Electromagnetic spectrum

Physics· Unit 7: Waves, Topic 5· 15 min read

1. Order and Common Properties of EM Waves★☆☆☆☆⏱ 5 min

All electromagnetic waves are transverse waves, meaning oscillations of electric and magnetic fields are perpendicular to the direction of energy transfer. They all travel at $c = 3.0 \times 1 0^{8} m s^{- 1}$ in a vacuum, and do not require a medium to propagate.

📘 Definition

Electromagnetic spectrum

The full continuous range of electromagnetic radiation ordered by wavelength (or frequency), grouped into categories based on their properties and uses.

📐 Worked Example

State the order of the following EM waves from lowest to highest frequency: infrared, gamma rays, radio waves, ultraviolet. Identify which wave has the longest wavelength.

1
1. Recall that wavelength and frequency are inversely proportional ( $c = f λ$ ), so lowest frequency = longest wavelength.
2
1. Using the mnemonic, order from lowest to highest frequency (same as longest to shortest wavelength):
3
$\text{Lowest} \rightarrow \text{Radio waves} < \text{Infrared} < \text{Ultraviolet} < \text{Gamma Rays} \rightarrow \text{Highest}$
4
1. Longest wavelength corresponds to lowest frequency.
5
Final answer: Order (lowest to highest frequency): Radio waves, Infrared, Ultraviolet, Gamma rays. Radio waves have the longest wavelength.

Exam tip:

Always double-check if the question asks for order by wavelength or frequency — easy marks are lost for reversing the order.

2. Approximate Wavelength Ranges★★☆☆☆⏱ 4 min

CIE expects you to remember order-of-magnitude wavelength ranges for each EM group. You do not need to memorise exact cut-off points, just the approximate range for each category:

EM Group	Approximate Wavelength Range (vacuum)
Radio	$> 1 0^{- 1} m$
Microwaves	$1 0^{- 3} m to 1 0^{- 1} m$
Infrared	$7 \times 1 0^{- 7} m to 1 0^{- 3} m$
Visible light	$4 \times 1 0^{- 7} m to 7 \times 1 0^{- 7} m$
Ultraviolet	$1 0^{- 8} m to 4 \times 1 0^{- 7} m$
X-rays	$1 0^{- 12} m to 1 0^{- 8} m$
Gamma rays	$< 1 0^{- 12} m$

✓ Quick check

Test your understanding of ranges:

Which of the following is the correct wavelength range for visible light?
- $1 0^{- 3} m to 1 0^{- 1} m$
- $4 \times 1 0^{- 7} m to 7 \times 1 0^{- 7} m$
- $1 0^{- 8} m to 4 \times 1 0^{- 7} m$
- $< 1 0^{- 12} m$
Reveal answer
1 —
Correct! Visible light spans 400 nm (violet) to 700 nm (red), which matches this range.

3. Common Uses★★☆☆☆⏱ 3 min

Examiners frequently ask to match EM waves to their common uses. You should be familiar with these key examples for CIE:

Radio waves: Broadcasting radio and TV signals
Microwaves: Cooking food, satellite and mobile communications
Infrared: Remote controls, thermal imaging, heating
Visible light: Fibre optic communications, illumination
Ultraviolet: Sterilising water, detecting forged bank notes
X-rays: Medical bone imaging, airport security scanning
Gamma rays: Sterilising medical equipment, cancer treatment

📐 Worked Example

Name the type of EM wave used for satellite communications, and explain why it is suitable for this purpose.

1
1. Recall common uses of microwaves include satellite communications.
2
1. Microwaves have wavelengths that can pass through the Earth's atmosphere without significant absorption or scattering.
3
1. They can carry large amounts of information, and can be transmitted directionally to satellites.
4
Final answer: Microwaves, because they can penetrate the atmosphere to reach satellites.

4. Hazards and Ionizing Ability★★☆☆☆⏱ 3 min

Higher frequency EM waves carry more energy per photon. Waves with frequency higher than ultraviolet are ionizing, meaning they can damage DNA in living cells and increase cancer risk.

📘 Definition

Ionizing radiation

Radiation with enough energy to remove electrons from atoms, causing ionization that can damage DNA and lead to cancer or cell death.

Example:

Gamma rays, X-rays, and high-energy ultraviolet are ionizing.

Microwaves: Internal heating of body tissue, can cause burns at high intensity
Infrared: Skin burns from excessive heating
Ultraviolet: Sunburn, skin cancer, damage to eye retinas
X-rays/Gamma rays: DNA mutation, cell damage, increased long-term cancer risk

5. Common Pitfalls

Wrong move:

Claiming EM waves require a medium to travel.

Why:

EM waves are non-mechanical, unlike sound, they travel through vacuum.

Correct move:

Always state that EM waves do not require a medium, and travel at $3 \times 1 0^{8} m s^{- 1}$ in vacuum.

Wrong move:

Reversing the order of wavelength and frequency.

Why:

Candidates often misread the question and give the opposite order to what is asked, losing easy marks.

Correct move:

Check the question wording, then use your mnemonic to confirm the order before answering.

Wrong move:

Claiming all EM waves have the same frequency.

Why:

Only speed in a vacuum is constant, frequency varies across the spectrum.

Correct move:

Remember that only speed in a vacuum is identical for all EM waves, frequency and wavelength vary.

Wrong move:

Memorizing exact cut-off values for wavelength ranges.

Why:

CIE does not ask for exact values, so this wastes valuable revision time.

Correct move:

Learn only the order-of-magnitude for each range, and the order of groups.

6. Quick Reference Cheatsheet

Group	Approx Wavelength	Key Use	Main Hazard
Radio	10 cm	Broadcasting	Low hazard
Microwaves	1 mm - 10 cm	Satellite comms	Internal heating
Infrared	700 nm - 1 mm	Remote controls	Skin burns
Visible	400 - 700 nm	Fibre optics	None (low intensity)
Ultraviolet	10 nm - 400 nm	Sterilisation	Skin cancer
X-rays	1 pm - 10 nm	Medical imaging	Ionizing, cancer risk
Gamma rays	< 1 pm	Cancer treatment	Highly ionizing

7. Frequently Asked

Do I need to memorize exact wavelength cut-offs for each group?

No, CIE examiners only require you to know approximate order-of-magnitude ranges and the order of groups by wavelength/frequency.

Do all electromagnetic waves travel at the same speed?

Yes, all EM waves travel at $c = 3.0 \times 1 0^{8} m s^{- 1}$ in a vacuum. Speed changes only when entering a new medium.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · 12
Order EM spectrum by wavelength
2022 · 22
Uses and hazards of EM waves
2021 · 11
Common properties of EM waves

Going deeper

syllabusOfficial CIE 9702 SyllabusConfirm latest requirements for wavelength ranges

What's Next

Mastery of the electromagnetic spectrum is foundational for later topics on photon energy, quantum physics, and medical physics, where you will build on the properties of different EM waves. It is a high-frequency topic in CIE exams, so regularly revising the order, ranges, uses and hazards will secure you easy marks. You can now continue building your knowledge of waves and related topics with the links below.