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 – Core & Supplement Topics

3.3.1 Order of the Spectrum (Core)

From longest wavelength (lowest frequency) to shortest wavelength (highest frequency):

RadioMicrowavesInfra‑red (IR)VisibleUltraviolet (UV)X‑raysGamma‑rays
λ ≈ 10 m – 10 kmλ ≈ 1 mm – 1 mλ ≈ 700 nm – 1 mmλ ≈ 400 nm – 700 nmλ ≈ 10 nm – 400 nmλ ≈ 0.01 nm – 10 nmλ ≈ < 0.01 nm

3.3.2 Speed of Electromagnetic Waves (Core)

  • All EM waves travel at the same speed in vacuum: c = 3.00 × 10⁸ m s⁻¹.

  • In air the speed is slightly lower: ≈ 2.998 × 10⁸ m s⁻¹ (≈ 0.999 c).

Example calculation (AO2): How long does a radio wave (frequency 100 MHz) take to travel 1 km in air?

  1. Distance = 1 km = 1 000 m.

  2. Time = distance ÷ speed ≈ 1 000 m / 2.998 × 10⁸ m s⁻¹ ≈ 3.34 × 10⁻⁶ s (3.34 µs).

3.3.3 Typical Uses & Communication Systems (Core + Supplement)

RegionTypical Use (Why this region?)Real‑World Example
Radio (≈ 1 mm – 10 km)Long‑range broadcasting; low‑frequency waves diffract around obstacles and travel long distances.AM/FM radio stations, maritime distress beacons.
Microwaves (≈ 1 mm – 1 m)High‑frequency, short‑wavelength → can carry large data rates and pass through the atmosphere with little attenuation.Satellite TV, GPS, point‑to‑point microwave links, radar.
Infra‑red (≈ 700 nm – 1 mm)Strong absorption by water and organic material → useful for heating and short‑range data transmission.Heating lamps, TV remote controls, thermal‑imaging cameras.
Visible (≈ 400 nm – 700 nm)Matches the human eye sensitivity; low atmospheric loss.LED lighting, fibre‑optic communication, digital displays.
Ultraviolet (≈ 10 nm – 400 nm)High‑energy photons capable of breaking chemical bonds → used for sterilisation and fluorescence.UV‑C germicidal lamps, tanning beds, fluorescence spectroscopy.
X‑rays / Gamma‑rays (≈ 0.01 nm – 10 nm)Very high‑energy, ionising radiation → penetrates matter, useful for imaging and cancer treatment.Medical X‑ray imaging, CT scans, radiotherapy.

3.3.4 Signal Types (Supplement)

  • Analogue signals: continuous variation of amplitude, frequency or phase (e.g., traditional AM/FM radio).

  • Digital signals: discrete binary levels (0 or 1) representing data (e.g., digital DAB radio, mobile‑phone data).

  • Digital transmission is less susceptible to noise and allows error‑correction, which is why most modern communication systems use it.

3.3.5 Harmful Effects of Excessive Exposure (Core)

RegionPrimary Biological EffectTypical SourcesHealth Outcome (Excessive Exposure)Safety Tip (AO3)
MicrowavesInternal heating of body cells (water‑molecule absorption)Microwave ovens, radar, satellite linksCore temperature > 42 °C → thermal stress, tissue damage, possible burnsUse only microwave‑safe containers; keep a safe distance from operating radar/antenna equipment.
Infra‑red (IR)Surface heating → skin burns (1st‑ to 3rd‑degree)Heating lamps, industrial furnaces, solar IRLocalized burn injury; prolonged exposure can cause tissue damage.Never look directly at high‑power IR sources; use protective shields or gloves when near industrial IR heaters.
Ultraviolet (UV)DNA/chemical‑bond damage to surface cells and eyes → skin cancer, cataractSunlight, tanning beds, germicidal UV‑C lampsSunburn, premature ageing, photokeratitis, cataract, basal‑cell & squamous‑cell skin cancers.Apply broad‑spectrum sunscreen (SPF ≥ 30); wear UV‑blocking sunglasses and protective clothing; avoid direct exposure to UV‑C lamps.
X‑rays / Gamma‑raysIonising radiation → DNA mutations, cell deathMedical imaging, industrial radiography, cosmic raysIncreased cancer risk (leukaemia, thyroid, etc.); acute radiation syndrome at very high doses.Use lead shielding; limit exposure time; follow ALARA principle (As Low As Reasonably Achievable).

3.3.6 Key Summary Points (Core)

  1. Energy of EM radiation rises as wavelength shortens; this governs the type of biological damage.

  2. Non‑ionising regions (microwaves, IR) mainly cause heating; ionising regions (UV‑C, X‑rays, γ‑rays) can break chemical bonds and damage DNA.

  3. Protective measures differ by region – shielding, sunscreen, safe‑container use, and exposure‑time limits are essential.

  4. Understanding the hazards informs safe design of everyday technology and occupational practices.

3.3.7 Practice Questions (AO2 & AO3)

  1. Speed & Time: A mobile‑phone base‑station transmits a microwave signal (frequency = 2 GHz). Calculate the time taken for the wave to travel 5 km through air.

    Solution outline*: Use c ≈ 2.998 × 10⁸ m s⁻¹; time = distance ÷ speed = 5 000 m / 2.998 × 10⁸ ≈ 1.67 × 10⁻⁵ s (16.7 µs).

  2. Safety Assessment: A laboratory uses a 250 W IR heating lamp for material testing. List three safety precautions that should be implemented before the lamp is switched on.

    Suggested answer*: (i) Wear heat‑resistant gloves and eye protection; (ii) Position the lamp on a heat‑resistant stand and keep flammable materials > 1 m away; (iii) Use a temperature‑controlled timer or interlock to prevent overheating.