, try the EM Spectrum Explorer.
Heating: over-exposure to radio waves, microwaves and infrared
When tissue absorbs electromagnetic waves, it gains energy. Radio waves, microwaves and infrared do not ionise atoms, but at high enough intensity the absorbed energy raises the temperature of the tissue.
- Microwaves are absorbed by water in the body and can heat internal tissue. The eyes are especially at risk.
- Infrared is absorbed by the skin. Too much can burn it.
- Very intense visible light, such as a laser beam, can damage the eye.
This is why a microwave oven has a metal case and a metal mesh in its door that keep the microwaves inside, and why it switches off when the door is opened.
“Non-ionising” does not mean harmless. The risk depends on how intense the waves are, how long the exposure lasts and which tissue absorbs them.
Check your understanding 3
A firefighter uses a thermal imaging camera to find a person in a smoke-filled room. (a) Explain why the camera uses infrared. (b) A colleague worries that the camera will harm the person. Is the worry justified?
Show answer
(a) The person is warmer than the room around them and gives off more infrared, which the camera detects; it does not need visible light, which the smoke blocks. (b) No. The camera only detects infrared that the person gives off. It sends no waves towards them, so it adds no exposure.
Ionising effects: over-exposure to X-rays and gamma rays
X-rays and gamma rays carry enough energy to knock electrons out of atoms and molecules. This is ionisation. In living cells, ionisation can:
- damage cells and their DNA;
- cause mutations that may lead to cancer;
- kill cells, at high doses.
Ultraviolet also harms living tissue. Too much exposure causes sunburn and damage to the eyes, and raises the risk of skin cancer. The ultraviolet region spans a range of wavelengths, and not all of it is ionising, so describe ultraviolet’s damage to skin and eyes rather than calling all ultraviolet ionising. The World Health Organization’s account of ultraviolet effects explains the harm to skin and eyes.
The same property often makes a wave useful and hazardous. Gamma rays sterilise food and treat cancer because they damage cells; that same damage is the hazard to anyone exposed. X-rays form images because they pass through the body; the part that is absorbed can ionise the cells it passes.
Check your understanding 4
A friend wears goggles that block ultraviolet on a sunbed and says: “My eyes are protected, so I can stay under the lamps as long as I like.” Which part of the risk has the goggles reduced, and which risk remains?
Show answer
The goggles reduce ultraviolet exposure to the eyes. They do not cover the skin, which still receives ultraviolet: sunburn and an increased risk of skin cancer remain. Staying longer increases that exposure, so eye protection does not make unlimited time under the lamps safe.
Matching a control to the exposure
A good safety explanation names the source, how the waves reach a person, and what they do to tissue. A control then blocks that path, removes the source or limits the exposure. “Be careful” is not a control.
Worked example 3
An ultraviolet water-treatment unit
Problem
An ultraviolet water-treatment unit shines ultraviolet into flowing water. The ultraviolet should reach the water, not an operator’s eyes or skin. Identify the hazard and the controls.
Show full solution
Source and path
Method
Find where the radiation comes from and how it could reach a person.Reason
A control must block the path or remove the source.Working
Answer
The ultraviolet lamp, with a direct path to anyone who opens the unit.
Harm
Method
State what the radiation can do to a person.Reason
The harm decides what needs protecting.Working
Answer
Ultraviolet can damage the skin and eyes.
Controls
Working
Answer
A closed casing that ultraviolet cannot pass through blocks the path. Switching off the lamp before the casing is opened removes the source.
The fact that ultraviolet kills microorganisms does not mean it leaves human tissue unaffected.
Check your understanding 5
Someone says an intense infrared heater is harmless because infrared is not ionising. What is wrong with the argument? Give one control that reduces the risk.
Show answer
Not being ionising means infrared does not cause ionisation; it can still be absorbed and heat the skin enough to burn it. Controls include a guard or barrier between the heater and people, keeping people further away, lowering the heater’s output or limiting the time spent close to it.
Check your understanding 6
An X-ray machine inspects parcels on a conveyor belt inside an enclosure. Explain the benefit, the harm to a person who reaches into the beam, and why the X-rays switch off when the enclosure is opened.
Show answer
Different contents let through different amounts of X-rays, so the detector forms an image of what is hidden inside. X-rays reaching a person can ionise atoms in their cells and damage them, even though the person feels nothing. Switching the X-rays off whenever the enclosure is open removes the source when someone could be in the beam.
Check your understanding 7 Compare absorbed power with total heating
Two identical samples start at the same temperature. A absorbs 120 J of infrared in 4.0 s; B absorbs 240 J in 12 s. Assume no energy loss or state change. Calculate each average absorbed power. Which sample finishes hotter?
Show answer
A’s average absorbed power is 120/4.0 = 30 W; B’s is 240/12 = 20 W. A absorbs energy faster during its interval, but B absorbs twice the total energy over its longer interval. With identical samples, the same starting temperature, no losses and no state change, B has the larger temperature rise and finishes hotter. Power is energy per second, not total energy. Infrared transfers energy by heating without ionising the sample.
Challenge yourself
During an X-ray of a broken arm, the patient stays by the machine while the radiographer steps behind a screen. Explain the benefit and the risk of the X-rays for each person, and why they are treated differently.
Check your thinking
The X-rays pass through soft tissue more easily than through bone, so the image shows the break. That benefit goes to the patient. Every exposure can ionise some cells, so it carries a small risk. The patient accepts that risk once, for an image that guides treatment. The radiographer takes many X-rays every day and gains nothing from them, so the screen blocks her exposure each time.
Common mistakes
- Saying all electromagnetic waves are ionising. Radio waves, microwaves and infrared do not ionise; their main hazard is heating.
- Saying non-ionising means safe. Intense microwaves and infrared can burn, and intense light can damage the eye.
- Naming a use when asked to explain it. Name the region, what the wave does and how that gives the result.
- Saying an X-ray image shows X-rays that bounce off bone. Bone lets through fewer X-rays than soft tissue, so it shows up on the detector.
- Treating detecting and treating cancer as the same use of gamma rays. Detection records gamma rays coming out of the body; treatment kills cells with a controlled beam.
- Saying food sterilised with gamma rays becomes radioactive. The gamma rays pass through it; no radioactive substance is added.
Spotted an error? Report a correction
Syllabus and review details
- SEC G2 Science Physics component 2027 · 2027
Physics section, PDF pages 8–23
Last reviewed: