Describe the harmful effects on people of excessive exposure to electromagnetic radiation, including: (a) microwaves; internal heating of body cells (b) infrared; skin burns (c) ultraviolet; damage to surface cells and eyes, leading to skin cancer an

3.3 Electromagnetic Spectrum – Harmful Effects of Excessive Exposure

Cambridge IGCSE Physics (0625) – key syllabus points covered

  • Identify the main health hazards of microwaves, infrared (IR), ultraviolet (UV), X‑rays and γ‑rays.

  • Explain why microwaves, IR and UV are non‑ionising whereas X‑rays and γ‑rays are ionising.

  • Give typical everyday and industrial sources for each region.

  • State simple safety measures (including symbols and legal exposure limits) that reduce the risk of damage.

  • Recall the basic wave‑property terminology (frequency \$f\$, wavelength \$\lambda\$, amplitude) and the relationship \$c = \lambda f\$ (with \$c = 3.0\times10^{8}\,\$m s⁻¹ in vacuum).

  • Understand the photon‑energy threshold for ionisation (≈ 10 eV) and be able to calculate photon energy \$E = hf\$.

Basic Electromagnetic‑Wave Properties

  • Speed in vacuum: \$c = 3.0\times10^{8}\,\$m s⁻¹ (the same for all EM waves).

  • Frequency (\$f\$): number of wave cycles per second (Hz). Higher frequency ⇒ higher photon energy.

  • Wavelength (\$\lambda\$): distance between successive crests (m). \$\lambda = c/f\$.

  • Amplitude: related to the intensity (power per unit area) of the wave.

  • Photon‑energy threshold for ionisation: \$E{\text{ion}} \approx 10\,\$eV. Using \$E = hf\$, the corresponding frequency is \$f{\text{ion}} = E{\text{ion}}/h \approx 2.4\times10^{15}\,\$Hz (≈ UV‑C). Waves with \$f > f{\text{ion}}\$ (X‑rays, γ‑rays) can remove tightly‑bound electrons from atoms.

Electromagnetic Spectrum – Order of Regions (by increasing frequency / decreasing wavelength)

RegionWavelength \$\lambda\$Frequency \$f\$Typical Photon Energy \$E=hf\$Ionising?
Radio≥ 1 m≤ 300 MHz≤ 1 meVNo
Microwave1 mm – 1 m300 MHz – 300 GHz≈ 10⁻³ – 10⁻¹ eVNo
Infrared (IR)700 nm – 1 mm300 GHz – 430 THz≈ 10⁻³ – 1 eVNo
Visible400 nm – 700 nm430 THz – 750 THz≈ 1.8 – 3.1 eVNo
Ultraviolet (UV‑A, UV‑B, UV‑C)10 nm – 400 nm750 THz – 30 PHz≈ 3 – 30 eVUV‑C (≥ 10 eV) ionising; UV‑A/UV‑B non‑ionising but chemically active
X‑rays0.01 nm – 10 nm30 PHz – 30 EHz≥ 100 eVYes
Gamma (γ) rays< 0.01 nm> 30 EHz> 1 MeVYes

1. Microwaves (≈ 1 mm – 1 m; 300 MHz – 300 GHz)

  • Why non‑ionising: Photon energy \$< 10^{-1}\,\$eV – far below the 10 eV ionisation threshold.

  • Interaction with the body: Water molecules have a permanent dipole; the alternating electric field forces them to rotate, converting EM energy into heat.

  • Harmful effect: Internal heating of body cells. Deep tissue temperature can rise enough to cause burns, interfere with nerve function, or affect the cardiovascular system.

  • Typical everyday & industrial sources (and key applications):

    • Microwave ovens (≈ 2.45 GHz) – cooking.

    • Radar (air‑traffic control, weather) – detection.

    • Mobile‑phone base stations, Wi‑Fi routers (2.4 GHz, 5 GHz) – communication.

    • Satellite communication links – broadcasting.

    • Industrial microwave dryers – material processing.

  • Safety measures & legal limits:

    • Maintain a safe distance (≥ 1 m) from high‑power transmitters.

    • Use metal shielding (oven door, Faraday cage) to contain the radiation.

    • Never operate a microwave oven when empty – prevents reflected power spikes.

    • Exposure limit (ICNIRP, 2020): Power density ≤ 10 W m⁻² for the general public.

    • Safety symbol: the standard “microwave radiation” pictogram (a wave inside a rectangle).

2. Infrared (IR) (≈ 700 nm – 1 mm; 300 GHz – 430 THz)

  • Why non‑ionising: Photon energy \$< 1\,\$eV – well below ionisation threshold.

  • Interaction with the body: IR is strongly absorbed by the outer layers of skin and by the water in tissues, converting EM energy directly into heat.

  • Harmful effect: Skin burns and superficial tissue damage from prolonged or intense exposure.

  • Typical everyday & industrial sources (and key applications):

    • Electric heaters, ceramic heat panels – space heating.

    • Heat lamps (e.g., food‑warming, reptile enclosures) – thermal radiation.

    • Remote‑control transmitters (≈ 38 kHz carrier modulated by IR) – consumer electronics.

    • Thermal‑imaging cameras, night‑vision devices – detection.

    • Sunlight – largest natural IR component (≈ 50 % of solar energy).

    • Industrial IR dryers – paint and textile processing.

  • Safety measures & legal limits:

    • Avoid direct, prolonged exposure to high‑temperature IR emitters.

    • Wear heat‑resistant gloves, aprons or protective clothing when working with IR heaters.

    • For very intense sources (e.g., industrial IR furnaces) use IR‑blocking goggles rated for the wavelength range.

    • Exposure limit (ICNIRP, 2020): Radiant heat flux ≤ 10 kW m⁻² for the general public.

    • Safety symbol: a wavy line with a heat symbol (often shown as a red‑orange wave).

3. Ultraviolet (UV) (≈ 10 nm – 400 nm; 750 THz – 30 PHz)

  • Why non‑ionising (UV‑A & UV‑B) but chemically active: Photon energies 3–10 eV are below the ionisation threshold but high enough to break chemical bonds (e.g., in DNA).

  • Why ionising (UV‑C): Photon energy > 10 eV; can eject electrons from atoms, similar to X‑rays.

  • Interaction with the body: UV photons are absorbed in the outermost skin cells (epidermis) and the corneal surface of the eye.

  • Harmful effects:

    • Sunburn (erythema) – acute damage to skin cells.

    • Premature skin ageing, DNA mutations → increased risk of skin cancers (melanoma, basal‑cell carcinoma, squamous‑cell carcinoma).

    • Eye damage – photokeratitis (“snow‑blindness”), cataracts, possible contribution to macular degeneration.

  • Typical everyday & industrial sources (and key applications):

    • Sunlight – natural source (UV‑A 315‑400 nm, UV‑B 280‑315 nm, UV‑C 100‑280 nm, the latter largely filtered by the atmosphere).

    • Tanning beds – intense UV‑A/UV‑B for cosmetic tanning.

    • Fluorescent and compact‑fluorescent lamps – contain a small UV component.

    • Welding arcs and industrial curing lamps – strong UV‑B/UV‑C.

    • UV‑sterilisation devices (e.g., water purifiers, air disinfectors) – use UV‑C to destroy microorganisms.

  • Safety measures & legal limits:

    • Apply broad‑spectrum sunscreen (SPF 30 + ) and re‑apply every two hours; choose “UVA‑UVB” labelled products.

    • Wear UV‑blocking sunglasses (CE‑marked) and protective clothing (wide‑brimmed hats, long sleeves).

    • Limit exposure during peak solar UV hours (10 am – 4 pm) and seek shade.

    • For artificial sources, use shielding glass or appropriate filters (e.g., polycarbonate for welding helmets).

    • Exposure limit (ICNIRP, 2020): Effective UV dose ≤ 30 J m⁻² (UV‑A) and ≤ 10 J m⁻² (UV‑B) for the general public per day.

    • Safety symbol: the “UV radiation” pictogram – a sun with three wavy lines.

4. X‑rays and Gamma (γ) Rays (≈ 0.01 nm – < 0.01 nm; > 30 PHz)

  • Why ionising: Photon energies > 10 keV (X‑rays) up to several MeV (γ‑rays) far exceed the 10 eV threshold, easily ejecting tightly‑bound electrons.

  • Interaction with the body: Ionisation along the track of the photon damages DNA, proteins and other cellular structures throughout the body.

  • Harmful effects:

    • Cell mutation → increased risk of cancers (skin, lung, thyroid, leukaemia, etc.).

    • Genetic damage that may affect future generations.

    • Acute radiation syndrome at very high doses (nausea, vomiting, hair loss, bone‑marrow failure).

  • Typical everyday & industrial sources (and key applications):

    • Medical imaging – X‑ray radiography, CT scans.

    • Radiotherapy – high‑dose X‑rays/γ‑rays for cancer treatment.

    • Industrial radiography – non‑destructive testing of welds, pipelines.

    • Security scanners – airport baggage scanners.

    • Radioactive isotopes (e.g., \$^{60}\$Co, \$^{137}\$Cs) – nuclear medicine, sterilisation.

    • Cosmic‑ray background – natural source of γ‑rays.

  • Safety measures & legal limits:

    • Use lead (or equivalent) shielding around the source and for the operator.

    • Minimise exposure time and maximise distance – follow the “time, distance, shielding” rule.

    • Apply the ALARA principle (As Low As Reasonably Achievable).

    • Wear personal dosimeters where required; monitor cumulative dose.

    • Exposure limits (ICRP, 2022): 1 mSv yr⁻¹ for the general public; 20 mSv yr⁻¹ for occupational workers (averaged over 5 years).

    • Safety symbols: the trefoil radiation warning sign (three black blades on a yellow background) for ionising radiation.

Link to Energy Resources (Syllabus Supplement)

  • Solar energy is harvested from the visible and UV portions of the spectrum (photovoltaic cells) and from the IR portion (solar thermal collectors).

  • Microwave and radio‑frequency technologies are used in wireless power transmission and in the generation of useful energy from the Sun (e.g., microwave beaming concepts for space‑based solar power).

  • Understanding the harmful effects helps explain why shielding, filtering and safe operating distances are essential when converting EM radiation into useful energy.

Summary Table – Harmful Effects & Safety

Radiation TypeWavelength / FrequencyTypical Sources & ApplicationsPrimary Harmful EffectKey Safety Measures (incl. symbols & limits)
Microwaves1 mm – 1 m (300 MHz – 300 GHz)Microwave ovens, radar, mobile‑phone base stations, Wi‑Fi, satellite links, industrial dryersInternal heating of body cells (deep burns, physiological disruption)Distance ≥ 1 m, metal shielding, never run empty oven; limit ≤ 10 W m⁻² (ICNIRP); microwave‑radiation pictogram
Infrared (IR)700 nm – 1 mm (300 GHz – 430 THz)Electric heaters, heat lamps, remote‑control emitters, thermal cameras, sunlight, industrial dryersSuperficial skin burns and tissue heatingAvoid prolonged close exposure, wear heat‑resistant clothing/gloves, IR‑blocking goggles for intense sources; limit ≤ 10 kW m⁻²; IR‑heat symbol
Ultraviolet (UV‑A/UV‑B/UV‑C)10 nm – 400 nm (750 THz – 30 PHz)Sunlight, tanning beds, fluorescent lamps, welding arcs, UV‑sterilisation devicesDamage to surface skin cells & eyes → sunburn, skin cancer, cataracts, photokeratitisSunscreen (SPF 30+), UV‑blocking sunglasses, protective clothing, limit exposure 10 am‑4 pm, filters for artificial sources; dose ≤ 30 J m⁻² (UV‑A), ≤ 10 J m⁻² (UV‑B); UV‑radiation pictogram
X‑rays / Gamma rays0.01 nm – < 0.01 nm (≥ 30 PHz)Medical imaging, radiotherapy, industrial radiography, security scanners, radioactive isotopes, cosmic raysIonisation → DNA damage, cancer, genetic mutations, acute radiation syndromeLead shielding, minimise time, maximise distance, ALARA principle, dosimeters; public limit 1 mSv yr⁻¹, occupational 20 mSv yr⁻¹; trefoil radiation warning sign

Suggested diagram: Electromagnetic spectrum showing wavelength ranges, typical sources, photon‑energy values, and the ionising/non‑ionising boundary (≈ 10 eV).