Leaving Cert Higher Level Physics: Waves, Sound, and Light Revision Notes

Chapter 3: Waves, Sound, and Light

Study Time: 2.5–3.5 hoursDifficulty: HL Exam StandardExam Focus: Graphs, ray diagrams, calculations

1. Objectives (What you need to know)

Simple explanation: Treat these objectives as your finish line. If you can do each one without help, the chapter is in strong shape for the exam.

Define wave motion and distinguish between transverse and longitudinal waves.
Use the wave equation to solve problems involving speed, frequency, and wavelength.
Explain sound as a longitudinal wave and describe resonance, harmonics, acoustics, and the Doppler effect.
Draw and interpret ray diagrams for reflection, refraction, total internal reflection, mirrors, and lenses.
Apply Snell's Law and explain diffraction, interference, and polarization as evidence for the wave nature of light.

2. Definitions (Exam-ready terminology)

Simple explanation: Learn these definitions in short, exact sentences. In physics, one missing word can turn a full-mark definition into a half-mark answer.

Term	Definition	Exam Note
Wave	A disturbance that transfers energy from one place to another without the net transfer of matter.	Do not say particles travel across the whole wave.
Transverse wave	A wave in which the vibrations are perpendicular to the direction of energy transfer.	Example: light, water waves, waves on a string.
Longitudinal wave	A wave in which the vibrations are parallel to the direction of energy transfer.	Example: sound in air.
Frequency (f)	Number of waves passing a point per second.	Unit: hertz (Hz).
Wavelength (λ)	The distance between two consecutive points in phase, such as crest to crest or compression to compression.	Unit: metre (m).
Amplitude	Maximum displacement of a particle from its rest position.	For sound, larger amplitude means louder sound.
Resonance	A large increase in amplitude when a system is forced to vibrate at its natural frequency.	Common in sound experiments and musical instruments.
Refraction	The bending of light when it changes speed as it passes from one medium into another.	Always mention change in speed.
Total internal reflection	Complete reflection of light inside a denser medium when the angle of incidence is greater than the critical angle.	Two conditions are needed.
Diffraction	The spreading of waves as they pass through a gap or around an obstacle.	Most noticeable when gap size is similar to wavelength.

3. Concepts (The core theory)

Simple explanation: This chapter is about patterns of vibration and how energy travels through waves, sound, and light.

Waves transfer energy. They do not carry matter across the full distance. In a water wave, for example, the water particles mainly move up and down while the wave travels forward.

Sound is a mechanical longitudinal wave, so it needs a material medium such as air, water, or a solid. Light is an electromagnetic transverse wave, so it can travel through a vacuum.

At Higher Level, this chapter is very diagram-heavy. A correct ray diagram can earn several marks even before calculation. Accuracy matters: use a ruler, mark the normal, label angles from the normal, and include arrowheads on rays.

4. Visual Learning Zone

Simple explanation: Read the diagram like a story. Ask what is changing, what stays constant, and which label explains the main physics idea.

Visual 1: Transverse wave anatomy

In a transverse wave, vibration is at right angles to the direction of wave travel.

Visual 2: Longitudinal wave — compressions and rarefactions

Sound in air travels as alternating compressions and rarefactions.

Visual 3: Reflection from a plane mirror

Angles are measured from the normal, not from the mirror surface.

Visual 4: Refraction through glass

Entering a denser medium: light slows down and bends towards the normal.

Visual 5: Total internal reflection in a glass block

Total internal reflection is used in optical fibres and prisms.

Visual 6: Converging lens ray diagram

A ray parallel to the principal axis passes through the focus after refraction.

Visual 7: Doppler effect

A moving source compresses wavefronts in front and stretches them behind.

5. Formula Toolbox

Simple explanation: Do not treat the formula box like decoration. First decide what each symbol means in the question, then check that your units make sense.

v = fλ
wave speed = frequency × wavelength

n = sin i / sin r
refractive index for air into a medium

T = 1 / f
period and frequency

sin c = 1 / n
critical angle relationship

Symbols: v = wave speed (m/s), f = frequency (Hz), λ = wavelength (m), T = period (s), n = refractive index, i = angle of incidence, r = angle of refraction, c = critical angle.

Interactive Simulators

Teacher voice: Wave questions become easier when you can see the relationship between speed, frequency, wavelength, and travel time instead of learning them as isolated facts.

Wave Tool 1

Wave Speed Calculator

Use v = fλ and watch how changing frequency or wavelength changes the speed.

Frequency f (Hz) Wavelength λ (m)

Wave speed

10.00 m/s

Wave Tool 2

Echo Distance Finder

This helps with sound questions where the wave travels out and back.

Echo time (s) Speed of sound (m/s)

Distance to reflector

136.00 m

6. Worked Examples (Step-by-step walkthroughs)

Simple explanation: Move through the example line by line. The goal is not only to get the answer, but to see why that method was the sensible choice.

Example 1: Wave equation
A wave has frequency 250 Hz and wavelength 1.4 m. Calculate its speed.

Step 1: Write the formula.
v = fλ

Step 2: Substitute.
v = 250 × 1.4

Step 3: Calculate.
v = 350 m/s

Final answer: 350 m/s

Example 2: Snell's Law
Light enters glass from air. The angle of incidence is 40° and the angle of refraction is 25°. Calculate the refractive index of the glass.

Step 1: Use the formula.
n = sin i / sin r

Step 2: Substitute.
n = sin 40° / sin 25°

Step 3: Calculate.
n = 0.643 / 0.423 = 1.52

Final answer: n = 1.52

Example 3: Critical angle
The refractive index of glass is 1.50. Calculate its critical angle.

sin c = 1 / n
sin c = 1 / 1.50 = 0.667
c = sin^-1(0.667)
c = 41.8°

Final answer: critical angle = 41.8°

7. Examiner Tips (How to maximise marks)

Simple explanation: These are the small habits that protect marks under pressure, especially when the maths is easy but the wording is sneaky.

Ray diagrams: Always draw the normal as a dashed line and measure angles from the normal, not the surface.

Wave equation: Convert centimetres to metres before using v = fλ.

Total internal reflection: You must state both conditions: light travels from denser to rarer medium, and angle of incidence is greater than critical angle.

Sound: Remember that sound cannot travel through a vacuum because it needs particles to vibrate.

8. Common Mistakes (Where students drop marks)

Simple explanation: This section matters because students often understand the science but still lose marks through a sign error, unit slip, or definition mix-up.

1. Confusing frequency and wavelength: Frequency is waves per second; wavelength is distance between matching points on a wave.

2. Measuring refraction angles from the surface: Angles must be measured from the normal.

3. Saying light needs a medium: Light can travel through vacuum; sound cannot.

4. Forgetting units: Frequency is Hz, wavelength is m, speed is m/s.

5. Calling diffraction “reflection”: Diffraction is spreading, not bouncing.

9. Examiner Traps (Hidden catches)

Simple explanation: Pause on these trigger words. Examiners like to hide the method inside one or two ordinary-looking phrases.

"Crests are 4 cm apart" → wavelength = 0.04 m, not 4 m.
"The source moves towards the observer" → observed pitch/frequency increases.
"Angle with the surface is 30°" → angle with the normal is 60°.
"Light reaches the critical angle" → refracted ray travels along the boundary.
"Gap much larger than wavelength" → little diffraction.

10. Practical Skills (Lab experiments)

Simple explanation: In a practical answer, imagine the examiner cannot see your setup. You need to state the apparatus, the measurement, the precaution, and the reason clearly.

Experiment 1: Measuring the speed of sound in air using resonance tube

Hold a vibrating tuning fork above a resonance tube.
Adjust the water level until a loud sound is heard.
Measure the air column length at resonance.
Use the wavelength relationship for the resonance condition and then calculate speed using v = fλ.
Precaution: Take several readings and avoid parallax error when reading the scale.

Experiment 2: Measuring refractive index using a glass block

Trace the glass block on paper.
Shine a narrow ray into the block at a known angle of incidence.
Mark the incident and emergent rays, then draw the refracted ray inside the block.
Measure i and r from the normal and calculate n = sin i / sin r.

11. Exam Questions (Structured)

Simple explanation: Treat these as rehearsal, not decoration. Before solving, say out loud what topic the question is really testing.

Q1. Wave equation [6 marks]
A water wave has wavelength 0.80 m and frequency 2.5 Hz.
(a) Define wavelength. [2]
(b) Calculate the speed of the wave. [2]
(c) State whether water waves are transverse or longitudinal. [1]
(d) Explain why waves transfer energy but not matter. [1]

Q2. Refraction and total internal reflection [8 marks]
A ray of light travels from glass into air.
(a) Explain why the ray bends away from the normal. [2]
(b) State the two conditions needed for total internal reflection. [2]
(c) The refractive index of the glass is 1.48. Calculate the critical angle. [3]
(d) Name one application of total internal reflection. [1]

Q3. Sound [7 marks]
(a) Explain why sound is described as a longitudinal wave. [2]
(b) What is resonance? [2]
(c) A sound wave has speed 340 m/s and frequency 680 Hz. Calculate its wavelength. [2]
(d) State what happens to the pitch heard when a sound source moves towards an observer. [1]

12. MCQs (Quick Check with explanations)

Simple explanation: Use MCQs to test understanding, not guessing speed. Try to explain why each wrong option is wrong before you check the answer.

1. Which wave can travel through a vacuum?
A. Sound B. Light C. Water wave D. Seismic P-wave

Answer: B. Light is an electromagnetic wave and does not need a material medium.

A, C, and D need matter or a material medium.

2. A wave has frequency 50 Hz and wavelength 3 m. Its speed is:
A. 16.7 m/s B. 53 m/s C. 150 m/s D. 300 m/s

Answer: C. v = fλ = 50 × 3 = 150 m/s.

3. Total internal reflection occurs when light travels:
A. from air to glass at any angle
B. from denser to rarer medium and i > c
C. from rarer to denser medium and i < c
D. along the normal only

Answer: B. Both conditions are required.

A is refraction into glass. C is the wrong direction. D gives no bending.

4. The Doppler effect explains why an approaching ambulance siren sounds:
A. lower pitched B. higher pitched C. quieter only D. unchanged

Answer: B. Wavefronts are compressed, so the observed frequency and pitch increase.

13. Past Paper Spotlight (Authentic-style ILC HL)

Simple explanation: This is where theory turns into exam judgement. Notice which facts are given, which facts are implied, and which formula or principle unlocks the question.

Question: A student uses a glass block to investigate refraction. The angle of incidence is 55° and the angle of refraction is 34°.

(a) Draw a labelled ray diagram showing the incident ray, refracted ray, normal, angle of incidence, and angle of refraction. [5]

(b) Calculate the refractive index of the glass. [3]

14. Higher-Level Challenge

Simple explanation: This is the stretch section. The maths or reasoning is a bit sharper here, but the same core ideas still do the heavy lifting.

Challenge: Explain why diffraction becomes more noticeable when the gap size is similar to the wavelength of the wave. Then describe how this explains why long-wavelength radio waves can bend around buildings more easily than visible light.

15. Last-Minute Revision Sheet

Simple explanation: Use this as your final scan before an exam: key facts, key equations, and the traps you are most likely to forget.

If the exam is tomorrow, remember:

v = fλ is the key wave equation.
Transverse: vibration perpendicular to travel direction.
Longitudinal: vibration parallel to travel direction.
Sound needs a medium; light can travel in vacuum.
Angles in reflection and refraction are measured from the normal.
Total internal reflection needs denser to rarer medium and i > critical angle.
Black-and-white rule for ray diagrams: ruler, arrows, labels, normal.
Diffraction, interference, and polarization prove the wave nature of light.

16. Checklist (Self-assessment)

Simple explanation: Be honest with the checklist. It is much better to spot a weak area now than in the exam hall.

I can define amplitude, frequency, wavelength, and wave speed.
I can distinguish between transverse and longitudinal waves.
I can use v = fλ correctly with units.
I can explain resonance and the Doppler effect.
I can draw reflection and refraction diagrams with the normal labelled.
I can calculate refractive index using Snell's Law.
I can state the conditions for total internal reflection.
I can explain diffraction, interference, and polarization.

17. Answers (Mark Schemes)

Simple explanation: Study the mark scheme wording carefully. It shows what examiners reward, not just what students hope will be accepted.

Q1. Wave equation [6]
(a) Wavelength is the distance between two consecutive points in phase, e.g. crest to crest. [2]
(b) v = fλ = 2.5 × 0.80 = 2.0 m/s. [2]
(c) Transverse. [1]
(d) Particles oscillate about fixed positions; energy travels through the wave. [1]

Q2. Refraction and total internal reflection [8]
(a) Light speeds up when moving from glass to air, so it bends away from the normal. [2]
(b) From denser to rarer medium [1]; angle of incidence greater than critical angle [1].
(c) sin c = 1/n = 1/1.48 = 0.676; c = sin^-1(0.676) = 42.5°. [3]
(d) Optical fibres / prism binoculars / periscopes. [1]

Q3. Sound [7]
(a) Particles vibrate parallel to the direction of energy transfer; compressions and rarefactions form. [2]
(b) Resonance is a large amplitude vibration when the forcing frequency equals the natural frequency. [2]
(c) v = fλ, so λ = v/f = 340/680 = 0.50 m. [2]
(d) Pitch increases. [1]

Past Paper Spotlight
(a) Correct block outline [1], normal [1], incident ray [1], refracted ray [1], angles labelled from normal [1].
(b) n = sin55 / sin34 = 0.819 / 0.559 = 1.47. [3]
(c) The ray bends towards the normal entering and away from the normal leaving; parallel faces cause equal opposite deviation. [2]

Leaving Cert Higher Level Physics

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