How to Revise Topic 3: Waves

Topic 3 covers a wide range of wave phenomena. It is diagram-heavy and requires both conceptual understanding and calculation skills.

What Topic 3 covers

1. General wave properties (transverse and longitudinal, amplitude, wavelength, frequency, period, wave speed)
2. Light (reflection, refraction, total internal reflection, lenses, dispersion)
3. Electromagnetic spectrum (types, properties, uses, dangers)
4. Sound (production, properties, speed, echoes, ultrasound)

Priority areas

Must-know calculations

• v = f x lambda (wave speed equation)
• T = 1/f (period and frequency)
• n = sin(i) / sin(r) (Snell’s Law for refraction)
• n = 1 / sin(c) (refractive index and critical angle)
• Magnification = image height / object height

Must-know diagrams

• Transverse and longitudinal wave diagrams with labels (amplitude, wavelength, compression, rarefaction)
• Ray diagrams for reflection (plane mirror)
• Ray diagrams for refraction through glass blocks
• Ray diagrams for converging lenses (real and virtual images)
• Total internal reflection at the critical angle
• The electromagnetic spectrum in order

Must-know concepts

• The difference between transverse and longitudinal waves
• Why frequency stays the same when a wave changes medium (but speed and wavelength change)
• How total internal reflection works and its applications (optical fibres, prisms)
• The order of the EM spectrum and properties of each type
• How sound is produced by vibrations and why it cannot travel through a vacuum
• How to use an oscilloscope trace to find frequency and amplitude

Revision strategy for Topic 3

4-day plan

• Day 1: Wave basics. Define amplitude, wavelength, frequency, period. Practise v = f x lambda. Draw and label transverse and longitudinal waves.
• Day 2: Light: reflection law, refraction (Snell’s Law), total internal reflection. Practise ray diagrams for each. Critical angle calculations.
• Day 3: EM spectrum (learn the order, properties, uses, dangers of each type). Converging lens ray diagrams (real and virtual images).
• Day 4: Sound (production, speed, echoes, ultrasound). Mixed past paper questions on all of Topic 3.

Common mistakes in Topic 3

• Drawing the normal incorrectly (it must be perpendicular to the surface at the point of incidence, not parallel to the surface).
• Measuring angles from the surface instead of from the normal.
• Confusing amplitude with wavelength on a wave diagram.
• Listing the EM spectrum in the wrong order.
• Saying sound can travel through a vacuum.
• Drawing freehand ray diagrams (use a ruler for light rays).

Self-test checklist

After revision, you should be able to:

• Define and calculate wave speed, frequency, period, amplitude and wavelength.
• Draw accurate ray diagrams for reflection, refraction and lenses.
• State and apply Snell’s Law.
• Calculate the critical angle from the refractive index.
• List the EM spectrum in order and state a use and danger for each type.
• Explain how sound is produced and detected.
• Read frequency and amplitude from an oscilloscope trace.