3.30625 · Section 3: Waves

Electromagnetic spectrum

Key learnings

EM spectrum order and shared properties
The EM spectrum in order of increasing frequency (decreasing wavelength): radio → microwave → infrared (IR) → visible → ultraviolet (UV) → X-ray → gamma. Mnemonic: Royal Mail Is Very Useful eXcept Gambling.

All seven are transverse, all travel at the same high speed in a vacuum ((E) $c = 3.0 \times 10^8$ m/s), and none needs a medium. Visible colours by frequency: red → orange → yellow → green → blue → indigo → violet (ROYGBIV), red lowest, violet highest.
Cambridge uses and harmful-effects list
Each region has Cambridge-specified uses and one harmful effect — give the exact spec example, not a generic word. Harmful effects (3.3.4): microwaves → internal heating of cells; IR → skin burns; UV → damage to surface cells and eyes, skin cancer; X-rays and gamma → mutation or damage to cells. Example uses: radio (broadcasting, astronomy, RFID), UV (sterilising water, detecting fake banknotes), gamma (sterilising equipment, cancer treatment).

Common mistakes

Drawn from real examiner reports.

Spectrum order: do not reverse the middle

Order by decreasing wavelength (= increasing frequency): radio > microwave > IR > visible > UV > X-ray > gamma. The extremes (radio longest, gamma shortest) are usually fine; the middle (IR, visible, UV) gets reversed — a widely-guessed trap where candidates put X-rays at the long-wavelength end. Check: higher frequency → shorter wavelength, since $c = f\lambda$ .

June 2024 Paper 0625/12 Core MCQ Q21: responses evenly distributed across all four options — most candidates guessing. "Some candidates incorrectly thought that X-rays have the longest wavelength and radio waves or microwaves have the shortest wavelength."

Vague uses score zero

"Medicine" for X-rays, "kills things" for gamma, "heating" for IR and "cleaning" for UV all score zero. Cambridge needs the specific spec example: X-rays → medical scanning of bones/teeth OR airport security scanners; gamma → sterilising equipment OR cancer treatment; IR → electric grills OR intruder alarms; UV → sterilising water OR detecting fake banknotes.

November 2023 P52 Q5(b)(i): vague uses ("heating", "security") scored zero; November 2023 P22 Q5(b): "uses such as 'medicine' that were too vague for credit".

Do not swap IR and UV harmful effects

Confusing hazards loses marks. IR → skin burns (thermal), not skin cancer. UV → damage to surface cells and eyes, skin cancer — not burns. Microwaves → internal heating of cells, NOT mutations. X-rays and gamma → mutation or damage to cells, not heating. Microwaves heat tissue thermally; they do not ionise, so "microwaves cause mutations" scores zero.

November 2023 P22 Q5(c): "A common error was to confuse the harmful effects of infrared and ultraviolet radiation." June 2024 P12 general comments: misconceptions about which EM regions produce ionising radiation.

EM waves share speed only in a vacuum

All seven EM regions travel at the same speed in a vacuum — but not in a material. In water or glass each region slows by a different amount (which is why glass disperses white light). Saying "all EM waves travel at the same speed in water" is wrong (a flagged error in Nov 2023 Paper 22 Q22): they share one speed only in a vacuum, and very nearly in air.

(E) Wave-speed sums: mind the powers of ten

(E) For $c = f\lambda$ ( $c = 3.0 \times 10^8$ m/s in a vacuum), two errors dominate: rearranging wrongly and mishandling powers of ten. Convert units FIRST — MHz → Hz (×10⁶), GHz → Hz (×10⁹), nm → m (×10⁻⁹) — then rearrange ( $f = c/\lambda$ or $\lambda = c/f$ ) and substitute. In Nov 2023 Paper 32 Q6(b)(i) many gave 150 Hz from a power-of-ten slip.

Name a real region longer than infrared

Asked for a region with wavelength longer than infrared, many name one that is not — only microwaves and radio waves are longer than IR. Invented names also score zero: in Nov 2023 Paper 52 Q5(a)(i) "Y-rays", "sound waves" and "alpha waves" were common wrong answers — sound is not an EM wave and alpha is a particle. Use only the seven real region names.

Exam tips

Write the full spectrum before answering
For spectrum-completion questions, write the full seven-region sequence (radio → gamma) on your working space first, then fill the blank boxes. This one habit stops the mid-spectrum reversals (IR/visible/UV) that lose the most marks.
Quote the exact spec use or hazard
For "state a use" or "state a harmful effect", quote the exact word from the syllabus list: "heating" scores zero, but "internal heating of cells" scores the mark. Never give a generic word like "medicine" or "security".
(E) Convert units before the wave-speed sum
(E) For $c = f\lambda$ ( $c = 3.0 \times 10^8$ m/s): convert units first (MHz → Hz ×10⁶, nm → m ×10⁻⁹), then write the rearranged formula, substitute, and add the unit (Hz or m). Powers of ten are the main failure point.
(E) Explain comms choices in clear steps
(E) "Why microwaves for satellite comms?": they penetrate the atmosphere and can be aimed at a geostationary satellite by a dish. "Why do optical fibres carry more data?": they use visible/IR light through transparent glass, and high frequency gives a high data rate.

Revision notes