What is the difference between light waves and sound waves in Physics?

Light waves are electromagnetic waves that can travel through a vacuum, while sound waves are mechanical waves that require a medium such as air, water, or solids to propagate. Light waves travel much faster than sound waves, with light traveling at approximately 300,000 kilometers per second, whereas sound travels at about 343 meters per second in air. This distinction is crucial for understanding wave behavior in different contexts, as covered in the Edexcel IGCSE Physics curriculum.

How do frequency and wavelength relate to the properties of light and sound?

In both light and sound waves, frequency refers to the number of wave cycles that pass a point per second, measured in Hertz (Hz). Wavelength is the distance between consecutive crests of a wave. For light, higher frequencies correspond to shorter wavelengths and are perceived as blue or violet, while lower frequencies have longer wavelengths and appear red. In sound, higher frequencies result in higher-pitched sounds, while lower frequencies produce lower-pitched sounds. Understanding these relationships is essential for Edexcel IGCSE Physics students studying wave properties.

Why does sound travel faster in solids than in gases?

Sound travels faster in solids than in gases because particles in solids are more closely packed together, allowing sound waves to transfer energy more efficiently between particles. In gases, particles are more spread out, which slows down the transmission of sound. This concept is important in the study of wave propagation and is a key topic in the Edexcel IGCSE Physics syllabus.

What is the principle of reflection and refraction in light waves?

Reflection occurs when light waves bounce off a surface, while refraction is the bending of light as it passes from one medium to another with a different density. The angle of incidence equals the angle of reflection, and the change in speed when entering a new medium causes refraction. These principles are fundamental in optics, a subtopic of waves in the Edexcel IGCSE Physics curriculum, and are crucial for understanding how lenses and mirrors work.

How do humans perceive different frequencies of sound and light?

Humans perceive different frequencies of sound as varying pitches, with higher frequencies producing higher pitches and lower frequencies resulting in lower pitches. In terms of light, different frequencies correspond to different colors, with higher frequencies appearing as blue or violet and lower frequencies as red. This perception is due to the way our ears and eyes are structured to detect these frequencies, a concept explored in the Edexcel IGCSE Physics course under the study of waves and their properties.

Why is "Measuring angles from the surface instead of the normal" a common mistake in Light and Sound?

Why it happens: Diagrams sometimes show the surface horizontally without a normal drawn. How to avoid it: ALWAYS draw the normal first — a dashed line perpendicular to the surface AT THE POINT of incidence. Then measure both angles from this normal. Source: 4PH1/1P Examiner Reports 2022-2024 — frequent mark loss

Why is "Quoting refractive index with a unit (e.g. 'n = 1.5 m/s')" a common mistake in Light and Sound?

Why it happens: Tendency to attach a unit to every quantity. How to avoid it: Refractive index is the RATIO of two speeds (or sines) — it is DIMENSIONLESS. Never include a unit. Source: 4PH1 Examiner Reports 2022-2024

Why is "Applying $\sin c = 1/n$ when light goes from AIR into glass" a common mistake in Light and Sound?

Why it happens: Memorising a formula without the context. How to avoid it: Critical angle applies only when light moves from DENSE to LESS DENSE. Light entering glass from air will refract; no critical angle exists for that direction.

Why is "Saying TIR occurs because the surface is 'shiny' or 'reflective'" a common mistake in Light and Sound?

Why it happens: Vague understanding of the mechanism. How to avoid it: TIR occurs because the angle of incidence inside the denser medium EXCEEDS the critical angle. The boundary need not be polished or coated. Source: 4PH1 Examiner Reports 2022-2024

Why is "Saying loudness changes with frequency (or pitch changes with amplitude)" a common mistake in Light and Sound?

Why it happens: Casual everyday phrasing. How to avoid it: Spec 3.28P / 3.29P (Paper 2): pitch ↔ FREQUENCY; loudness ↔ AMPLITUDE. Keep them strictly separated. Source: 4PH1/2P Examiner Reports 2022-2024

Why is "Calculator left in RADIAN mode" a common mistake in Light and Sound?

Why it happens: Forgotten setting from a previous question. How to avoid it: Before any / / work, check 'DEG' is shown on the calculator display. 30° should give 0.500.

WavesEdexcel IGCSE Physics (4PH1)

Light and Sound

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Light and Sound — Pearson Edexcel International GCSE Physics 4PH1 Study Notes (2026 onwards, Spec Issue 4)

Reflection and refraction of light, refractive index $n = \sin i / \sin r$ , critical angle $\sin c = 1/n$ , total internal reflection (TIR) for optical fibres and prisms; sound as a longitudinal wave that reflects/refracts. Includes TWO required practicals (specs 3.17, 3.19) and Paper-2-only material: human hearing range 20-20 000 Hz, oscilloscope traces and pitch/loudness on screen.

What you’ll learn

Mapped to the Cambridge IGCSE 4PH1 syllabus (2026 onwards).

3.14 — Know that light waves are transverse and that they can be reflected and refracted.
3.15 — Use the law of reflection: angle of incidence = angle of reflection.
3.16 — Construct ray diagrams to illustrate reflection and refraction.
3.17 — Required practical: investigate the refraction of light using rectangular blocks, semi-circular blocks and triangular prisms.
3.18 — Know and use the relationship between refractive index, angle of incidence and angle of refraction: $n = \sin i / \sin r$ .
3.19 — Required practical: investigate the refractive index of glass using a glass block.
3.20 — Describe the role of total internal reflection in transmitting information along optical fibres and in prisms.
3.21 — Explain the meaning of the term critical angle $c$ .
3.22 — Know and use the relationship between the critical angle and the refractive index: $\sin c = 1/n$ .
3.23 — Know that sound waves are longitudinal and that they can be reflected and refracted.
3.24P — Know that the frequency range for human hearing is 20 Hz to 20 000 Hz (Paper 2).
3.25P — Required practical: investigate the speed of sound in air (Paper 2).
3.26P — Understand how an oscilloscope and microphone can be used to display a sound wave (Paper 2).
3.27P — Required practical: investigate the frequency of a sound wave using an oscilloscope (Paper 2).
3.28P — Know that the pitch of a sound is related to the frequency of the source (Paper 2).
3.29P — Know that the loudness of a sound is related to the amplitude of vibration of the source (Paper 2).

Reflection of light (spec 3.14-3.16)

Angle of incidence = angle of reflection.

The law (spec 3.15). When a wave reflects off a flat surface, the angle of incidence and the angle of reflection are equal — both measured from the NORMAL (the line drawn perpendicular to the surface at the point of incidence).

Drawing a reflection ray diagram (spec 3.16).

Draw the mirror as a straight line, with shading (hatching) on the back side.
At the point where the ray hits, draw the NORMAL — a dashed line perpendicular to the mirror.
Mark the angle of incidence $\theta_i$ between the incident ray and the normal.
Draw the reflected ray on the other side of the normal at the SAME angle $\theta_r = \theta_i$ .
Add arrows to show the direction of travel of both rays.

Common pitfall. Examiners deduct marks when candidates measure angles from the SURFACE rather than from the normal. Always draw the normal first.

Image in a plane mirror. The image is virtual (behind the mirror), upright, the same size as the object, and laterally inverted (left-right flipped). The image distance behind the mirror = the object distance in front.

Both angles measured from the NORMAL.
Angle of incidence = angle of reflection.
Image in a plane mirror is virtual, upright, laterally inverted.

Refraction of light (spec 3.14, 3.17, 3.18)

Speed changes → ray bends.

Why it happens. Light slows when it enters an optically denser medium (e.g. air → glass). The change of speed at the boundary causes a CHANGE OF DIRECTION — refraction.

The rules.

Air → glass (denser medium): ray bends TOWARDS the normal.
Glass → air (less dense medium): ray bends AWAY from the normal.
If the ray hits ALONG the normal (angle of incidence = 0°), there is NO bending — only slowing.

Snell's law (spec 3.18). For light entering a denser medium of refractive index $n$ from air:

$n = \dfrac{\sin i}{\sin r}$

$i$ = angle of incidence in the LESS dense medium (typically air).
$r$ = angle of refraction in the DENSER medium (typically glass).
$n$ is DIMENSIONLESS.

Typical values. Vacuum/air: $n = 1.00$ . Water: $n = 1.33$ . Glass: $n \approx 1.5$ . Diamond: $n = 2.42$ . Higher $n$ means a slower speed in the medium.

Required practical (spec 3.17). Investigate refraction through different shapes:

Rectangular block: ray enters one face, refracts towards normal, travels through, exits parallel to the incoming ray (refracts away from normal on exit).
Semi-circular block: ray entering on the FLAT face along the centre hits the curved surface at 90° (along the normal) → emerges undeviated. Used to demonstrate the critical-angle condition by varying the angle on the flat face.
Triangular prism: ray refracts at BOTH faces; the prism splits white light into a spectrum because $n$ depends slightly on wavelength (dispersion).

Required practical (spec 3.19). Measure the refractive index of glass:

Trace around a rectangular glass block on plain paper.
Shine a ray of light at one face. Mark the entry point with a cross.
Mark several points along the incident ray AND along the emerging ray.
Remove the block; join the marks to reconstruct the path inside.
Draw the normal at the entry point. Measure $i$ and $r$ with a protractor.
Repeat for 5+ different incident angles.
Plot $\sin i$ (y-axis) vs $\sin r$ (x-axis). Straight line through origin → Snell's law verified; GRADIENT = refractive index $n$ .

Air → glass: ray bends TOWARDS normal.
Glass → air: ray bends AWAY from normal.
$n = \sin i / \sin r$ , dimensionless.
Required practical (3.19): graph of $\sin i$ vs $\sin r$ ; gradient = $n$ .

See the full worked example for light and sound →

Required practical detail: refractive index of glass (spec 3.19)

Plot $\sin i$ vs $\sin r$ ; gradient = $n$ .

Edexcel 4PH1 explicitly names spec 3.19 as a REQUIRED PRACTICAL. Exam questions can ask you to describe the method, identify variables and process the data.

Apparatus. Ray box with single slit, rectangular glass block, plain paper, sharp pencil, protractor, ruler.

Variables.

Independent: angle of incidence $i$ .
Dependent: angle of refraction $r$ .
Control: same glass block, same light wavelength (white light), same paper.

Procedure.

Place glass block on paper; draw around it.
Shine a ray at one face. Mark the incident ray with two crosses (close to and far from the block) and likewise mark the emerging ray.
Remove the block. Use a ruler to join the crosses → incident and emerging rays. Join the entry and exit points on the block outline → the refracted ray inside.
Draw the NORMAL at the entry point.
Measure $i$ and $r$ with a protractor.
Repeat for at least 5 incident angles between 15° and 75°.

Processing. Plot $\sin i$ on the y-axis against $\sin r$ on the x-axis. The graph should be a straight line through the origin. The GRADIENT = refractive index $n$ .

Why use a graph? Averaging out random error across many readings. A single calculation $n = \sin i / \sin r$ from one pair is much less reliable.

Sources of error. Thick pencil lines, mis-aligned protractor, parallax error reading the angle, scratched block surface.

Trace block; mark entry and exit rays; reconstruct inside.
Repeat for 5+ incident angles.
Plot $\sin i$ vs $\sin r$ → gradient = $n$ .
Graph method beats single-pair calculation for accuracy.

See the full worked example for light and sound →

Total internal reflection (spec 3.20-3.22)

Above $c$ : ray fully reflected.

Critical angle (spec 3.21). The angle of incidence INSIDE the denser medium for which the angle of refraction in the less-dense medium would be 90°.

At incidence angles slightly less than $c$ : most of the light refracts out at a grazing angle, but some weak reflection occurs. At angles GREATER than $c$ : refraction is impossible → ALL the light is reflected back inside (total internal reflection).

Formula (spec 3.22).

$\sin c = \dfrac{1}{n}$

Where $n$ is the refractive index of the denser medium relative to the less-dense one. For glass-air ( $n = 1.5$ ): $\sin c = 1/1.5 = 0.667 \Rightarrow c \approx 41.8°$ . For water-air ( $n = 1.33$ ): $c \approx 48.8°$ .

Uses (spec 3.20).

Optical fibres. A thin glass core surrounded by glass cladding of lower refractive index. Light entering the core at a shallow angle hits the core-cladding boundary at an angle > $c$ → totally internally reflected. Repeats along the entire length, even when the fibre is bent. Used for high-speed broadband and endoscopes.
Prisms in periscopes and binoculars. A right-angled prism with a 45°-45°-90° cross-section: light entering one short face hits the hypotenuse at 45°, which exceeds the critical angle for glass-air. The light is totally internally reflected through 90° and exits perpendicular to the second short face. Periscopes use TWO such prisms; binoculars use them to fold the optical path and produce upright images.

Why TIR over mirrored surfaces? Mirrors absorb some light at each reflection (~5%). TIR reflects ~100% — much better for high-quality optics.

$\sin c = 1/n$ — memorise.
Only happens from DENSER to LESS DENSE.
Used in optical fibres, prism reflectors, endoscopes.
TIR ≈ 100% reflection, beats silvered mirrors.

See the full worked example for light and sound →

Optical fibres in detail (spec 3.20)

Core + cladding + repeated TIR.

Construction. A thin transparent CORE (high $n$ ) surrounded by a slightly lower- $n$ CLADDING. The core may be only a few micrometres across.

Operation.

Light enters one end of the core at a shallow angle to the fibre axis.
It hits the core-cladding boundary at an angle of incidence > critical angle.
Total internal reflection → light bounces back into the core.
It travels along the fibre, repeating TIR at every boundary contact.
Almost no light is lost (the fibre is highly transparent and TIR is ~100% efficient), so signals can travel many kilometres before needing amplification.

Why curve-tolerant. Even when the fibre is gently bent, the angle of incidence at the boundary remains above the critical angle (the bend is gradual relative to the fibre diameter).

Uses.

Telecommunications. High-speed internet, telephone networks. Digital data are encoded as light pulses (0 = no pulse, 1 = pulse).
Medical endoscopes. Two bundles of fibres — one carries illumination INTO the body, the other carries the image BACK OUT. Allows non-invasive examination of organs.
Sensors. Some industrial sensors use fibres to carry light to/from harsh environments (high temperature, electromagnetic interference).

Core has higher $n$ than cladding.
Light hits boundary > critical angle → TIR.
Used in telecom and endoscopes.

See the full worked example for light and sound →

Sound — a longitudinal wave (spec 3.23)

Sound reflects (echoes) and refracts.

Spec 3.23. Sound waves are LONGITUDINAL (air molecules oscillate parallel to the direction of energy transfer) and they can be reflected and refracted.

Reflection of sound = echoes. A hard surface like a wall reflects sound. The angle of incidence = angle of reflection, same law as for light. Echoes can be used to estimate distances if you know the speed of sound (spec 3.25P method).

Refraction of sound. Sound speed depends on the medium temperature. Warm air (faster $v$ ) and cold air (slower $v$ ) bend sound waves. On a still night, temperature inversions can let sound travel further than usual.

Required practical — speed of sound in air (spec 3.25P, Paper 2 only).

Method (one common approach):

Two students stand a measured large distance $d$ apart (e.g. 100 m).
Student A bangs two wooden blocks together.
Student B starts a stopwatch when she SEES the blocks meet (light is effectively instantaneous) and stops it when she HEARS the bang.
Time interval $t$ . Calculate $v = d / t$ .
Repeat and average to reduce reaction-time error.

Typical result: $v_{\text{sound, air}} \approx 330 - 340\,\text{m/s}$ at room temperature.

Alternative method. Two microphones a fixed distance apart connected to an oscilloscope or data logger; clap source; measure the time delay between the two traces.

Sound = longitudinal wave (spec 3.23).
Reflects (echoes) — same law as light.
Refracts when temperature varies with height.
Speed in air ~340 m/s.

Paper 2 only — hearing, oscilloscope, pitch, loudness (spec 3.24P-3.29P)

Oscilloscope traces + audible range + amplitude/frequency.

Paper 2 only. Spec statements 3.24P-3.29P are flagged with the bold P and appear ONLY on 4PH1/2P.

Human hearing range (spec 3.24P). A healthy young human ear detects frequencies from 20 Hz to 20 000 Hz. Older listeners typically have a reduced upper limit (closer to 12-15 kHz).

Oscilloscope display (spec 3.26P). A microphone converts a sound wave into a voltage signal that is then fed to an oscilloscope. The trace on the screen shows VOLTAGE vs TIME — but because the voltage is proportional to the instantaneous air-pressure variation, the trace shows the SHAPE of the sound wave.

Reading the trace.

Period $T$ = horizontal distance for ONE complete wave × time-base setting (e.g. 0.5 ms per division).
Frequency $f = 1/T$ .
Amplitude = vertical distance from the rest line up to a peak.

Required practical — measuring sound frequency (spec 3.27P). Connect a microphone to an oscilloscope. Play a tone (signal generator + speaker, or tuning fork). Adjust the time-base so 2-3 complete waves fit across the screen. Measure $T$ from the trace. Calculate $f = 1/T$ . Compare with the source frequency.

Pitch and loudness (spec 3.28P, 3.29P).

Pitch ↔ FREQUENCY. Higher $f$ → higher pitch. On the oscilloscope: more cycles in the same horizontal distance.
Loudness ↔ AMPLITUDE. Larger amplitude → louder. On the oscilloscope: taller wave (greater vertical excursion).

Worked sketch. Asked to compare two oscilloscope traces:

Same time-base, same y-scale.
Trace A has 4 complete waves in 8 divisions and amplitude 2 divisions.
Trace B has 2 complete waves in 8 divisions and amplitude 4 divisions.

Then: trace A has DOUBLE the frequency of B (higher pitch), but HALF the amplitude (quieter).

Audible range: 20 Hz to 20 000 Hz (spec 3.24P).
Oscilloscope trace shape = pressure variation.
Period from trace → $f = 1/T$ .
Pitch ↔ frequency; loudness ↔ amplitude (spec 3.28P, 3.29P).
All this content is Paper 2 only.

See the full worked example for light and sound →

How it’s examined

Light and Sound is one of the highest-yielding topics on 4PH1: 12-20 marks total across Paper 1 and Paper 2. Highest-frequency items on PAPER 1: Snell's law calculations (3-4 marks); critical angle calculations (3-4 marks); TIR diagram + explanation (5-6 marks); the required-practical 3.19 method as a 6-mark extended-response. On PAPER 2 the Paper-2-only material (3.24P-3.29P) typically adds another 6-10 marks: oscilloscope reading (3-4 marks); pitch/loudness comparison (2-3 marks); audible range recall (1-2 marks).

Sources: Pearson Edexcel International GCSE Physics 4PH1 specification — Issue 4, September 2024 (valid for 2026 onwards); Pearson Edexcel 4PH1 Examiner Reports 2022-2024; 4PH1/1P May/Jun 2024 question paper and mark scheme; 4PH1/2P May/Jun 2024 question paper and mark scheme; 4PH1/1P January 2024 question paper and mark scheme. Last reviewed 2026-05-12.

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Step-by-step worked examples — Light and Sound

Step-by-step solutions to past-paper-style questions on light and sound, written exactly the way a tutor would explain them at the board.

1Refractive index from angles (4 marks, Paper 1)
Core• Adapted from 4PH1/1P May/Jun 2024• refraction, snell, spec-3.18, spec-3.19, Paper 1
Question
A ray of light enters a glass block from air. The angle of incidence in air is 50° and the angle of refraction in the glass is 30°. (a) Calculate the refractive index of the glass. (b) Calculate the speed of light in the glass, given $c = 3.00 \times 10^{8}$ m/s. (4 marks)
Step-by-step solution
1. Step 1
  (a) Write Snell's law (spec 3.18).
  $n = \dfrac{\sin i}{\sin r}$
2. Step 2
  Substitute.
  $n = \dfrac{\sin 50°}{\sin 30°} = \dfrac{0.766}{0.500}$
3. Step 3
  Evaluate.
  $n = 1.53$
4. Step 4
  (b) Speed in glass. Using $n = c / v_{\text{glass}}$ → $v_{\text{glass}} = c/n = (3.00 \times 10^{8})/1.53 = 1.96 \times 10^{8}\,\text{m/s}$ .
Answer
(a) $n = 1.53$ (no unit). (b) $v_{\text{glass}} = 1.96 \times 10^{8}$ m/s.
Examiner tip
Edexcel mark schemes accept 1.50-1.55 for part (a). Refractive index is DIMENSIONLESS — quoting a unit loses 1 mark. ECF: a wrong $n$ still earns the division mark in part (b).
2Critical angle of glass (3 marks, Paper 1)
Core• Adapted from 4PH1/1P January 2024• TIR, critical angle, spec-3.21, spec-3.22, Paper 1
Question
The refractive index of a glass block is 1.50. Calculate the critical angle for light passing from the glass into air. (3 marks)
Step-by-step solution
1. Step 1
  Write the relationship (spec 3.22).
  $\sin c = \dfrac{1}{n}$
2. Step 2
  Substitute.
  $\sin c = \dfrac{1}{1.50} = 0.667$
3. Step 3
  Take inverse sine.
  $c = \sin^{-1}(0.667) = 41.8°$
Answer
Critical angle = 41.8° (≈ 42°).
Examiner tip
Make sure the calculator is in DEGREES mode. Edexcel mark schemes accept 41 - 42°. The formula $\sin c = 1/n$ ONLY applies when light goes from a DENSE medium (glass, water) into a LESS DENSE medium (air). It does NOT apply going the other way.
3Required practical: refractive index of glass (6 marks, Paper 1)
Core• Adapted from 4PH1/1P May/Jun 2024• required practical, spec-3.19, Paper 1
Question
Describe a method to measure the refractive index of a glass block using a ray box. State how the data should be processed to give a value of $n$ with reduced random error. (6 marks)
Step-by-step solution
1. Step 1
  Apparatus (1 mark). Ray box, single slit, rectangular glass block, sharp pencil, protractor, plain paper.
2. Step 2
  Method (1 mark). Place the block on the paper; draw around it. Shine a ray of light at one face. Mark the incident ray, the entry point and the emerging ray with crosses.
3. Step 3
  Measure angles (1 mark). Remove the block; draw the normal at the entry point. Measure the angle of incidence $i$ between the incident ray and the normal, and the angle of refraction $r$ between the refracted ray inside the block and the normal.
4. Step 4
  Repeat for different angles (1 mark). Repeat the experiment for at least 5 different incident angles between, say, 15° and 75°.
5. Step 5
  Process (1 mark). Plot a graph of $\sin i$ (y-axis) against $\sin r$ (x-axis). A straight line through the origin confirms Snell's law. The gradient gives $n$ .
6. Step 6
  Reliability (1 mark). Using a graph effectively averages many readings → reduces random error. Sharp pencil + accurate protractor reduce systematic error.
Answer
Trace block, mark entry/exit rays, draw normal, measure $i$ and $r$ . Repeat for 5+ angles. Plot $\sin i$ vs $\sin r$ ; gradient = $n$ .
Examiner tip
Spec 3.19 is one of two required practicals in this subtopic (the other is 3.17 — investigating refraction through different shapes). Edexcel rewards: graph method > single-pair calculation. Always plot $\sin i$ vs $\sin r$ and read $n$ from the gradient.
4TIR in an optical fibre (5 marks, Paper 1)
Extended• Adapted from 4PH1/1P January 2024• TIR, optical fibre, spec-3.20, spec-3.21, Paper 1
Question
Explain how light is transmitted along an optical fibre. Include in your answer the meaning of the critical angle and a condition that must be satisfied at every reflection inside the fibre. (5 marks)
Step-by-step solution
1. Step 1
  Setup. A fibre consists of a thin glass core surrounded by glass cladding of lower refractive index. Light enters one end of the core.
2. Step 2
  Total internal reflection (spec 3.20). Light striking the core-cladding boundary is REFLECTED back inside (no refraction out), provided the angle of incidence exceeds the critical angle.
3. Step 3
  Critical angle (spec 3.21). The critical angle $c$ is the angle of incidence at the boundary, inside the denser medium, for which the angle of refraction in the less-dense medium would be 90°.
4. Step 4
  Condition. Angle of incidence INSIDE the fibre must be GREATER than the critical angle at EVERY internal reflection.
5. Step 5
  Result. Light is repeatedly totally internally reflected — bouncing along the fibre's length with almost no loss, even when the fibre is bent.
Answer
Light enters glass core; strikes core-cladding boundary at $> c$ ; total internal reflection; bounces along the fibre with negligible loss.
Examiner tip
Edexcel mark scheme rewards: (1) angle of incidence > critical angle, (2) total internal reflection occurs, (3) no light refracts out, (4) repeated at every reflection. Mention the cladding's lower refractive index for the 5th mark.
5Frequency from an oscilloscope trace (Paper 2 only, 4 marks)
Extended• Adapted from 4PH1/2P May/Jun 2024• oscilloscope, frequency, spec-3.26P, spec-3.27P, Paper 2
Question
A microphone is connected to an oscilloscope. A tuning fork is struck and held near the microphone. On the oscilloscope screen the time-base is set to 0.5 ms/division, and one complete wave spans 4.0 horizontal divisions. (a) Calculate the period of the sound wave. (b) Calculate the frequency. (4 marks) [Paper 2 only]
Step-by-step solution
1. Step 1
  (a) Period from trace. 1 complete wave occupies 4.0 divisions × 0.5 ms/div = 2.0 ms.
  $T = 4.0 \times 0.5 = 2.0\ \text{ms} = 2.0 \times 10^{-3}\ \text{s}$
2. Step 2
  (b) Frequency from $f = 1/T$ (spec 3.6).
  $f = \dfrac{1}{T} = \dfrac{1}{2.0 \times 10^{-3}}$
3. Step 3
  Evaluate.
  $f = 500\ \text{Hz}$
4. Step 4
  Check (spec 3.24P). 500 Hz is well inside the audible range 20 Hz - 20 000 Hz, so the wave would be heard as a clear note (similar pitch to middle C, 261 Hz).
Answer
(a) $T = 2.0 \times 10^{-3}$ s (= 2.0 ms). (b) $f = 500$ Hz.
Examiner tip
Spec 3.26P / 3.27P are Paper 2 only. Edexcel mark scheme: 1 mark counting divisions, 1 mark applying time-base, 1 mark using $f = 1/T$ , 1 mark final answer with unit. Always check your final $f$ falls inside the audible range (20 Hz - 20 kHz) — a sanity check spotted by examiners.

Key Formulae — Light and Sound

The formulae you need to memorise for light and sound on the Pearson Edexcel IGCSE 4PH1 paper, with every variable defined in plain English and a note on when to use it.

Law of reflection (spec 3.15)
$\theta_i = \theta_r$
When to use
Angle of incidence (between incident ray and normal) = angle of reflection (between reflected ray and normal). Both angles are measured from the NORMAL, never from the surface. Applies to all waves including light and sound.
Snell's law / refractive index (spec 3.18)
$n = \dfrac{\sin i}{\sin r}$
When to use
Light passing from AIR into a denser medium. $i$ = angle of incidence in air, $r$ = angle of refraction in the medium. $n$ is dimensionless. For glass ≈ 1.5; water ≈ 1.33.
Critical angle (spec 3.22)
$\sin c = \dfrac{1}{n}$
When to use
Light passing from a DENSER medium (refractive index $n$ ) into a LESS DENSE medium (e.g. air). For angles of incidence > $c$ , total internal reflection occurs.

Key Definitions and Keywords — Light and Sound

Definitions to memorise and the exact keywords mark schemes credit for light and sound answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE 4PH1 sitting.

Reflection (spec 3.14, 3.15)
Examiner keyword
When a wave bounces off a surface back into the original medium. Law: angle of incidence = angle of reflection (both measured from the normal). The incident ray, reflected ray and normal lie in the same plane.
Refraction (spec 3.14, 3.17)
Examiner keyword
Change of direction (bending) of a wave when it crosses a boundary into a medium where its speed changes. Light entering glass slows down → bends towards the normal. Light leaving glass speeds up → bends away from the normal.
Refractive index $n$ (spec 3.18)
Examiner keyword
A measure of how much a medium slows light. $n = c / v_{\text{medium}} = \sin i / \sin r$ . Dimensionless. Higher $n$ → light slows more → ray bends more towards the normal on entry.
Critical angle $c$ (spec 3.21)
Examiner keyword
The angle of incidence, measured INSIDE a denser medium, at which the angle of refraction in the less-dense medium would be 90°. At incidence angles GREATER than $c$ , refraction is impossible — total internal reflection occurs instead.
Total internal reflection (TIR) (spec 3.20)
Examiner keyword
When light, inside a denser medium and striking the boundary with a less-dense medium at an angle greater than the critical angle, is completely reflected back into the denser medium. Used in optical fibres and reflecting prisms in periscopes.
Human hearing range (spec 3.24P)
Examiner keyword
Roughly 20 Hz to 20 000 Hz for a healthy young human ear. Sounds below 20 Hz = infrasound; above 20 kHz = ultrasound. (Paper 2)
Pitch and loudness (spec 3.28P, 3.29P)
Examiner keyword
Pitch corresponds to FREQUENCY (higher $f$ = higher pitch). Loudness corresponds to AMPLITUDE (larger amplitude = louder). On an oscilloscope trace: higher pitch = more cycles per division; louder = taller wave. (Paper 2)

Common Mistakes and Misconceptions — Light and Sound

The traps other students keep falling into on light and sound questions — taken from recent Pearson Edexcel IGCSE 4PH1 examiner reports and mark schemes — and how to avoid them.

✕Measuring angles from the surface instead of the normal
4PH1/1P Examiner Reports 2022-2024 — frequent mark loss
Why it happens
Diagrams sometimes show the surface horizontally without a normal drawn.
How to avoid it
ALWAYS draw the normal first — a dashed line perpendicular to the surface AT THE POINT of incidence. Then measure both angles from this normal.
✕Quoting refractive index with a unit (e.g. 'n = 1.5 m/s')
4PH1 Examiner Reports 2022-2024
Why it happens
Tendency to attach a unit to every quantity.
How to avoid it
Refractive index is the RATIO of two speeds (or sines) — it is DIMENSIONLESS. Never include a unit.
✕Applying $\sin c = 1/n$ when light goes from AIR into glass
Why it happens
Memorising a formula without the context.
How to avoid it
Critical angle applies only when light moves from DENSE to LESS DENSE. Light entering glass from air will refract; no critical angle exists for that direction.
✕Saying TIR occurs because the surface is 'shiny' or 'reflective'
4PH1 Examiner Reports 2022-2024
Why it happens
Vague understanding of the mechanism.
How to avoid it
TIR occurs because the angle of incidence inside the denser medium EXCEEDS the critical angle. The boundary need not be polished or coated.
✕Saying loudness changes with frequency (or pitch changes with amplitude)
4PH1/2P Examiner Reports 2022-2024
Why it happens
Casual everyday phrasing.
How to avoid it
Spec 3.28P / 3.29P (Paper 2): pitch ↔ FREQUENCY; loudness ↔ AMPLITUDE. Keep them strictly separated.
✕Calculator left in RADIAN mode
Why it happens
Forgotten setting from a previous question.
How to avoid it
Before any $\sin / \cos / \tan$ work, check 'DEG' is shown on the calculator display. $\sin 30°$ should give 0.500.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Video lesson

Short walkthrough of the concepts students most often get stuck on.

Light and Sound — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Light and Sound

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Refractive index from angles (4 marks, Paper 1)

2Critical angle of glass (3 marks, Paper 1)

3Required practical: refractive index of glass (6 marks, Paper 1)

4TIR in an optical fibre (5 marks, Paper 1)

5Frequency from an oscilloscope trace (Paper 2 only, 4 marks)

Law of reflection (spec 3.15)

Snell's law / refractive index (spec 3.18)

Critical angle (spec 3.22)

Reflection (spec 3.14, 3.15)

Refraction (spec 3.14, 3.17)

Refractive index nnn (spec 3.18)

Critical angle ccc (spec 3.21)

Total internal reflection (TIR) (spec 3.20)

Human hearing range (spec 3.24P)

Pitch and loudness (spec 3.28P, 3.29P)

✕Measuring angles from the surface instead of the normal

✕Quoting refractive index with a unit (e.g. 'n = 1.5 m/s')

✕Applying sin⁡c=1/n\sin c = 1/nsinc=1/n when light goes from AIR into glass

✕Saying TIR occurs because the surface is 'shiny' or 'reflective'

✕Saying loudness changes with frequency (or pitch changes with amplitude)

✕Calculator left in RADIAN mode

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Light and Sound — frequently asked questions

Refractive index $n$ (spec 3.18)

Critical angle $c$ (spec 3.21)

✕Applying $\sin c = 1/n$ when light goes from AIR into glass