Waves Common Exam Traps

Overview

This page is a fast revision warning sheet for Waves.

Focus on avoiding:

• misconceptions
• graph-reading mistakes
• phase errors
• formula misuse
• wrong wave classification
• careless interpretation errors

This is not a full lesson note. Use it for quick exam revision.

Why It Matters

Wave questions are often lost through misreading graphs, mixing up definitions, or using correct formulas on the wrong quantities. A short traps page is useful because these mistakes are common, repetitive, and easy to fix once made explicit.

Definition

This page is a revision support note collecting common misconceptions and quick corrections for foundational wave ideas, especially graph interpretation, phase, wave classification, and the physical meaning of standard relations.

Key Representations

Core forms to keep straight:

$$v=f\lambda$$ $$f=\frac{1}{T}$$ $$\Delta \varphi =\frac{2\pi \Delta x}{\lambda }$$ $$\Delta \varphi =\frac{2\pi \Delta t}{T}$$ $$I=\frac{P}{A}$$

Trap 1: Confusing Particle Motion with Wave Propagation

Wrong idea: Particles move forward with the wave.

Correct: In most mechanical waves, particles oscillate about equilibrium while the disturbance travels.

Example:

• sound wave travels forward
• air molecules mainly vibrate back and forth

Reminder: Energy moves; matter does not move overall.

Trap 2: Mixing Up Displacement-Distance and Displacement-Time Graphs

Wrong idea: Both graphs show the same thing.

Correct:

Displacement-Distance Graph

• snapshot of many particles at one instant
• use to find wavelength

Displacement-Time Graph

• motion of one particle over time
• use to find period and frequency

$$f=\frac{1}{T}$$

Reminder: Space graph gives $\lambda$, time graph gives $T$.

Trap 3: Confusing Amplitude with Wavelength

Wrong idea: Amplitude and wavelength are both “wave size”.

Correct:

• amplitude = maximum displacement from equilibrium
• wavelength = distance between adjacent in-phase points

Reminder: Vertical scale often gives amplitude; horizontal spacing often gives wavelength.

Trap 4: Misusing $v=f\lambda$

Wrong idea: Increasing frequency always increases speed.

Correct: In a given medium, wave speed is usually fixed.

$$v=f\lambda$$

So if $f$ increases in same medium, $\lambda$ decreases.

Reminder: Frequency is set by source; speed depends on medium.

Trap 5: Getting Phase Difference Wrong

Wrong idea: Any two peaks are always $\pi$ out of phase.

Correct:

• one full cycle = $2\pi$
• half cycle = $\pi$
• quarter cycle = $\frac{\pi }{2}$

By distance:

$$\Delta \varphi =\frac{2\pi \Delta x}{\lambda }$$

By time:

$$\Delta \varphi =\frac{2\pi \Delta t}{T}$$

Reminder: Same position in repeating cycle = in phase.

Trap 6: Thinking All Waves Need a Medium

Wrong idea: Every wave requires particles.

Correct:

Mechanical Waves

Need medium:

• sound
• water waves

Electromagnetic Waves

Do not need medium:

• light
• radio
• X-rays

Reminder: Light travels through vacuum.

Trap 7: Saying Sound Is Transverse

Wrong idea: Sound has crests and troughs like water waves.

Correct: Sound in air is longitudinal.

It consists of:

• compressions
• rarefactions

Particle oscillation is parallel to wave travel.

Trap 8: Forgetting Only Transverse Waves Can Be Polarised

Wrong idea: Any wave can be polarised.

Correct: Only transverse waves can be polarised.

Therefore:

• light can be polarised
• sound in air cannot

Reminder: Polarisation is strong evidence of transverse nature.

Trap 9: Confusing Intensity with Amplitude

Wrong idea: Intensity and amplitude are the same quantity.

Correct:

Intensity is power per unit area:

$$I=\frac{P}{A}$$

For many waves:

$$I\propto A^2$$

where $A$ here means amplitude.

Reminder: Double amplitude gives four times intensity.

Trap 10: Misreading the Inverse-Square Relationship

Wrong idea: Double distance gives half intensity.

Correct:

For point-source spreading:

$$I\propto \frac{1}{r^2}$$

So doubling distance gives:

$$\frac{1}{4}$$

of original intensity.

Tripling distance gives:

$$\frac{1}{9}$$

Trap 11: Mixing Frequency and Period

Wrong idea: Frequency increases when period increases.

Correct:

$$f=\frac{1}{T}$$

So larger period means lower frequency.

Reminder: Slow oscillation = long $T$ = small $f$.

Trap 12: Thinking Destructive Interference Means Energy Vanishes

Wrong idea: Waves cancel and energy disappears.

Correct: Energy is redistributed to other regions (constructive areas), not destroyed.

See Interference and Diffraction.

Quick Checklist

Before submitting answers, ask:

• Is this graph in space or in time?
• Did I confuse amplitude with wavelength?
• Is speed determined by source or medium?
• Is this wave transverse or longitudinal?
• Can it be polarised?
• Did I use $\Delta \varphi$ correctly?
• Did I apply inverse-square law properly?
• Did I mix frequency and period?

Related Links

• Waves
• Stationary Waves
• Interference and Diffraction
• Sound Measurement Practicals
• Superposition of Waves
• Oscillations and Simple Harmonic Motion

Links

• Main topic: Waves
• Related concept: Stationary Waves
• Related concept: Interference and Diffraction
• Related concept: Sound Measurement Practicals
• Related topic: Superposition of Waves

Final Memory Line

Most wave mistakes come from confusing:

• particle motion vs wave travel
• space graph vs time graph
• amplitude vs wavelength
• frequency vs speed
• wave type vs wave behaviour