🌡️ Describe how infrared radiation transfers thermal energy and its everyday applications
☀️ Explain the effects of ultraviolet radiation on the human body including tanning and cell damage
🩻 Explain how X-rays are used in medical imaging and the risks they carry
☢️ Describe how gamma rays are used in cancer treatment (radiotherapy) and sterilisation
📡 Use the wave equation v = fλ to solve problems involving EM waves including IR, UV, X-rays and gamma
⚖️ Compare the properties, wavelengths, frequencies and dangers of IR, UV, X-rays and gamma across the EM spectrum
🌡️ Infrared Radiation
Infrared (IR) radiation is electromagnetic radiation with wavelengths between approximately 700 nm and 1 mm, just beyond the red end of visible light. All objects above absolute zero (0 K) emit infrared radiation.
The hotter an object, the more infrared radiation it emits per second, and the shorter the peak wavelength of that radiation. This is why a red-hot metal bar emits both infrared and visible red light, while the human body (at ~37°C) emits only infrared.
How IR Transfers Energy
Infrared is a form of radiation — it transfers thermal energy through a vacuum without needing a medium. Unlike conduction (particle vibration) or convection (fluid movement), infrared can travel through empty space. This is how the Sun heats the Earth.
Wave equation: v = fλ
Speed of all EM waves in vacuum: v = 3.0 × 10⁸ m/s
f = frequency (Hz), λ = wavelength (m)
Applications of Infrared
Thermal imaging cameras: Detect IR emitted by warm bodies; used by emergency services to find people in smoke or darkness.
Remote controls: TV and other device remotes use near-IR pulses to transmit signals.
Optical fibre communications: IR signals carry data along glass fibres at high speed.
Grills and heaters: Radiant heaters convert electrical energy into IR to warm rooms.
Night-vision equipment: Military and security cameras detect IR from warm bodies at night.
IR wavelength range: ~700 nm – 1 mm · Frequency range: ~3×10¹¹ Hz – 4×10¹⁴ Hz
Health Considerations
Prolonged exposure to intense IR can cause burns to skin and damage to the retina of the eye (e.g. looking at the Sun or welding arcs). However, everyday IR from warm objects at normal temperatures poses no significant health risk.
Property
Value / Detail
Position in EM spectrum
Between microwaves and visible light
Typical wavelength
700 nm – 1 mm
Speed in vacuum
3.0 × 10⁸ m/s
Detected by
Skin, thermometers, IR cameras
Key uses
Thermal imaging, remote controls, fibre optics
☀️ Ultraviolet Radiation
Ultraviolet (UV) radiation has wavelengths between approximately 10 nm and 400 nm, just beyond the violet end of visible light. The Sun is the main natural source; UV lamps produce it artificially.
UV carries more energy per photon than visible light or IR, because energy is proportional to frequency (E = hf). This higher energy makes UV capable of ionising molecules in living tissue, which is what gives it both its useful and harmful properties.
Effects on the Human Body
Tanning: UV-B radiation (280–315 nm) stimulates melanocytes in the skin to produce melanin, a brown pigment that absorbs UV and protects deeper layers. This appears as a suntan. Melanin acts as the body's natural sunscreen.
Vitamin D synthesis: UV-B also triggers the production of vitamin D in the skin, essential for bone health. Moderate sun exposure is therefore beneficial.
Sunburn: Overexposure to UV causes redness, inflammation and peeling of skin as cells are damaged.
Skin cancer: UV radiation can cause mutations in DNA within skin cells. Unrepaired mutations can lead to uncontrolled cell division — skin cancer. UV-A (315–400 nm) penetrates more deeply; UV-B causes more direct DNA damage.
Eye damage: UV can cause cataracts (clouding of the lens) and damage the cornea. UV-blocking sunglasses are important protection.
Beneficial Uses of UV
Sterilisation: UV lamps kill bacteria on surfaces, in water treatment and in hospital environments by damaging bacterial DNA.
Fluorescence: UV causes certain materials to fluoresce (emit visible light); used in security markings and detecting forged documents.
Black lights: Used in entertainment and forensic investigations.
Treating skin conditions: Controlled UV exposure can treat psoriasis and eczema under medical supervision.
UV is ionising radiation — it has enough energy to remove electrons from atoms and break chemical bonds in DNA. This makes it potentially dangerous but also useful for sterilisation.
UV Type
Wavelength
Effect
UV-A
315 – 400 nm
Tanning, deeper skin penetration, ageing
UV-B
280 – 315 nm
Sunburn, vitamin D, DNA damage, skin cancer risk
UV-C
100 – 280 nm
Most harmful; absorbed by ozone layer
🩻 X-rays
X-rays are high-frequency, short-wavelength electromagnetic waves with wavelengths between approximately 0.01 nm and 10 nm. They are produced when high-speed electrons are rapidly decelerated, typically by firing them at a metal target in an X-ray tube.
X-rays carry much more energy per photon than UV, visible or IR radiation. They are strongly ionising, meaning they can strip electrons from atoms they pass through. This makes them both useful and potentially dangerous.
Medical Imaging
X-rays are widely used in medicine to image the internal structures of the body:
Transmission X-ray imaging (radiography): A beam of X-rays is directed through the body onto a detector (traditionally photographic film, now digital). Dense materials like bone absorb X-rays strongly and appear white on the image; soft tissue absorbs less and appears grey; air (e.g. in lungs) absorbs very little and appears black.
CT scans (Computed Tomography): Multiple X-ray images are taken from different angles and processed by a computer to produce a detailed 3D cross-sectional image of the body.
Contrast agents: Substances like barium sulfate can be swallowed or injected to make soft tissues more visible on X-ray images (e.g. barium meal to image the digestive system).
Industrial inspection: Checking welds and structural components for cracks without destroying them (non-destructive testing).
Crystallography: X-ray diffraction is used to determine the atomic structure of crystals and biological molecules (including DNA).
Risks of X-rays
Because X-rays are ionising, they can damage or kill living cells and cause mutations in DNA, which may lead to cancer. Medical use is carefully controlled to keep doses as low as reasonably achievable (ALARA). Radiographers stand behind lead screens or leave the room during exposures to minimise their cumulative dose.
Lead absorbs X-rays very effectively due to its high density and atomic number. Lead aprons protect patients' reproductive organs and thyroid glands during X-ray procedures.
Property
Value / Detail
Wavelength
0.01 nm – 10 nm
Frequency
~3×10¹⁶ Hz – 3×10¹⁹ Hz
Ionising?
Yes — strongly ionising
Penetration
Passes through soft tissue; absorbed by dense materials (bone, lead)
Key uses
Medical imaging, CT scans, airport security, industrial testing
☢️ Gamma Rays
Gamma rays (γ-rays) are the highest-energy, shortest-wavelength electromagnetic waves, with wavelengths below approximately 0.01 nm (10 pm). They are produced by nuclear processes — specifically by radioactive nuclei decaying from excited nuclear energy states.
Gamma rays occupy the extreme end of the EM spectrum. They share their wavelength range with the most energetic X-rays, and in practice the distinction is made by origin: X-rays come from electron processes; gamma rays come from the nucleus.
Cancer Treatment: Radiotherapy
Gamma rays can be focused onto tumours to destroy cancer cells. This is called radiotherapy. Key features:
Gamma rays are highly ionising — they damage the DNA of cancer cells so severely that the cells can no longer replicate.
The beam is rotated around the patient so it always passes through the tumour, but healthy tissue on the outside receives only a fraction of the total dose.
The common source is cobalt-60, a radioactive isotope that emits gamma rays.
Modern techniques such as intensity-modulated radiotherapy (IMRT) and the Gamma Knife use multiple beams to minimise damage to surrounding healthy tissue.
Side effects include fatigue, hair loss and damage to surrounding healthy cells.
Sterilisation
Gamma rays are used to sterilise medical instruments, surgical equipment and food:
Medical sterilisation: Sealed packages of syringes, scalpels and dressings are exposed to gamma rays from a cobalt-60 source. The gamma rays kill all bacteria, viruses and spores without heating the equipment, making it safe for sealed plastic items that cannot be autoclaved.
Food irradiation: Exposing food (e.g. spices, strawberries) to gamma radiation kills bacteria like Salmonella and extends shelf life. It does NOT make the food radioactive.
Other Uses
Tracer studies: Gamma-emitting radioisotopes (e.g. technetium-99m) are injected into the body. A gamma camera tracks the tracer to diagnose disease.
PET scans: Positron-emitting tracers produce gamma rays that are detected to create 3D images of metabolic activity.
Gamma rays are the most penetrating EM radiation. Several centimetres of lead or metres of concrete are needed to reduce their intensity significantly. They are both the most dangerous and the most medically useful form of EM radiation.
📊 Comparing IR, UV, X-rays and Gamma Across the EM Spectrum
All electromagnetic waves travel at the same speed in a vacuum: c = 3.0 × 10⁸ m/s. They differ in wavelength and frequency, and these differences determine their properties and uses.
c = fλ
c = 3.0 × 10⁸ m/s (speed of light in vacuum)
f = frequency in Hz · λ = wavelength in m
Rearrangements:
f = c ÷ λ λ = c ÷ f
As you move from IR → visible → UV → X-rays → gamma rays across the EM spectrum:
Wavelength decreases
Frequency increases
Energy per photon increases (E = hf)
Ionising ability increases
Penetrating power generally increases
Radiation
Wavelength
Ionising?
Key Use
Key Risk
Infrared
700 nm – 1 mm
No
Thermal imaging, remote controls
Burns (intense IR)
Ultraviolet
10 – 400 nm
Yes (low-medium)
Sterilisation, fluorescence
Skin cancer, cataracts
X-rays
0.01 – 10 nm
Yes (high)
Medical imaging, CT scans
Cell damage, cancer
Gamma rays
< 0.01 nm
Yes (very high)
Radiotherapy, sterilisation
Cell/DNA damage, cancer
Non-ionising radiation (like IR) does not have enough energy to remove electrons from atoms. Ionising radiation (UV, X-rays, gamma) does — this is why ionising radiation is more dangerous to living tissue.
Example 1: A medical X-ray machine produces X-rays with a frequency of 3.0 × 10¹⁸ Hz. Calculate the wavelength of these X-rays. State whether this wavelength is in the X-ray range.
1 Write down the known values.
f = 3.0 × 10¹⁸ Hz · c = 3.0 × 10⁸ m/s · λ = ?
2 Select and write the wave equation.
c = fλ → λ = c ÷ f
4 Convert to nm for comparison.
λ = 1.0 × 10⁻¹⁰ m = 0.1 nm
5 Check against X-ray range (0.01 – 10 nm).
0.1 nm is within the X-ray range ✓
λ = 1.0 × 10⁻¹⁰ m (0.1 nm) — this is in the X-ray range.
Example 2: An infrared heater emits radiation with a wavelength of 2.5 × 10⁻⁶ m. Calculate the frequency of this radiation and confirm it is in the infrared part of the EM spectrum.
1 Write down known values.
λ = 2.5 × 10⁻⁶ m · c = 3.0 × 10⁸ m/s · f = ?
2 Rearrange c = fλ for frequency.
f = c ÷ λ
3 Substitute.
f = (3.0 × 10⁸) ÷ (2.5 × 10⁻⁶)
f = 1.2 × 10¹⁴ Hz
4 Compare wavelength to IR range (700 nm – 1 mm).
2.5 × 10⁻⁶ m = 2500 nm
This is between 700 nm and 1 mm ✓ — confirmed infrared.
f = 1.2 × 10¹⁴ Hz — this is infrared radiation (wavelength 2500 nm is within the IR range).
Example 3: A hospital uses gamma rays from cobalt-60 to sterilise surgical equipment. Explain why gamma rays are effective at sterilisation and why the sterilised equipment is safe to use on patients immediately after treatment. [4 marks]
1 State the relevant property of gamma rays.
Gamma rays are highly ionising — they carry enough energy to break chemical bonds and ionise atoms in biological molecules.
2 Explain the mechanism of sterilisation.
When gamma rays pass through bacteria, viruses or spores on the equipment, they cause severe damage to the DNA of these microorganisms. This prevents the organisms from replicating, effectively killing them.
3 Explain penetration.
Gamma rays are penetrating, so they can pass through sealed packaging and reach all surfaces of the equipment, sterilising everything inside without opening the packaging.
4 Explain safety for patients.
Gamma rays are a form of electromagnetic radiation — they do not remain in the equipment after the source is removed. The equipment does not become radioactive, so it is completely safe for immediate use on patients.
Gamma rays kill microorganisms by ionising and damaging their DNA. They are penetrating, reaching all surfaces through sealed packaging. Since gamma rays are EM radiation (not particles), they do not make the equipment radioactive — it is safe to use immediately.
Example 4: A UV lamp used for sterilisation emits UV radiation at a wavelength of 254 nm. A gamma-ray source emits gamma rays at a wavelength of 1.0 × 10⁻¹² m. Calculate the frequency of each and state which carries more energy per photon. (h = 6.63 × 10⁻³⁴ J·s)
1 Calculate frequency of UV (λ = 254 nm = 2.54 × 10⁻⁷ m).
f_UV = c ÷ λ = (3.0 × 10⁸) ÷ (2.54 × 10⁻⁷)
f_UV = 1.18 × 10¹⁵ Hz ≈ 1.2 × 10¹⁵ Hz
2 Calculate frequency of gamma rays (λ = 1.0 × 10⁻¹² m).
f_γ = c ÷ λ = (3.0 × 10⁸) ÷ (1.0 × 10⁻¹²)
f_γ = 3.0 × 10²⁰ Hz
4 Compare and conclude.
E_γ ÷ E_UV ≈ 2.6 × 10⁵ — gamma rays carry about 260,000 times more energy per photon than these UV rays.
f_UV ≈ 1.2 × 10¹⁵ Hz · f_γ = 3.0 × 10²⁰ Hz · Gamma rays carry far more energy per photon (2.0 × 10⁻¹³ J vs 7.8 × 10⁻¹⁹ J), making them far more ionising.
Q1. Which of the following correctly orders these EM waves from longest to shortest wavelength?
Q2. A remote control emits infrared radiation at a frequency of 4.0 × 10¹³ Hz. Calculate its wavelength in metres. Give your answer in standard form to 2 significant figures.
Q3. Which statement about UV radiation and skin is correct?
Q4. Explain why lead aprons are used to protect patients during X-ray examinations. Give two reasons. (Write 2–3 sentences)
Q5. A gamma-ray source has a wavelength of 5.0 × 10⁻¹² m. Calculate its frequency in Hz in standard form.
Challenge Q1 [6 marks]: A hospital radiotherapy department uses a cobalt-60 source to treat a patient's tumour with gamma radiation. The gamma rays have a frequency of 2.85 × 10²⁰ Hz.
(a) Calculate the wavelength of these gamma rays. [2]
(b) The beam is rotated around the patient during treatment. Explain why this technique is used rather than directing a single fixed beam at the tumour. [2]
(c) Explain why gamma rays are effective at destroying cancer cells but also carry a risk to healthy tissue. [2]
(a) λ = c ÷ f = (3.0 × 10⁸) ÷ (2.85 × 10²⁰) = 1.05 × 10⁻¹² m ≈ 1.1 × 10⁻¹² m [2]
(b) Rotating the beam means the tumour always lies at the intersection of all beam paths and receives the full accumulated dose [1]. Healthy tissue surrounding the tumour only passes through the beam for a fraction of the rotation, receiving a much smaller total dose, so damage to healthy cells is minimised [1].
(c) Gamma rays are highly ionising [1] — they damage DNA in cells so severely that the cells cannot replicate and die. Cancer cells are dividing rapidly and are particularly vulnerable. However, gamma rays cannot distinguish between cancerous and healthy cells, so surrounding healthy tissue also risks DNA damage, mutation and secondary cancer [1].
Challenge Q2 [5 marks]: Compare and contrast the use of UV radiation and gamma radiation for sterilisation of medical equipment. Your answer should include: mechanism of sterilisation, ability to penetrate packaging, suitability and any risks. [5 marks extended answer]
Model Answer (5 marks):
Both UV and gamma radiation kill microorganisms by ionising their DNA, preventing replication [1].
UV radiation has a longer wavelength (10–400 nm) and lower frequency than gamma, so it is less penetrating. It is effective for sterilising exposed surfaces (e.g. workbenches, water) but cannot penetrate sealed packaging — items must be unwrapped and re-wrapped after treatment [1].
Gamma radiation (λ < 0.01 nm) is highly penetrating and can pass through sealed plastic packaging to sterilise contents without opening them, making it ideal for pre-packaged surgical instruments [1].
Neither method makes the equipment radioactive — both are EM waves and leave no residual radiation [1].
Risk: both are ionising. Workers handling UV sources risk skin cancer and eye damage (cataracts). Gamma sources require thick lead or concrete shielding and must be stored in licensed facilities due to the far greater hazard [1].
Challenge Q3 [4 marks]: An X-ray imaging system uses X-rays of wavelength 0.20 nm. A proposed new system uses X-rays of wavelength 0.050 nm.
(a) Calculate the ratio of the frequencies of the two X-ray beams. [2]
(b) The new system delivers a higher quality image but medical staff are concerned about patient safety. Explain their concern and suggest how the risk could be managed. [2]
(a) Since c = fλ and c is constant, f ∝ 1/λ.
f₁ = c ÷ 0.20×10⁻⁹ = 1.5 × 10¹⁸ Hz
f₂ = c ÷ 0.050×10⁻⁹ = 6.0 × 10¹⁸ Hz
Ratio f₂ : f₁ = 6.0 × 10¹⁸ ÷ 1.5 × 10¹⁸ = 4 : 1 [2]
(b) The shorter-wavelength X-rays have 4× higher frequency and therefore 4× more energy per photon (E = hf) [1]. They are more ionising and more likely to damage DNA in healthy tissue, increasing the risk of radiation-induced cancer. Risk management: use the minimum exposure time necessary; use lead shielding for sensitive organs; limit the number of scans per patient per year; ensure radiographers are not in the room during exposure [1].
Challenge Q4 [5 marks]: A student claims: "Infrared radiation is safe because it is non-ionising, but UV radiation is always harmful." Evaluate this claim, including quantitative reference to wavelengths and frequencies where possible. [5 marks]
Model Answer (5 marks):
The claim is partly correct but oversimplified in both parts.
IR part — partially correct but not absolute: IR (700 nm – 1 mm, f ≈ 3×10¹¹ – 4×10¹⁴ Hz) is non-ionising — photons lack sufficient energy to remove electrons from atoms [1]. In most everyday situations IR is safe. However, intense IR (e.g. from industrial furnaces, laser sources) can cause severe skin burns and permanent retinal damage without triggering pain receptors quickly enough to warn the user [1]. So "safe" is context-dependent.
UV part — incorrect to say always harmful: UV (10–400 nm, f ≈ 7.5×10¹⁴ – 3×10¹⁶ Hz) is ionising and does carry risks including sunburn, DNA mutations and skin cancer [1]. However, UV has important beneficial effects: it stimulates vitamin D production in skin (essential for bone health and immunity) and is used medically to treat skin conditions such as psoriasis. UV sterilisation of surfaces and water is a beneficial application [1].
Conclusion: Both the harmlessness of IR and the harmfulness of UV depend on intensity, duration and context. The key distinction is that UV is ionising (can damage DNA) while IR is not — but both can be harmful under extreme conditions, and UV has important biological and medical benefits [1].