Edexcel IGCSE Physics: Waves — Revision Notes

Condensed for the final weeks. For the full explanation, use the Waves study guide.

Wave properties

Transverse wave: oscillates perpendicular to the direction of travel (light, water). Longitudinal wave: oscillates parallel to the direction of travel (sound). Waves transfer energy and information, not matter.

v = f λ              f = 1/T

Worked example. A sound wave of frequency 500 Hz travels at 340 m/s in air: λ = v/f = 340/500 = 0.68 m.

Amplitude = maximum displacement from equilibrium. Wavefront = a line joining points in phase. Wavelength = distance between successive identical points. Period = time for one full oscillation.

The Doppler effect

Source moving towards an observer: wavefronts bunch up ahead of it, so wavelength shortens and observed frequency rises. Source moving away: wavefronts spread out behind it, so wavelength lengthens and observed frequency falls. This is why a siren sounds higher-pitched approaching and lower-pitched receding.

The electromagnetic spectrum

All electromagnetic waves travel at the same speed in free space; only frequency and wavelength differ between them.

Order, decreasing wavelength / increasing frequency: radio -> microwave -> infrared -> visible -> ultraviolet -> x-ray -> gamma.

Type	Use	Hazard at excessive exposure
Radio	Broadcasting	—
Microwave	Cooking, communications	Internal tissue heating
Infrared	Thermal imaging, remote controls	Skin burns
Visible	Photography, optical fibres	—
Ultraviolet	Fluorescent lamps, sterilising	Skin/surface cell damage, blindness
X-ray	Medical imaging	Cell/tissue damage
Gamma	Sterilisation, medical treatment	Cancer, mutation

Reflection and refraction

Law of reflection: angle of incidence = angle of reflection.

n = sin i / sin r          sin c = 1/n

Light refracts (changes speed and direction) at a boundary between media. Above the critical angle c, light undergoes total internal reflection instead of refracting out — the basis of optical fibres, periscopes and binoculars (prisms lose no light to absorption, unlike ordinary mirrors).

Worked example. A material has refractive index n = 1.5. Its critical angle: sin c = 1/1.5 = 0.667, so c = 41.8° (1 d.p.).

Refraction worked example

A light ray strikes a glass block at an angle of incidence of 40 deg and refracts to an angle of refraction of 25 deg.

n = sin i / sin r = sin 40 / sin 25 = 0.643 / 0.423 = 1.52

Once n is known, the same block’s critical angle follows directly: sin c = 1/n = 1/1.52 = 0.658, so c = 41.1 deg (1 d.p.). This two-step pattern – find n from one measured angle pair, then use n to find c – is the standard route through a Topic 3 refraction question, and mixing up which formula answers which part (n from two angles; c from n alone) is a common source of lost marks under time pressure.

Reading a ray diagram correctly

In a ray diagram for refraction, both the angle of incidence and the angle of refraction are always measured from the normal – the line perpendicular to the boundary at the point where the ray meets it – never from the boundary surface itself. Light travelling from a less dense medium (such as air) into a denser medium (such as glass or water) bends towards the normal, so the angle of refraction is smaller than the angle of incidence; travelling the other way, out of the denser medium, it bends away from the normal instead. Getting this direction backwards is one of the most common diagram-labelling errors examiners report for this topic, and checking whether the ray is entering or leaving the denser medium before drawing the bend is the fastest way to avoid it.

Sound

Sound is longitudinal; it reflects and refracts like any wave (3.23, both Physics and the Science Double Award).

Physics only (3.24P–3.29P): Human hearing range: 20 Hz to 20,000 Hz. An oscilloscope + microphone displays a sound wave’s waveform. Pitch = frequency (higher frequency = higher pitch). Loudness = amplitude (greater amplitude = louder).

Exam traps

• Confusing transverse and longitudinal waves — light/water are transverse; sound is longitudinal.
• Using f = 1/T when the question actually needs v = fλ (or vice versa) — they answer different questions.
• Assuming electromagnetic waves travel at different speeds in a vacuum — they don’t; only λ and f differ.
• Forgetting total internal reflection only happens above the critical angle, and only travelling from a denser into a less dense medium.
• Mixing up pitch (frequency) and loudness (amplitude) — a common one-mark slip.
• Quoting the critical angle formula upside down — it’s sin c = 1/n (rearranged correctly, n = 1/sin c), not sin c = n; a sine can never exceed 1, so a value of sin c greater than 1 shows the formula has been inverted.

Self-test

1. State the two equations linking wave speed, frequency, wavelength and period.
2. A wave has frequency 250 Hz and wavelength 1.2 m. Calculate its speed.
3. Explain why a car horn sounds higher-pitched as the car approaches you.
4. Put the electromagnetic spectrum in order of increasing frequency.
5. State one use and one hazard of ultraviolet radiation.
6. A material has a critical angle of 42°. Calculate its refractive index.
7. Distinguish between pitch and loudness in terms of the physical quantities they correspond to.

Answers: 1. v = fλ and f = 1/T. 2. v = fλ = 250 x 1.2 = 300 m/s. 3. As the car approaches, each successive sound wavefront is emitted closer to you than the last, so the wavefronts bunch up, shortening the wavelength and raising the observed frequency (pitch). 4. Radio, microwave, infrared, visible, ultraviolet, x-ray, gamma. 5. Use: fluorescent lamps (or sterilising); hazard: damage to surface skin cells, or blindness. 6. n = 1/sin c = 1/sin 42° = 1/0.669 = 1.5 (2 s.f.). 7. Pitch corresponds to frequency (higher frequency = higher pitch); loudness corresponds to amplitude (greater amplitude = louder sound).