Waves

This guide covers Topic 3, Waves, in full — sub-topics (a) Units, (b) Properties of waves, (c) The electromagnetic spectrum and (d) Light and sound — from the Pearson Edexcel International GCSE in Physics (4PH1), Issue 4 specification. Statements marked “P” are Physics-only content.

Before studying this

No prior wave-specific study is required, though comfort with rearranging simple equations (from Topic 1) is assumed.

Syllabus coverage

PEARSON EDEXCEL INTERNATIONAL GCSE PHYSICS (4PH1) — Topic 3

(a) Units — using degree (°), hertz (Hz), metre (m), metre/second (m/s) and second (s).

(b) Properties of waves — explaining the difference between longitudinal and transverse waves; knowing amplitude, wavefront, frequency, wavelength and period; knowing waves transfer energy and information without transferring matter; using wave speed = frequency × wavelength; using frequency = 1/time period; applying these relationships to sound and electromagnetic waves; explaining the Doppler effect; explaining that all waves can be reflected and refracted.

(c) The electromagnetic spectrum — knowing the continuous electromagnetic spectrum (radio, microwave, infrared, visible, ultraviolet, x-ray, gamma) travels at the same speed in free space; knowing the order by decreasing wavelength/increasing frequency, including visible colours; explaining uses of each type of radiation; explaining detrimental effects of excessive exposure and protective measures.

(d) Light and sound — knowing light waves are transverse and can be reflected and refracted; using the law of reflection; drawing ray diagrams for reflection and refraction; using the refractive index relationship; investigating refraction and refractive index practically; describing total internal reflection in optical fibres and prisms; explaining the critical angle and its relationship to refractive index; knowing sound waves are longitudinal and can be reflected and refracted; [P] knowing the human hearing range (20-20,000 Hz); [P] investigating the speed of sound practically; [P] understanding oscilloscope/microphone use for displaying sound waves; [P] investigating sound frequency using an oscilloscope; [P] relating pitch to frequency and loudness to amplitude.

Wave properties

A transverse wave oscillates perpendicular to its direction of travel (e.g. light, water ripples); a longitudinal wave oscillates parallel to its direction of travel (e.g. sound). Key wave quantities: amplitude (maximum displacement from equilibrium), wavefront (a line joining points in phase), frequency (oscillations per second), wavelength (distance between successive identical points) and period (time for one complete oscillation). Waves transfer energy and information without transferring matter itself.

The core wave relationship connects speed, frequency and wavelength:

wave speed = frequency × wavelength
v = f × λ

and frequency relates to period by:

frequency = 1 / time period
f = 1/T

Worked example. A sound wave has frequency 500 Hz and travels at 340 m/s in air. Its wavelength:

λ = v/f = 340/500 = 0.68 m

The Doppler effect

When a wave source moves relative to an observer, the observed frequency and wavelength change — the Doppler effect. As a source moves towards an observer, each successive wavefront is emitted from a position slightly closer to the observer than the last, so the wavefronts bunch up ahead of the source, shortening the observed wavelength and raising the observed frequency. As the source moves away, each wavefront is emitted further back, so the wavefronts spread out behind it, lengthening the observed wavelength and lowering the observed frequency. This is why an ambulance siren sounds higher in pitch as it approaches and lower as it recedes.

The electromagnetic spectrum

Light is part of a continuous electromagnetic spectrum, all travelling at the same speed in free space, ordered by increasing frequency and decreasing wavelength: radio, microwave, infrared, visible (red through violet), ultraviolet, x-ray, gamma ray. Each has characteristic uses (broadcasting, cooking, night vision, optical fibres/photography, fluorescent lamps, medical imaging, sterilisation respectively) and, at excessive exposure, characteristic hazards — microwaves cause internal heating of body tissue, infrared causes skin burns, ultraviolet causes damage to surface cells and blindness, and gamma rays cause cancer and mutation — against which appropriate protective measures should be used.

Reflection and refraction

Light obeys the law of reflection (angle of incidence = angle of reflection) and can be refracted when passing between media in which it travels at different speeds, changing speed and direction at the boundary. The refractive index n relates the angles of incidence and refraction:

n = sin i / sin r

At sufficiently large angles inside a denser medium, light undergoes total internal reflection rather than refracting out — the principle behind transmitting light along optical fibres. The critical angle c is the angle beyond which total internal reflection occurs, related to refractive index by:

sin c = 1/n

Total internal reflection is also used inside glass prisms — for example, a right-angled prism can turn a light beam through 90° or 180° by reflecting it internally off a face at 45°, which is greater than the critical angle for glass. This is the principle behind periscopes and binoculars, which use prisms rather than mirrors because total internal reflection loses no light to absorption at the reflecting surface.

Sound (Physics only)

Sound waves are longitudinal, and like all waves can be reflected and refracted. Humans typically hear frequencies between 20 Hz and 20,000 Hz. An oscilloscope connected to a microphone can display a sound wave’s waveform: the pitch of a sound corresponds to its frequency (higher frequency = higher pitch), and its loudness corresponds to the amplitude of vibration (greater amplitude = louder sound).

Common mistakes

• Confusing transverse and longitudinal waves — light and water waves are transverse; sound is longitudinal.
• Mixing up f = 1/T with v = fλ — both are needed together but answer different questions (period vs speed).
• Assuming all electromagnetic waves have different speeds in free space — they all travel at the same speed (the speed of light) in a vacuum; only their frequency and wavelength differ.
• Forgetting total internal reflection only happens above the critical angle, and only when travelling from a denser to a less dense medium.

Quick revision checklist

• Transverse vs longitudinal waves; v = fλ and f = 1/T
• The Doppler effect
• The electromagnetic spectrum order, uses and hazards
• Law of reflection; n = sin i/sin r; total internal reflection and sin c = 1/n
• Pitch (frequency) and loudness (amplitude) of sound (Physics only)

Related resources

• Electricity — the previous topic
• Energy Resources and Energy Transfers — the next topic
• Pearson Edexcel International GCSE Physics hub

Written against the Pearson Edexcel International GCSE in Physics (4PH1) specification, Issue 4. Always check the current specification for your examination year.