Uses and Dangers of Electromagnetic Radiation

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 3.12 — Uses of each electromagnetic radiation
• 3.13 — Detrimental effects of excessive exposure to electromagnetic waves, and protective measures

Specification section 3(b) Properties of waves — also taught in What a Wave Is, The Wave Equation, The Doppler Effect, The Electromagnetic Spectrum.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

The uses look like a list to memorise and they are not, quite. Each band is used for what it is used for because of a physical property — how far it travels, what it passes through, what it is absorbed by, how much energy it carries. Learn the property and the use follows; learn the use alone and you will have to relearn it every time.

The seven bands, and the property behind each use

1. RADIO travels enormous distances, passes through walls and can be diffracted around hills and over the horizon — so it is used for BROADCASTING television and radio, and for long-range communication.
2. MICROWAVES pass easily through the atmosphere but are strongly absorbed by water molecules — so they are used for SATELLITE COMMUNICATION and mobile phone signals, and for COOKING, where the water in food absorbs them and heats it from within.
3. INFRARED is emitted by anything warm and is absorbed as heat — so it is used in THERMAL IMAGING and night-vision cameras, in remote controls, in optical fibres, and in grills and heaters. Absorbing any of these radiations increases the vibration of the particles in the absorbing material, which is what a rise in temperature is.
4. VISIBLE LIGHT is what the eye detects and what optical fibres carry with very little loss — so it is used for SEEING, for PHOTOGRAPHY, and for high-speed communication down fibres.
5. ULTRAVIOLET carries enough energy to cause chemical change and to kill microorganisms — so it is used for STERILISING water and equipment, for detecting forged banknotes with fluorescent inks, and in sunbeds.
6. X-RAYS pass through soft tissue but are absorbed by denser materials such as bone and metal — so they are used for MEDICAL IMAGING and for SECURITY SCANNING of luggage.
7. GAMMA RAYS are the most penetrating and the most energetic — so they are used to STERILISE surgical instruments and food, and to treat cancer by killing tumour cells, and as tracers in medical imaging.

Now the dangers, and they follow a pattern rather than being seven unrelated facts. Energy increases with frequency along the spectrum, so the harm increases in the same direction — and the KIND of harm changes at a particular point. Up to and including visible light, the effect of too much radiation is essentially HEATING. Beyond it, the radiation carries enough energy per unit to break chemical bonds and damage the DNA inside cells, and that is a different order of problem.

The dangers, in order along the spectrum

1. MICROWAVES are absorbed by water, and a body is largely water — so excessive exposure heats internal tissue. Microwave ovens are shielded by a metal case and a mesh in the door, and interlocks switch the magnetron off when the door opens.
2. INFRARED in excess causes skin BURNS, which is exactly what standing too close to a fire does.
3. ULTRAVIOLET damages surface cells, causing sunburn, premature ageing of the skin, and skin cancer, and it can damage the eyes. Protection: sunscreen, covering the skin, and eye protection when welding or using a sunbed.
4. X-RAYS and GAMMA RAYS are IONISING — they carry enough energy to knock electrons out of atoms, which can damage or mutate the DNA in a cell and cause cancer.
5. Protection against those two is by SHIELDING with lead or thick concrete, by limiting the DOSE and the exposure TIME, and by DISTANCE. Radiographers stand behind a screen or leave the room, and wear film badges to monitor the dose they receive over time.
6. Note also that everyone receives a small BACKGROUND dose of ionising radiation all the time, from rocks, from the air and from space — so no choice about radiation is ever a choice between some and none.

Think of it like hail against a window

Imagine hailstones striking a pane of glass. A great many tiny ones will rattle the window and warm it very slightly, and no matter how many arrive the glass will not break — that is radio, microwave and infrared, where more radiation means more heating and nothing else. Now imagine a single stone the size of a fist. It does not matter that only one arrives: it breaks something. That is the ionising end of the spectrum, where the energy carried by each individual unit is enough to do damage no amount of gentler radiation could do. This is why the danger is not simply about intensity, and why a very weak X-ray source is treated more carefully than a very strong radio transmitter.

One honest qualification, because this is a topic where public worry and physical evidence often diverge. Mobile phones emit microwaves at low power, and the established mechanism for harm from microwaves is heating. Extensive study has not established a health risk at those powers, and the specification asks you to know the mechanism rather than to take a position on the controversy. State what the radiation does and what the evidence shows, and you will be answering the question that has actually been set.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Statement 3.12 — the use, and the property it depends on

Statement 3.13 — the dangers, and the protection for each

• MICROWAVE — internal heating of tissue, because the body is largely water. Protection: metal shielding, mesh in the oven door, door interlocks
• INFRARED — skin burns. Protection: distance from the source, and shielding
• ULTRAVIOLET — sunburn, premature skin ageing, skin cancer, eye damage. Protection: sunscreen, covering the skin, eye protection
• X-RAY and GAMMA — ionising: they damage or mutate the DNA in cells, which can cause cancer. Protection: lead or concrete shielding, limited dose and time, distance, film badges to monitor exposure
• The pattern: up to VISIBLE the harm is HEATING; beyond it the radiation is IONISING and the harm is cellular damage

Learn this definition · Ionising radiation

Radiation carrying enough energy per unit to knock electrons out of atoms. Ultraviolet, X-rays and gamma rays are ionising, and can damage or mutate the DNA in living cells.

Taught in full in Alpha, Beta and Gamma.

The two kinds of harm, and where the boundary lies

A. BELOW ultraviolet — radio, microwave, infrared, visible: the harm is HEATING, and more radiation simply means more heat.

B. ULTRAVIOLET AND ABOVE — X-rays, gamma: the radiation is IONISING, so a single unit carries enough energy to break bonds and damage DNA. Intensity is not the only thing that matters.

Answering a 'why is this radiation used for this?' question

1. Name the physical property of that band — what it passes through, what absorbs it, how much energy it carries.
2. State the use.
3. Link them explicitly: the use follows FROM the property.
4. For a danger question, name the mechanism of harm and then a specific protective measure.

Model answer [4 marks]

Explain why X-rays are used to image bones, and state two ways a radiographer is protected. [4]

X-rays pass easily through soft tissue but are absorbed by denser materials such as bone, so a photographic plate or detector behind the patient is exposed everywhere except in the shadow of the bones, producing an image of them. X-rays are ionising and can damage the DNA in cells, so a radiographer stands behind a lead screen or leaves the room while the exposure is made, and wears a film badge that records the total dose received over time.

Model answer [3 marks]

Explain why microwaves are used for cooking food. [3]

Microwaves are strongly absorbed by water molecules, and food contains a great deal of water. The absorbed energy makes the water molecules vibrate more, which raises the temperature of the food. Because the microwaves penetrate a short distance into the food rather than only heating the surface, the food is heated from within as well as from the outside.

Not this: All electromagnetic radiation is dangerous in the same way, and more of it is always worse.

This: Below ultraviolet the harm is heating, so intensity is what matters. From ultraviolet upwards the radiation is ionising, and a single unit carries enough energy to damage DNA — so a weak ionising source needs more care than a strong radio transmitter.

Mark-losing trap. Give the PROPERTY that explains the use, not just the use. 'Absorbed by water' earns the mark.

Mark-losing trap. X-rays and gamma are IONISING — say they damage DNA and can cause cancer, not just 'they are harmful'.

Mark-losing trap. Name a SPECIFIC protection: lead shielding, film badge, sunscreen, distance, limited time.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — Which electromagnetic radiation is used in thermal imaging cameras?
2. Grade 7 · Explain [2 marks] — Explain why microwaves are able to cook food.
3. Grade 8 · Explain [5 marks] — Select every statement that belongs in a full-mark explanation of why X-rays can be used to produce an image of a broken bone.
4. Grade 9 · Explain [5 marks] — Select every statement that belongs in a full-mark explanation of why exposure to ultraviolet radiation is more dangerous than the same exposure to infrared.
5. 9+ · Analyse [6 marks] — A patient refuses a dental X-ray, saying: 'Radiation causes cancer, so no amount of it can be safe.' Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who learned the dangers by receiving them, and the one who used the radiation as an instrument rather than a picture. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Marie Curie — the one who learned the dangers by receiving them

Curie is on both halves of this page at once. She built mobile X-ray units and drove them to the front in the First World War, training women to operate them and bringing radiography to perhaps a million wounded soldiers — the use. And she worked for decades with radioactive materials that nobody knew to shield against, carrying tubes of radium in her pockets and keeping them by her bed; she died in 1934 of a blood disease very probably caused by that exposure, and her notebooks are still too radioactive to handle without protection.

• “What did you carry in your pockets that we would never allow now?”
• “How did you get X-ray machines to the front line?”
• “When did you realise the materials were harming you?”
• “What would you have done differently if you had known?”
• “Why are your notebooks still dangerous to open?”

Rosalind Franklin — the one who used the radiation as an instrument rather than a picture

This page treats X-rays as something that makes a shadow picture of a bone, which is the everyday use — but Franklin used them for something quite different and far harder. Fired at a crystal or a fibre, X-rays scatter off the regularly spaced atoms and interfere, producing a pattern of spots from which the arrangement can be reconstructed. Her Photograph 51 of DNA is the most famous result ever obtained that way. It is the clearest demonstration that a band of the spectrum can be an instrument for seeing what no microscope could reach.

• “How can a pattern of spots tell you the shape of a molecule?”
• “What made Photograph 51 so much clearer than earlier attempts?”
• “How long did an exposure take, and what were you doing while it ran?”
• “What precautions did you take working with X-rays?”
• “Why can an X-ray reach detail that a microscope cannot?”

Carry on through the course

• Previous lesson: The Electromagnetic Spectrum
• Next lesson: Reflection and Ray Diagrams
• What a Wave Is — Something crosses the room and nothing gets to the other side — the idea that makes the rest of the topic possible.
• The Wave Equation — Two equations that connect everything on the previous page — and one of them you could have worked out yourself.
• The Doppler Effect — Why an ambulance changes note as it passes — and why the siren itself never does.
• The Electromagnetic Spectrum — Seven names for one thing — and the two properties that change as you go along it.
• Reflection and Ray Diagrams — One rule, drawn correctly — and why the image in a mirror is behind it.
• All of Physics · Physics key terms · required practicals · equations · Incandio Science · The 4PH1 specification