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C.3.5—Superposition

Syllabus
First assessment 2025
Objective
—
Level
HL

Apply Superposition

Add overlapping displacements

When waves overlap, the resultant displacement at a point is the algebraic sum of the individual displacements: yresultant=y1+y2y_{\mathrm{resultant}}=y_1+y_2. The waves then continue propagating after the overlap.

Keep the signs

Displacements on the same side of equilibrium add; opposite displacements partially or completely cancel. Equal opposite pulses can produce zero displacement at an instant without destroying either wave.

Connect to interference

Repeated superposition of coherent waves can create stable maxima and minima. A diffraction pattern extending beyond a geometrical shadow is evidence that wave overlap and interference are involved.

Common trap

Do not add amplitudes as positive magnitudes only, and do not treat destructive interference as permanent disappearance of the waves.

C.3.5 Exam Analysis

Assessment in practice

1–3 marks
How it is assessed

Questions ask for a resultant displacement or explain why a diffraction pattern supports the wave model. The evidence rewards signed addition and an explicit link between maxima/minima and interference.

Command terms

Explain / What is

What earns marks

Add the signed displacements at the specified point and time. Use constructive addition for same-sign displacements and cancellation for opposite-sign displacements; then connect maxima/minima to interference or diffraction evidence.

Watch for

Adding amplitudes as magnitudes and ignoring the sign of each displacement at the stated time.

Representative question

Question 1

[Maximum number: 2]

Early theories of light suggest that a geometrical shadow of the slit will be observed on the screen. Explain how the diffraction pattern formed on the screen provides evidence for the wave theory of light.