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C.2.4—Nature of electromagnetic waves

Syllabus
First assessment 2025
Objective
—
Level
HL

Explain the Nature of Electromagnetic Waves

Oscillating fields

An electromagnetic wave consists of oscillating electric and magnetic fields. The two fields are perpendicular to each other and both are perpendicular to the direction of propagation and energy transfer, so the wave is transverse.

No material medium is required

Electromagnetic fields can propagate through a vacuum. Every electromagnetic wave travels in vacuum at c=3.00×108 m s−1c=3.00\times10^8\,\mathrm{m\,s^{-1}}, with c=fλc=f\lambda.

One spectrum, approximate regions

Radio, microwave, infrared, visible, ultraviolet, X-ray and gamma radiation are all electromagnetic waves. Use the approximate wavelength orders of magnitude supplied in the Physics data booklet; the named regions do not have perfectly sharp physical boundaries.

Common trap

Different spectrum regions do not have different vacuum speeds. They differ in frequency and wavelength while satisfying the same value of cc.

C.2.4 Exam Analysis

Assessment in practice

1–2 marks
How it is assessed

Questions ask for the definition of a transverse wave or the nature of an electromagnetic wave in vacuum. The evidence rewards the perpendicular relationship and correct wave classification.

Command terms

State / What is

What earns marks

State the direction of particle/field oscillation relative to energy propagation. For transverse waves use perpendicular; for longitudinal waves use parallel and identify compressions/rarefactions when relevant.

Watch for

Calling every mechanical wave transverse or defining transverse motion without referencing the direction of propagation.

Representative question

Question 1

[Maximum number: 1]

An ultraviolet wave is travelling in a vacuum.

What is the frequency and the nature of the wave?

Wave frequency / Hz

Nature of the wave

101510^{15}

transverse

101510{ }^{15}

longitudinal

10−710^{-7}

transverse

10−710^{-7}

longitudinal

Retrieve the C.2 Wave Model

Model the transfer

A travelling wave propagates a disturbance and transfers energy without a resultant transport of the medium. Use the local particle/field motion to describe oscillation, and the wave direction to describe propagation.

Connect the quantities

Describe waves with wavelength λ\lambda, frequency ff, period TT, amplitude and speed vv, linked by v=fλ=λ/Tv=f\lambda=\lambda/T. Measure wavelength between adjacent in-phase points.

Classify the wave

Transverse oscillations are perpendicular to propagation; longitudinal oscillations are parallel. Mechanical waves require a medium, while electromagnetic waves are transverse field oscillations and travel at cc in vacuum.

Final check

At a boundary, identify which quantity is fixed by the source and which properties change in the new medium. Keep units consistent and distinguish propagation speed from the local oscillation speed.