3.24PH1 · Section 3: Waves

The electromagnetic spectrum

Key learnings

EM spectrum order; all at c in vacuum
The EM spectrum in order of increasing frequency (decreasing wavelength): radio → microwaves → infrared (IR) → visible → ultraviolet (UV) → X-rays → gamma. Mnemonic: "Ripe Mangoes In Venus Under eXtra Gravity". All seven share three properties: they are transverse waves, they all travel at the speed of light $c = 3 \times 10^8$ m/s in a vacuum, and they can all pass through a vacuum. The wave equation applies to every region: in a vacuum $c = f\lambda$ .
Uses of each EM region
Signature spec uses: radio — broadcasting, satellite comms; microwaves — cooking (absorbed by water), phone and satellite signals; IR — remote controls, thermal imaging, optical fibres; visible — vision, photography, fibre broadband; UV — sterilising water/equipment, fluorescent lamps, security ink; X-rays — imaging bones and teeth, security scanners, checking welds; gamma — radiotherapy (killing cancer cells), sterilising equipment and food.
Ionising radiation: UV, X-ray, gamma
Ionising EM radiation has enough energy per photon to remove electrons from atoms: UV, X-rays and gamma (boundary between visible and UV). Non-ionising (radio, microwave, IR, visible) cannot ionise but can harm at high intensity. Hazards (spec 3.13): microwaves → internal heating; IR → skin burns; UV → surface cell damage, blindness; gamma → cancer and mutation. Precautions: lead/concrete shielding for X-rays and gamma; sunscreen/eyewear for UV; limit exposure.

Common mistakes

Drawn from real examiner reports.

Wrong spectrum order (middle three)

A common error puts visible light before infrared, or UV where X-ray belongs. Correct ascending-frequency order: radio → microwave → IR → visible → UV → X-ray → gamma. Higher frequency = shorter wavelength = more energetic. Correct extremes but a jumbled middle still loses marks. The mnemonic "Ripe Mangoes In Venus Under eXtra Gravity" fixes the middle.

June 2024 Paper 2PR general comments: candidates who misremembered the spectrum ordering lost marks on matching questions — the examiner noted that confusing the sequence was a consistent source of errors.

Speed of light without "in a vacuum"

Writing "all EM waves travel at $3 \times 10^8$ m/s" is incomplete. The full statement is " $c = 3 \times 10^8$ m/s in a vacuum (free space)". Two elements: the value AND "in a vacuum". EM waves slow in a denser medium (glass, water), which is why refraction happens. Like "opposition to current" for resistance, dropping the qualifier can score zero on a definition.

November 2024 Paper 2P examiner report summary: explicitly listed recalling and using units/qualifiers correctly as an area requiring improvement across the paper.

Microwaves are non-ionising

A persistent myth: "microwaves cause cancer because they are radiation". Microwaves are non-ionising — they heat tissue by vibrating water molecules (thermal effect), but cannot remove electrons or damage DNA directly. Only UV, X-rays and gamma are ionising. Related error: gamma used for cooking or phone signals — gamma is used in medicine and sterilisation only.

June 2024 Paper 2PR general comments: candidates who applied all hazards to a single EM region on matching questions scored zero — knowing which types are ionising and which hazards apply to each is essential.

Gamma is not faster than radio

All EM waves travel at the SAME speed, $c = 3 \times 10^8$ m/s in a vacuum. Gamma rays are not faster than radio waves — they have a higher frequency and shorter wavelength, but identical speed. Since $c = f\lambda$ is constant, as $f$ rises $\lambda$ falls in proportion. Do not equate "more energy / higher frequency" with "faster".

Swapping EM uses and hazards

Keep each region's USE separate from its HAZARD. Microwaves — use: cooking/comms; hazard: internal heating. UV — use: fluorescent lamps/sterilising; hazard: surface cell damage, blindness. Gamma — use: radiotherapy/sterilising; hazard: cancer, mutation. On matching questions, applying one hazard to every region, or mixing a use with a hazard, scores zero.

Convert MHz, GHz, nm before c = fλ

Before using $c = f\lambda$ , convert to base units: MHz → Hz (× $10^6$ ), GHz → Hz (× $10^9$ ), nm → m (× $10^{-9}$ ), cm → m (× $10^{-2}$ ). A frequency left in MHz or a wavelength in nm gives an answer off by many powers of ten. Convert first, substitute $c = 3 \times 10^8$ m/s, then add the unit (m or Hz).

Exam tips

Hazard answers: classify, harm, precaution
For a hazard question give three parts: (a) ionising or non-ionising; (b) the specific spec hazard — skin burns, DNA damage, cancer, blindness or internal heating; (c) the precaution. A "classify → harm → precaution" chain earns full marks; "it is dangerous" scores zero.
Ordering: give all seven + the basis
For ordering questions write the full seven-region sequence (radio → gamma) and, if asked for the basis, state "increasing frequency / decreasing wavelength". A partial list or a reversed middle (IR/visible/UV) loses marks. Use the mnemonic to lock the order.
c = fλ: convert units, add the unit
For $c = f\lambda$ use $c = 3 \times 10^8$ m/s and convert first: MHz → Hz (× $10^6$ ), GHz → Hz (× $10^9$ ), nm → m (× $10^{-9}$ ). Rearrange for the unknown ( $\lambda = c/f$ or $f = c/\lambda$ ) and write the unit (m or Hz) on the answer.
Why X-rays beat IR: energy chain
For "why are X-rays more hazardous than IR", give the chain: X-rays have higher frequency → higher energy per photon → enough to ionise atoms → breaks DNA bonds → mutations/cancer. IR is low-energy and non-ionising, so it only heats (burns). Name ionisation explicitly.

Revision notes