Physics • Year 12 • Module 7 • Lesson 1
The Electromagnetic Spectrum
Lock in the seven EM regions, the universal wave equation, and photon energy relationships before tackling harder questions.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: electromagnetic wave, photon, wavelength, frequency, speed of light, Planck's constant, infrared, ultraviolet, gamma ray, electronvolt. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | A transverse wave consisting of oscillating electric and magnetic fields perpendicular to each other and to the direction of propagation; requires no medium. | |
| 1.2 | A discrete quantum of electromagnetic energy; its energy is given by E = hf. | |
| 1.3 | The distance between successive crests of a wave, measured in metres (symbol λ). | |
| 1.4 | The number of complete wave cycles passing a point per second, measured in hertz (Hz). | |
| 1.5 | The universal constant c = 3.00 × 108 m/s; the speed at which all EM waves travel in a vacuum. | |
| 1.6 | The fundamental constant h = 6.63 × 10−34 J·s that relates photon energy to frequency. | |
| 1.7 | The region of the EM spectrum with wavelengths roughly 10−5 m, felt as heat and used in thermal imaging. | |
| 1.8 | The region of the EM spectrum with wavelengths roughly 10−8 m; can cause sunburn and damage DNA. | |
| 1.9 | The highest-energy region of the EM spectrum, with wavelengths around 10−12 m, produced by nuclear decay. | |
| 1.10 | A unit of energy equal to 1.60 × 10−19 J; convenient for expressing photon energies. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the corrected version on the line below it. 12 marks (1 T/F + 1 correction each)
2.1 All electromagnetic waves travel at different speeds in a vacuum, depending on their frequency. T / F
2.2 Radio waves have the lowest frequency and the longest wavelength in the EM spectrum. T / F
2.3 Visible light makes up the largest portion of the electromagnetic spectrum in terms of wavelength range. T / F
2.4 When light enters glass from air, its frequency decreases while its wavelength and speed both decrease. T / F
2.5 A photon of X-rays carries more energy per photon than a photon of visible light. T / F
2.6 Electromagnetic waves are longitudinal waves because the electric field oscillates along the direction of travel. T / F
3. Fill-in-the-blank paragraph
Use the word bank to complete the passage. Each word or phrase is used once. 8 marks (1 per blank)
Word bank:
transverse · c = fλ · vacuum · frequency · gamma · E = hf · ionising · wavelength
All electromagnetic waves are ___________ waves — the oscillating electric and magnetic fields are perpendicular to the direction of travel. In a ___________, all EM waves travel at the same speed c = 3.00 × 108 m/s regardless of their ___________. The fundamental wave equation ___________ relates the speed of light to wavelength and frequency. Within the EM spectrum, ___________ rays have the shortest ___________ and the highest photon energy. The photon energy equation ___________ shows that higher frequency corresponds to more energy per photon. High-energy photons such as X-rays and gamma rays are described as ___________ because they carry enough energy to remove electrons from atoms.
4. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
4.1 What two quantities does the wave equation c = fλ relate, and what happens to wavelength when frequency doubles (at constant speed)?
4.2 Explain why gamma rays are biologically dangerous while radio waves of the same intensity are harmless, using the photon energy equation.
4.3 What happens to the speed and wavelength of visible light when it passes from air into glass (n = 1.5)? Does the frequency change?
4.4 State the seven regions of the EM spectrum in order from lowest to highest frequency.
5. Label the electromagnetic spectrum
The diagram below shows the electromagnetic spectrum as a bar. Label boxes A–G with the correct region name. Then write the approximate wavelength range and one practical application for each region in the table. 14 marks (1 label + 1 application each, 2 for wavelength range column — any 3 rows)
| Box | Region name | Approx. wavelength range | One practical application |
|---|---|---|---|
| A | |||
| B | |||
| C | |||
| D | |||
| E | |||
| F | |||
| G |
Q1 — Term–definition match
1.1 electromagnetic wave • 1.2 photon • 1.3 wavelength • 1.4 frequency • 1.5 speed of light • 1.6 Planck's constant • 1.7 infrared • 1.8 ultraviolet • 1.9 gamma ray • 1.10 electronvolt.
Q2 — True / false with correction
2.1 False. All EM waves travel at the same speed c = 3.00 × 108 m/s in a vacuum, regardless of frequency.
2.2 True.
2.3 False. Visible light covers only a very narrow band (~400–700 nm) of the total EM spectrum. Radio waves span by far the largest wavelength range.
2.4 False. When light enters glass, the frequency stays constant (it is determined by the source). Both speed and wavelength decrease.
2.5 True. X-rays have higher frequency than visible light, so E = hf gives a higher photon energy.
2.6 False. EM waves are transverse waves — the electric and magnetic fields oscillate perpendicular to the direction of travel, not along it.
Q3 — Cloze paragraph
In order: transverse / vacuum / frequency / c = fλ / gamma / wavelength / E = hf / ionising.
Q4.1 — Wave equation
The wave equation c = fλ relates frequency (f) and wavelength (λ), with the speed of light c constant in vacuum. When frequency doubles, wavelength halves (they are inversely proportional at constant c).
Q4.2 — Biological danger
By E = hf, gamma rays (very high f) carry photon energies of order 10−13 J or more — enough to ionise atoms and break chemical bonds in DNA, causing cell damage. Radio waves (very low f) carry photon energies of order 10−25 J, which is far too small to ionise atoms or damage biological molecules.
Q4.3 — Light entering glass
Speed decreases to v = c/n = 3.00×108/1.5 = 2.00×108 m/s. Wavelength decreases by the same factor: λ' = λ/n. Frequency does NOT change — it is determined by the source and remains constant when entering any medium.
Q4.4 — Seven EM regions (low to high frequency)
Radio waves → Microwaves → Infrared → Visible light → Ultraviolet → X-rays → Gamma rays.
Q5 — Labelled EM spectrum
A: Radio waves — ~103–10−1 m — AM/FM radio, TV broadcasting, mobile phone communication.
B: Microwaves — ~10−1–10−3 m — Microwave ovens, radar, Wi-Fi.
C: Infrared — ~10−3–7×10−7 m — Thermal imaging, remote controls, night vision.
D: Visible light — ~700–400 nm (7×10−7–4×10−7 m) — Human vision, photography, fibre optics.
E: Ultraviolet — ~4×10−7–10−8 m — Sterilisation, forensic investigation, sunscreen design.
F: X-rays — ~10−8–10−11 m — Medical imaging (bone scans), airport security screening.
G: Gamma rays — ~10−11–10−14 m — Cancer radiotherapy, nuclear medicine, sterilisation of medical equipment.