Physics • Year 12 • Module 7 • Lesson 2
Properties of Electromagnetic Waves
Lock in the core vocabulary, the production of EM waves, and the inverse square law formula before tackling harder problems.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: electromagnetic wave, transverse wave, intensity, inverse square law, accelerating charge, electric field, magnetic field, reflection, refraction, absorption. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | A wave that is a self-sustaining disturbance in electric and magnetic fields; requires no medium and travels at c in vacuum. | |
| 1.2 | A wave in which the oscillations are perpendicular to the direction of energy transfer. | |
| 1.3 | Power per unit area; measured in W/m²; the equation is I = P/A. | |
| 1.4 | The relationship stating that intensity from a point source is inversely proportional to the square of the distance: I = P/(4πr²). | |
| 1.5 | The physical cause of electromagnetic wave production; when charges undergo this motion, changing fields propagate outward as a wave. | |
| 1.6 | A region of space in which a stationary electric charge experiences a force; denoted E. | |
| 1.7 | A region of space in which a moving charge or current experiences a force; denoted B. | |
| 1.8 | The bouncing of a wave off a surface; angle of incidence equals angle of reflection. | |
| 1.9 | The bending of a wave as it crosses a boundary between two media with different refractive indices; frequency is unchanged. | |
| 1.10 | The process by which wave energy is transferred to a medium, reducing wave intensity; causes heating of the absorbing material. |
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 A stationary electric charge produces electromagnetic waves because it has an electric field around it. T / F
2.2 In an electromagnetic wave, the electric field and magnetic field oscillate perpendicular to each other and to the direction of propagation. T / F
2.3 All electromagnetic waves travel at the same speed in vacuum, approximately 3.0 × 10&sup8; m/s. T / F
2.4 If the distance from a point source of EM radiation doubles, the intensity of the radiation is reduced by half. T / F
2.5 When an EM wave is refracted at a boundary, both its frequency and wavelength change. T / F
2.6 Radio waves are produced by accelerated electrons in antenna circuits; gamma rays are produced by nuclear transitions. T / F
3. Fill-in-the-blank paragraph
Use the word bank to complete the passage. Each word is used once. 8 marks (1 per blank)
Word bank:
accelerate · area · medium · perpendicular · point source · speed of light · transverse · vacuum
Electromagnetic waves are produced when electric charges ___________. Unlike sound waves, EM waves do not require a ___________ to travel through; they can propagate through ___________. All electromagnetic waves travel at the ___________ of approximately 3.0 × 10&sup8; m/s in vacuum. EM waves are classified as ___________ waves because the oscillating electric and magnetic fields are ___________ to the direction of energy transfer. When a ___________ radiates uniformly in all directions, its energy spreads over an ever-increasing spherical surface ___________, causing intensity to decrease with the square of distance.
4. Short recall questions
Answer each question in 1–2 sentences using precise scientific terms. 8 marks (2 each)
4.1 Why does a stationary charge NOT produce electromagnetic waves, while an oscillating charge does?
4.2 State the relationship between the amplitudes of the electric and magnetic fields in an electromagnetic wave and the speed of light.
4.3 Explain why skin absorbs ultraviolet radiation and can be sunburned, but glass is transparent to visible light.
4.4 A laser beam does not obey the inverse square law as its beam propagates through empty space. Why not?
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a linking phrase (e.g. “produces”, “decreases with”, “is a type of”). Aim for at least 6 labelled arrows. 6 marks (1 per valid labelled arrow)
Supplied terms: accelerating charge · electromagnetic wave · intensity · distance · transverse wave · inverse square law.
Q1 — Term–definition match
1.1 electromagnetic wave • 1.2 transverse wave • 1.3 intensity • 1.4 inverse square law • 1.5 accelerating charge • 1.6 electric field • 1.7 magnetic field • 1.8 reflection • 1.9 refraction • 1.10 absorption.
Q2 — True / false with correction
2.1 False. A stationary charge creates a static electric field but does NOT produce electromagnetic waves. Only an accelerating (changing velocity) charge creates the changing fields that propagate outward as an EM wave.
2.2 True. The E and B fields are mutually perpendicular and both are perpendicular to the direction of propagation — this is the defining property of a transverse electromagnetic wave.
2.3 True. All EM waves travel at c ≈ 3.0 × 10&sup8; m/s in vacuum, regardless of frequency or wavelength.
2.4 False. When distance doubles, intensity is reduced to one-quarter (not one-half), because intensity follows the inverse square law: I ∝ 1/r².
2.5 False. When refracted, frequency remains constant (it is determined by the source). Only wavelength (and speed) change as the wave crosses the boundary.
2.6 True. Electrons oscillating in antenna circuits produce radio waves; nuclear transitions (changes in nuclear energy levels) produce gamma rays.
Q3 — Cloze paragraph
In order: accelerate / medium / vacuum / speed of light / transverse / perpendicular / point source / area.
Q4.1 — Stationary vs oscillating charge
A stationary charge produces only a static (non-changing) electric field. No changing field means no propagating wave. An oscillating charge continuously changes its velocity (accelerates), creating a time-varying electric field that in turn induces a time-varying magnetic field — these self-sustaining, propagating fields are an EM wave.
Q4.2 — Field amplitude ratio
The ratio of the amplitudes of the electric and magnetic fields equals the speed of light: E/B = c. The fields are always in a fixed proportion; neither can exist in the wave without the other.
Q4.3 — UV absorption by skin; glass transparent to visible
Different materials absorb different wavelengths depending on their molecular and electronic structure. Skin contains molecules (such as melanin and DNA bases) that absorb UV photons efficiently, causing chemical damage (sunburn). Glass is transparent to visible light because its atomic structure lacks the resonant energy transitions that would absorb those wavelengths — but glass is opaque to UV, which is why ordinary windows block UV.
Q4.4 — Laser and inverse square law
The inverse square law applies only to a point source radiating uniformly in all directions (spherical spreading). A laser beam is highly collimated (parallel rays); the energy is confined to a narrow cylinder, not spread over an expanding sphere. Because the cross-sectional area of the beam stays approximately constant, the intensity does not decrease with distance in the same way.
Q5 — Sample concept map
Award 1 mark per valid labelled arrow. Correct maps should include arrows such as:
- accelerating charge — produces → electromagnetic wave
- electromagnetic wave — is a type of → transverse wave
- intensity — is governed by → inverse square law
- distance — inversely affects (squared) → intensity
- inverse square law — relates intensity to → distance
- electromagnetic wave — has property → intensity
Accept any scientifically correct linking phrase. Minimum 6 valid labelled arrows for full marks.