Reflection of waves; sound waves (physics only; sound HT)

SPEC 4.6.1.3 + 4.6.1.4 • REFLECTION & SOUND

Reflection & Sound Waves

Two topics in one lesson. Reflection applies to all waves with one universal rule — measured from the normal. Sound waves are how we hear: the ear converts wave vibrations into electrical signals over a limited frequency range.

⚠️ ENTIRE LESSON IS PHYSICS ONLY (not in Combined Science) • Sound waves portion is also HT ONLY

The Law Of Reflection Physics only

One rule, applied at every reflecting boundary — for light, sound, water waves, any wave.

Law of Reflection

Angle of incidence = angle of reflection. Both angles are measured between the ray and the normal — a line drawn perpendicular to the surface at the point where the ray hits.

Three Things To Always Draw

The surface (often shaded with hatching on the back to show it is a solid material)

The normal — a dashed line perpendicular to the surface, exactly at the point of incidence

The two rays — incident (coming in) and reflected (going out)

Why "From The Normal"?

Without the normal, "angle" is ambiguous. The convention — measure from the normal — gives a single unambiguous angle for every ray, even when the surface is curved or tilted. Mark schemes only accept angles from the normal.

Ray Diagram — Reflection Physics only

The spec requires you to be able to construct ray diagrams for reflection. The standard setup:

Rules of construction: draw the normal at the exact point of incidence; both angles are measured between each ray and the normal; arrows on rays show direction of travel.

Specular Reflection Physics only

What happens when light reflects from a smooth surface — like a polished mirror, still water, or polished metal.

Specular Reflection

Spec wording:"Reflection from a smooth surface in a single direction is called specular reflection."

In other words: a smooth surface reflects parallel incident rays as parallel reflected rays, producing a clear, sharp image.

Why a Clear Image?

On a smooth surface, the normal points in the same direction everywhere. Every ray reflects with the same angle from the same normal, so parallel rays stay parallel. Your eye reconstructs them into a coherent image.

Mark-Scheme Wording

Examiners want the exact word "specular". "Mirror reflection" and "regular reflection" are often rejected. Memorise specular = smooth surface, single direction.

Note: specular and diffuse reflection are formally defined in spec section 4.6.2.6 (Visible light, physics only) — they're covered here because they're fundamentally reflection concepts.

Diffuse Reflection Physics only

What happens when light reflects from a rough surface — like paper, a painted wall, or fabric.

Diffuse Reflection

Spec wording:"Reflection from a rough surface causes scattering: this is called diffuse reflection."

In other words: a rough surface scatters parallel incident rays in many directions, so no clear image forms.

The Law Still Applies

A rough surface doesn't break the law of reflection. The law applies at every single point — it's just that a rough surface has many normals pointing in many directions, so the rays scatter.

What Waves Do At Boundaries Physics only

When a wave hits a boundary between two materials, three things can happen — usually all three at once, in different proportions.

OutcomeWhat it meansExample

ReflectionThe wave bounces off the boundary back into the original mediumLight hitting a mirror; sound hitting a hard wall (echo)

TransmissionThe wave passes through the boundary into the second materialLight through glass; sound through a thin wall

AbsorptionThe wave's energy is transferred to the material — the wave doesn't continueLight hitting matt-black paint; sound absorbed by curtains or carpets

All Three Together

At any boundary, a wave is usually partially reflected, transmitted, and absorbed. A window mostly transmits light (you can see through), partially reflects (you see a faint image of yourself), and slightly absorbs (the glass warms up). The proportions depend on the wave type and the materials.

Examiner Alert Physics only

From Jun 2023 P2 Q07.1

A question asked students to name the type of reflection from a smooth surface. Only 28% wrote the correct word: specular.

Common rejected answers: "mirror reflection", "regular reflection", "smooth reflection", "direct reflection". Mark scheme accepted only "specular".

Memorise These Two Words

SPECULAR = smooth surface, parallel rays out, clear image

DIFFUSE = rough surface, scattered rays out, no clear image

Trick To Remember

"Specular" contains "spec" — like spectacles or a mirror you can look at. "Diffuse" contains "diff" — like different directions.

Sound Waves Physics only • HT only

For Higher Tier physics students only. Sound waves are the longitudinal waves you met in Lesson 6.01 — but now we study how they interact with solids and the ear specifically.

Recap

Sound waves are longitudinal vibrations of air particles (or particles in any medium). They consist of compressions (high pressure) and rarefactions (low pressure) travelling outwards from the source.

Spec Requirement

You need to be able to:

Describe processes which convert wave disturbances between sound waves and vibrations in solids (e.g. the eardrum)

Explain why these processes only work over a limited frequency range — and why this limits human hearing

Know that normal human hearing range is from 20 Hz to 20 kHz

Sound Through Solids Physics only • HT only

Sound waves don't only travel through air. They can travel through any medium where particles can transmit vibrations — including solids.

How It Works

When a sound wave hits a solid surface, the compressions and rarefactions push on the surface particles, making them vibrate. Those vibrations then travel through the solid as a wave. The reverse also happens: a vibrating solid (like a guitar string or a loudspeaker cone) sets the air next to it vibrating, producing sound.

ExampleWhat's happening

Pressing your ear to a desk and tapping it gentlySound travels through the solid desk to your ear — vibrations transferred from solid to ear

A speaker cone vibratingA vibrating solid (the cone) pushes air → air becomes a sound wave

The eardrum being struck by soundAir-borne sound makes a solid (the eardrum) vibrate — this is what we use to hear

Spec Wording

"Sound waves can travel through solids causing vibrations in the solid." — AQA 4.6.1.4. Use this phrasing if asked.

The Ear — Sound To Signal Physics only • HT only

The mechanism that lets you hear is a chain of conversions: air vibrations → solid vibrations → fluid vibrations → electrical signals.

The Spec Bullet

"Within the ear, sound waves cause the eardrum and other parts to vibrate which causes the sensation of sound." — AQA 4.6.1.4. Note the words "vibrate" and "sensation of sound" — both creditworthy phrases.

Why Hearing Has Limits Physics only • HT only

The conversion of sound to solid vibrations in the ear only works over a limited frequency range. Outside that range, the eardrum and bones can't follow the vibration — and you can't hear.

The Mechanism

Below 20 Hz: the vibrations are too slow — the eardrum doesn't respond as a wave; you might feel pressure changes instead of hearing

Above ~20 kHz: the vibrations are too fast — the eardrum, bones, and cochlea hair cells can't physically respond quickly enough to convert the vibration into nerve signals

In between: the system is "tuned" to convert these vibrations efficiently into electrical signals the brain can interpret as sound

Generalising

The same idea applies to any system that converts wave vibrations into solid vibrations. Each mechanical system has a frequency range it can follow — outside that range, the conversion fails. This is what restricts the limits of human hearing (spec wording).

Human Audible Range Physics only • HT only

You need to know these numbers exactly. They are spec-required values.

20 Hz

Lower limit

20 kHz

Upper limit

20 000

Hz in 20 kHz

Spec Wording

"Students should know that the range of normal human hearing is from 20 Hz to 20 kHz ."

How The Range Changes

The upper limit narrows with age. Most teenagers can hear close to 20 kHz; many adults can't hear much above 15 kHz; loss is gradual and starts from the highest frequencies. Prolonged loud noise exposure also reduces the upper limit (damages cochlea hair cells).

Sound Below And Above The Range

Infrasound = below 20 Hz (e.g. earthquakes, elephant communication). Ultrasound = above 20 kHz (covered in Lesson 6.04 — medical imaging, sonar). Both are still sound waves; we just can't hear them.

Examiner Alert — Universal Rule

The Most Common Reflection Error

Students consistently lose marks by measuring the angle of incidence or reflection from the surface instead of from the normal.

If the question gives an angle of "60°" measured from the surface, the angle from the normal is 30°. Get it wrong and the whole calculation goes wrong.

Universal Rule For All Ray Diagrams

This rule applies to every ray-optics question:

Reflection — angle of incidence = angle of reflection (from normal)

Refraction (next lessons) — angle of incidence, angle of refraction (both from normal)

Total internal reflection — critical angle (from normal)

Discipline

Always draw the normal first. Use a dashed line at the exact point where the ray hits the surface. Only then mark the angles. This habit alone saves marks across the whole module.

Weak Vs Strong — Angle Measurement

A ray hits a mirror at an angle. The figure shows 35° between the ray and the surface. What is the angle of incidence?

Weak

Answer: "Angle of incidence = 35°"

Why it fails: Measured from the surface, not from the normal. The actual angle of incidence is 90° − 35° = 55°. The mark scheme rejects this answer outright.

Strong

Answer: "Angle of incidence = 55°"

Why it works: Recognised that 35° was given from the surface and subtracted from 90° to get the angle from the normal. By the law of reflection, the angle of reflection is also 55°.

Sanity Check

The two angles measured from a flat surface always add up to 90°: (angle from surface) + (angle from normal) = 90°. If you spot one given from the surface, convert it instantly.

Common Errors

Students often writeCorrect versionWhy it matters

"Mirror reflection" / "regular reflection"Specular reflectionMark scheme only accepts "specular" (Jun 2023 P2 Q07.1)

Angle measured from the surfaceAngle measured from the normalWrong angle by 90° − x; rejected universally

Forgets to draw the normalAlways draw a dashed normal at the point of incidenceNo normal = no way to mark the angles correctly

"Sound waves are transverse"Sound waves are longitudinalFundamental error — Lesson 6.01 spec rule

"The eardrum hears the sound"The eardrum vibrates ; the brain interprets the signals as soundEardrum doesn't "hear" — it converts air vibrations into solid vibrations

"Sound above 20 kHz isn't a wave"It is still a sound wave (ultrasound) — humans just can't hear itAudible range is a hearing limit, not a wave limit

Practice Question

A student shines a laser onto a mirror. The angle between the incident ray and the mirror surface is 40°.

(a) State the law of reflection. 1 mark

(b) Calculate the angle of reflection (measured from the normal). 2 marks

(c) The student replaces the mirror with a piece of matt black paper. Describe what happens to the laser light, and name the type of reflection from the paper. 3 marks

(d) [HT, Physics only] A dog whistle produces sound at a frequency of 30 kHz. Explain why a human cannot hear the whistle but a dog can. 3 marks

(a) Angle of incidence = angle of reflection (both measured from the normal). [1 mark]

(b) Angle from normal = 90° − 40° = 50°. [1 mark for conversion] Angle of reflection = angle of incidence = 50°. [1 mark]

(c) Most of the laser light is absorbed by the matt black paper (so the spot looks dim). Any light that does reflect is scattered in many directions because the paper is rough. [2 marks] The reflection is diffuse. [1 mark]

(d) The human audible range is 20 Hz to 20 kHz. [1 mark] 30 kHz is above 20 kHz, so the conversion of sound to vibrations in the human eardrum and ear bones cannot happen at that frequency — the system can't respond fast enough. [1 mark] A dog's audible range extends higher than 20 kHz, so its ear can still convert the sound into nerve signals. [1 mark]

Lesson Summary

Lesson 6.03 complete. Tick yourself off against the spec checkpoints — note which apply to your tier.

✓ Reflection (Physics only — both tiers)

State the law of reflection : angle of incidence = angle of reflection (from the normal)

Construct ray diagrams with surface, normal, incident and reflected rays

Distinguish specular (smooth, parallel out, clear image) from diffuse (rough, scattered out, no image)

Describe the three things a wave can do at a boundary: reflect, transmit, absorb

Always measure angles from the normal , never from the surface

✓ Sound Waves (Physics only • HT only)

Describe how sound waves cause vibrations in solids (eardrum example)

Describe the ear pathway: air → eardrum → bones → cochlea → nerve → brain

Explain why the conversion only works over a limited frequency range — and how this limits hearing

Know the audible range: 20 Hz to 20 kHz

Next Up

Lesson 6.04 — Waves for detection and exploration (HT, Physics only). Ultrasound and seismic waves: how we "see" things hidden from view by timing reflected pulses.