The Electromagnetic Spectrum

The electromagnetic spectrum is a SINGLE CONTINUOUS RANGE of transverse waves. We divide it into named groups by approximate wavelength purely for convenience.

Group	Approx wavelength	Approx frequency
Radio	> 0.1 m	< 3 GHz
Microwaves	1 mm – 10 cm	3–300 GHz
Infrared (IR)	700 nm – 1 mm	0.3 – 400 THz
Visible light	400 – 700 nm	430 – 750 THz
Ultraviolet (UV)	10 – 400 nm	$10^{15}$ Hz region
X-rays	0.01 – 10 nm	$10^{17}$ Hz region
Gamma rays	< 0.01 nm	> $10^{19}$ Hz

Key properties shared by ALL EM waves (spec 3.10).

1. They are TRANSVERSE.
2. They travel at $c = 3 \times 10^{8}\,\text{m/s}$ in vacuum (spec 3.10).
3. They obey $v = f\lambda$ (spec 3.5).
4. They can be reflected and refracted (spec 3.9).
5. They DO NOT require a medium to travel (unlike sound).

In a material medium (glass, water, air), EM waves travel SLOWER than $c$. The slowdown causes refraction. Numerical exam questions almost always set $v = c$ unless told otherwise.

Decreasing wavelength / increasing frequency (spec 3.11):

$\text{radio} \to \text{microwaves} \to \text{infrared} \to \text{visible} \to \text{ultraviolet} \to \text{X-ray} \to \text{gamma}$

Use any memorable mnemonic, e.g. "Roman Men Invented Very Unusual X-ray Guns".

Visible-light sub-order. Inside visible, again in order of DECREASING wavelength (= increasing frequency):

$\text{red} \to \text{orange} \to \text{yellow} \to \text{green} \to \text{blue} \to \text{indigo} \to \text{violet}$

Standard mnemonic: "Richard Of York Gave Battle In Vain".

Quantitative reminder. A wave with wavelength 600 nm (orange light) has frequency

$f = \dfrac{c}{\lambda} = \dfrac{3 \times 10^{8}}{6 \times 10^{-7}} = 5 \times 10^{14}\,\text{Hz}$

Compare radio at $f \sim 10^{6}$ Hz and gamma at $f \sim 10^{20}$ Hz — fourteen orders of magnitude span.

Edexcel awards a mark per correct pairing. The spec 3.12 list:

• Radio waves — broadcasting (radio, TV) and communications. They diffract around hills and through buildings; long wavelength = robust signal.
• Microwaves — cooking food (~2.45 GHz absorbed by water molecules); satellite TV and phone communications (penetrate the atmosphere).
• Infrared (IR) — electric heaters, grills (cooking by IR radiation); night-vision equipment (warm objects glow in IR); short-range remote controls.
• Visible light — optical fibres (light pulses carry digital data); photography (cameras capture reflected visible light).
• Ultraviolet (UV) — fluorescent lamps (UV excites a phosphor coating which then emits visible light); detecting forged bank notes (security marks fluoresce in UV).
• X-rays — observing the internal structure of objects (bones in medical imaging; weld inspection in industry); airport security (luggage scanning).
• Gamma rays — sterilising food and medical equipment (gamma kills bacteria and viruses); radiotherapy (treating cancer tumours).

Why each band suits its use. A pattern question on Paper 1 may ask you to JUSTIFY the choice:

• Radio: long wavelength → diffracts around obstacles → broadcast range.
• Microwave: ~2.45 GHz resonates with water → heats food efficiently.
• IR: absorbed by surfaces → surface heating; emitted by all warm objects → detected by thermal cameras.
• UV: high energy per photon → ionises some atoms → fluorescence.
• X-ray: penetrates soft tissue, absorbed by dense matter (bone) → image contrast.
• Gamma: very high energy → kills cells, deeply penetrating → sterilisation.

Each band has a DIFFERENT mechanism for harming tissue:

• Microwaves — INTERNAL HEATING. Microwaves absorbed by water inside cells → cellular damage / burns from inside out. Microwave ovens have metal mesh in the door (holes smaller than $\lambda$) to keep the radiation in.
• Infrared — SKIN BURNS. Absorbed at the SURFACE → surface skin burns. Use of insulating gloves / safety screens near hot objects.
• Ultraviolet — SURFACE CELL DAMAGE AND BLINDNESS. UV photons damage DNA in surface skin cells → sunburn, premature ageing, skin cancer. UV damage to the retina can cause blindness. Protection: sunscreen, hats, UV-blocking sunglasses, protective clothing, avoid midday sun.
• X-rays / Gamma rays — CANCER AND MUTATION. Very high-energy photons IONISE atoms inside cells → damaged DNA → mutations → cancer. Workers wear LEAD APRONS; sources are stored in lead-lined containers; patients receive only short exposures.

Spec 3.13 phrasing to memorise. "describe simple protective measures against the risks". The protective measures expected:

• Lead aprons / lead-lined containers — for X-ray and gamma.
• Sunscreen, hats, sunglasses, long sleeves — for UV.
• Microwave-oven metal mesh / interlock so oven cannot run with door open — for microwaves.
• Insulating gloves / screens / distance — for IR.

Examiner pattern. A favourite question asks for the HAZARD and a SPECIFIC protective measure. Cluster your answer: hazard (one tissue-level effect) + matching protection.

A favourite Paper-1 4-mark item compares MICROWAVE and IR heating of food:

Microwave (inside-out). Microwaves penetrate food and are absorbed primarily by WATER molecules INSIDE. Water molecules vibrate → kinetic energy spreads as heat. Food heats from within.

Infrared (outside-in). IR is absorbed at the SURFACE. Surface heats first; heat then CONDUCTS inwards. Toasters, grills and electric heaters work this way.

Hazard contrast. Microwave hazard = INTERNAL heating of body tissue (e.g. eyes, blood-rich tissue). IR hazard = SKIN/SURFACE burns.

Practical implication. Microwaves cook unevenly because thicker pieces don't fully absorb the wave before it reaches the centre — hence the rotating turntable in domestic ovens. IR grills can leave the inside cold even when the surface is browned.