Applications of electromagnetic waves
Key idea: Each part of the electromagnetic spectrum has uses that depend on how materials absorb or transmit it. X-ray images work because bone absorbs X-rays more than soft tissue does.
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Guided steps
- Link each region’s uses to transmission, detection, absorption or biological interaction.
- Read why detection of emission differs from sending a coded signal.
- Explain direct absorption and possible later heating by contact.
- Write the source-to-detector explanation using differing transmitted intensity.
- Identify the first mechanism error and explain the relevant genetic-material effect.
- Write the interaction and medical use, then compare the complete public mark points.
- Explain how ultraviolet reveals a difference between the signature paint and the surrounding paint.
- Explain X-ray contrast, gamma sterilisation and infrared detection using the interaction each device needs.
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Every region of the electromagnetic spectrum is put to work somewhere, from radio broadcasts to cancer treatment. Each use depends on how that region behaves: what it passes through, what absorbs it, what gives it out, or what it does to living cells. To explain a use, name the region and then the property that makes it suitable.
Uses of each region
| Region | Typical uses | Why this region works |
|---|---|---|
| Radio waves | radio and television broadcasting, radio astronomy, RFID tags | carry signals through air and some walls, depending on frequency and material; made and detected with suitable aerials |
| Microwaves | mobile phones, satellite television, microwave ovens | suitable communication frequencies can transmit through the atmosphere in directed beams; absorption in food transfers energy and heats it |
| Infrared | remote controls, intruder alarms, thermal imaging | given out by every warm object, more strongly the hotter it is; easily made and detected |
| Visible light | photography, optical fibres in medicine and telecommunications | detected by eyes and cameras; travels along glass fibres by total internal reflection |
| Ultraviolet | sunbeds, checking bank notes, disinfecting water | makes some substances glow (fluoresce); damages the genetic material of bacteria and viruses |
| X-rays | medical radiology, airport security, finding cracks in metal | very penetrating: pass through skin, muscle, clothing and luggage |
| Gamma rays | sterilising food and medical equipment, detecting and treating cancer | penetrate material; can kill cells in treatment or sterilisation, while detected tracer emissions support imaging |
The secret marks on a bank note are printed in an ink that is invisible in ordinary light but glows under an ultraviolet lamp. A missing or unexpected pattern suggests that the note needs further checks.
Gamma rays serve different jobs in detection and treatment. In nuclear-medicine imaging, emissions associated with a tracer can leave the body and be detected to build information about its distribution; suitable tracer patterns can help detect cancer. Treatment instead aims to damage cancer cells through absorbed radiation. Detection is not explained merely by saying “gamma rays kill cells”. See NIBIB’s account of nuclear medicine.
Worked example 1
Name the region, then the reason
Problem
State the region of the electromagnetic spectrum used in each device, and give the property that makes it suitable: (a) a TV remote control; (b) a thermal-imaging camera used to find people in smoke.
Show full solution
(a) Remote control
Method
Infrared.Reason
A small infrared LED makes the signal and a sensor on the TV detects it. The beam is invisible and ordinarily does not pass through an opaque wall. Its path, reflections and the receiver’s signal code determine whether the TV responds.
Working
Answer
Infrared: easily made and detected over a short range.
(b) Thermal-imaging camera
Method
Infrared.Reason
A warm body gives out more infrared than its cooler surroundings, so a thermal camera can distinguish the thermal contrast. Infrared also passes through smoke better than visible light.
Working
Answer
Infrared: given out by warm objects.
Quick check
Check your understanding 1 Heating food
In a hypothetical oven comparison, moist food absorbs the microwaves strongly while the specified plate absorbs very little. Explain why the food heats directly and the plate may initially stay cooler. Could the plate later become hot even with little direct microwave absorption?
Show answer
Absorption transfers electromagnetic energy into the moist food, increasing its internal energy. The specified low-absorbing plate initially receives less direct microwave energy, but it can still be heated by contact with hot food. Different plate materials absorb microwaves differently.
Guided practice 1
Why bones show up on an X-ray
Problem
Explain why X-rays can be used to take an image of a broken bone.
What do bone and flesh do to X-rays?
Hints
Hint 1
Two materials
An image needs bone and soft tissue to look different.
Hint 2
Which absorbs more?
Bone attenuates more of the incident X-rays than surrounding soft tissue in this comparison.
Show solution step by step
Compare bone and soft tissue
Method
A greater fraction of the incident X-rays passes through soft tissue than through bone.
Reason
Bone’s density and composition cause greater attenuation for the imaging beam; thickness and beam energy also affect transmission.
Working
Answer
Soft tissue: more transmitted. Bone: more attenuated; fewer reach the detector.
Form the image
Method
The detector behind the body receives different transmitted X-ray intensities. Fewer reach it behind bone than behind soft tissue. That contrast forms the image; a break can interrupt the bone’s pattern.
Reason
Different amounts of X-ray reaching the detector make the picture.
Working
Answer
Different absorption → image of the bone.
Quick check
Spot the mistake 1
Does UV cook the bacteria?
Learner claim
A student writes: “Ultraviolet lamps disinfect drinking water by heating the bacteria until they die.” What is wrong?
How does UV kill bacteria?
Show solution step by step
The region is right
Method
Ultraviolet is used to disinfect water.Reason
It is one of the syllabus uses of ultraviolet.Working
Answer
Ultraviolet: disinfecting water.The reason is wrong
Method
UV damages the genetic material of the bacteria, so they die or cannot reproduce.
Reason
Disinfection depends on sufficient exposure to the appropriate UV, which inactivates microorganisms by damaging genetic material; heating the water to kill them is not this method’s principal mechanism.
Working
Answer
UV → damaged genetic material → water disinfected.
Common mistake
Exam-style question 1
Sterilising packed food
Examination question
Some foods are sterilised with gamma rays after they have been sealed in their packets. (a) Explain why gamma rays are suitable for this. [2 marks] (b) State one use of gamma rays in medicine. [1 mark]
Penetration, what it does, one medical use
Show solution step by step
(a) Why gamma rays
2 marksMethod
Gamma rays are very penetrating, so they pass through the sealed packet and the food. They kill the bacteria inside.
Reason
The packet does not have to be opened, so an intact packet reduces later contamination while it remains sealed.
Working
Answer
Penetrates the packet; kills bacteria.(b) A medical use
1 markMethod
Treating cancer, by killing cancer cells; or sterilising medical equipment.
Reason
Gamma rays kill living cells.Working
Answer
Radiotherapy for cancer.
Self-mark with the mark scheme
Tick each mark point that your answer includes.
Give one mark for each point your answer states.
Common mistake
Saying the gamma rays make the food radioactive loses the second mark: they kill the bacteria without making the food radioactive.
Check your understanding 2 A forged signature
A museum suspects that the signature on an old painting was added recently. Modern paints glow under one region of the spectrum, but the old paints on the canvas do not. Which region should the museum shine on the painting, and what would show the signature is new?
Show answer
Ultraviolet can excite fluorescence, the same principle used for security marks on bank notes. Under the question’s specified paint distinction, fluorescence in the signature but not in the original paint supports the modern-paint hypothesis. The glow identifies a difference in paint; material analysis can help date it.
Check your understanding 3 Explain three different radiation uses
An X-ray detector receives more radiation through a thin part of a metal component than through a thick part, with the beam unchanged. Explain the contrast. Then explain why gamma rays can sterilise food in sealed packets, while a thermal camera uses infrared to detect a warm person.
Show answer
X-ray inspection: thinner metal attenuates less of the beam, so more reaches the detector. The contrast comes from different transmitted intensities, not from the detector heating the component.
Gamma sterilisation: gamma rays penetrate the sealed packet and damage or kill microorganisms inside. Keeping the packet sealed reduces later contamination.
Thermal imaging: a warm person emits infrared. The camera detects this emission and distinguishes it from the surroundings. A remote control instead sends a coded infrared signal; it is an emitter, not a thermal camera.
Common mistakes
- Giving a use without naming the region, or naming the region without the property that makes it suitable.
- Saying microwaves or infrared are ionising. Their main effect is heating.
- Saying UV disinfects by heating. It damages the genetic material of bacteria.
- Writing “X-rays are penetrating” for an X-ray image without saying that bone or metal absorbs more than the material around it.
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