The Electromagnetic Spectrum

The electromagnetic spectrum is one family of transverse waves, from radio waves to gamma rays. Cambridge tests it every session because it packs three mark types into one subtopic: ordered recall, applications and hazards, and a $v = f\lambda$ calculation with very large numbers.

What is the order of the electromagnetic spectrum?

In order of increasing frequency (and decreasing wavelength): radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. Learn it in one direction with a mnemonic and always rebuild it the same way. Within visible light, red has the longest wavelength and violet the shortest.

All electromagnetic waves share three properties. They are transverse. They can travel through a vacuum. They all travel at the same speed in a vacuum: $3.0 \times 10^{8}\ \text{m/s}$, which is approximately their speed in air too. Extended candidates must quote that value from memory.

Region	Typical use	Main hazard
Radio	Broadcasting, communications	None
Microwaves	Satellite TV, mobile (cell) phones, cooking	Internal heating of body tissue
Infrared	Remote controls, thermal imaging, electric grills	Skin burns
Visible	Vision, photography, illumination	None
Ultraviolet	Sterilising water, detecting forged banknotes, sunbeds	Skin cancer, eye damage
X-rays	Medical imaging, security scanning	Mutation/damage to cells
Gamma	Sterilising medical equipment, cancer treatment	Mutation/damage to cells, cancer

High-frequency radiation (UV, X-rays, gamma) carries the serious hazards. Pair every use with its region exactly. “Microwaves for cooking” scores; “radiation for cooking” does not.

Which electromagnetic waves are used for communication?

This is the Extended (Supplement) strand. Satellite communication and mobile phones use microwaves, because microwaves pass through the ionosphere to reach satellites. Bluetooth and Wi-Fi use microwaves and high-frequency radio. Optical fibres carry visible light or infrared for cable TV and high-speed broadband, because glass transmits these with little absorption. Extended candidates also distinguish digital from analogue signals: digital transmission gives higher quality, because a degraded digital signal can be regenerated exactly, and carries more information per second.

Worked Exam Question

A radio station broadcasts at a frequency of 100 MHz. Electromagnetic waves travel at $3.0 \times 10^{8}\ \text{m/s}$ in air. Calculate the wavelength of the broadcast. [3]

Worked solution:

1. Equation: $v = f\lambda$, rearranged to $\lambda = v \div f$
2. Convert: $100\ \text{MHz} = 100 \times 10^{6}\ \text{Hz} = 1.0 \times 10^{8}\ \text{Hz}$
3. Substitute: $\lambda = (3.0 \times 10^{8}) \div (1.0 \times 10^{8})$
4. Answer: $\lambda = 3.0\ \text{m}$ (2 significant figures)

Mark scheme:

• M1: $\lambda = v \div f$ stated or implied
• M1: correct conversion of MHz to Hz and substitution
• A1: $3.0\ \text{m}$ with unit

Common Mistakes

• Reversing the spectrum order under pressure. Decide once: rebuild it from radio (lowest frequency) every time.
• Forgetting the MHz or GHz conversion. 100 MHz is $10^{8}\ \text{Hz}$; skipping the $10^{6}$ factor wrecks the wavelength by a million.
• Saying different EM waves travel at different speeds in a vacuum. They all travel at $3.0 \times 10^{8}\ \text{m/s}$ in a vacuum.
• Writing “radiation causes cancer” for every region. Only ionising, high-frequency regions (UV, X-ray, gamma) earn the cancer/mutation mark.
• Calling EM waves longitudinal. The whole spectrum is transverse.

Exam Technique Tip

For standard-form calculations, process powers of ten separately from the leading digits. Compute $3.0 \div 1.0$ first, then $10^{8} \div 10^{8}$, and combine. Writing both steps protects the method mark when a calculator slip occurs. Examiners follow your working, and a visible correct method limits the damage to one mark.

How This Is Examined

Expect this subtopic on every paper. Papers 1 and 2 ask order questions, use-region matching and one-step calculations. Papers 3 and 4 set the full $\lambda = v \div f$ calculation plus short answers on uses and dangers. Extended-only material includes the memorised speed value, communication systems and digital versus analogue signals. There is no associated practical, so Papers 5 and 6 leave it alone. The table looks like a memory burden, but it compresses fast. Most students lock the order, one use and one hazard per region inside a single 1-to-1 lesson.