Form Three Physics Notes
Chapter One: Waves · Ordinary Level (Form Three) · Study Notes. Read this chapter free, then continue every chapter in the Click Online Academy app.
All Form Three Physics Chapters
- Chapter 1WavesRead below · Free
- Chapter 2Newton’s LawsContinue in the app
- Chapter 3EquilibriumContinue in the app
- Chapter 4FrictionContinue in the app
- Chapter 5Simple MachinesContinue in the app
Learning Outcomes
By the end of this chapter, you should be able to:
- define a wave and explain how waves transfer energy without transferring matter;
- distinguish mechanical from electromagnetic waves, and transverse from longitudinal waves;
- define and use amplitude, wavelength, frequency, period and wave speed;
- use the equation v = fλ;
- describe reflection, refraction, diffraction and interference of waves;
- apply wave ideas to everyday situations in Tanzania (sound, radio, water waves, medical uses).
Key Terms and Concepts
| Term | Meaning |
|---|---|
| Wave | A progressive disturbance that carries energy from one place to another without the medium as a whole moving along with it. |
| Mechanical wave | A wave that needs a material medium (sound, water waves, waves on a rope). |
| Electromagnetic wave | A wave of vibrating electric and magnetic fields that can travel through a vacuum (light, radio waves, X-rays). |
| Transverse wave | Particles vibrate at right angles to the direction of travel (crests and troughs). |
| Longitudinal wave | Particles vibrate parallel to the direction of travel (compressions and rarefactions). |
| Amplitude (A) | Maximum displacement from the rest position. SI unit: metre (m). |
| Wavelength (λ) | Distance between two successive crests (or compressions). SI unit: metre (m). |
| Frequency (f) | Number of complete waves per second. SI unit: hertz (Hz). |
| Period (T) | Time for one complete oscillation. SI unit: second (s). T = 1/f. |
| Wave speed (v) | Distance travelled by a wave per unit time. v = fλ. SI unit: m/s. |
| Reflection | Bouncing back of a wave from a barrier. |
| Refraction | Change in direction of a wave when its speed changes on entering a new medium. |
| Diffraction | Spreading of a wave when it passes through a gap or around an obstacle. |
| Interference | Overlapping (superposition) of two or more waves, giving a combined pattern. |
1. Introduction to Waves
Beat a drum and a person far away hears it. Drop a stone in a pond and ripples spread out, but a floating leaf only bobs up and down. In each case a disturbance moves and carries energy. This is a wave.
A wave is a progressive disturbance that carries energy from one point to another. The particles of the medium vibrate about their rest positions and pass the disturbance to their neighbours; they do not travel with the wave.
Mechanical and electromagnetic waves
| Mechanical waves | Electromagnetic waves | |
|---|---|---|
| Medium | Need a material medium | Do not need a medium; can travel in a vacuum |
| Nature | Vibrating particles | Vibrating electric and magnetic fields |
| Speed | Depends on the medium (sound in air ≈ 340 m/s) | 3 × 10⁸ m/s in a vacuum |
| Examples | Sound, water waves, waves on a rope, seismic waves | Radio, microwaves, infrared, light, ultraviolet, X-rays, gamma rays |
Transverse and longitudinal waves
- Transverse: particles vibrate perpendicular to the direction of travel. Examples: waves on a rope, water ripples, light.
- Longitudinal: particles vibrate parallel to the direction of travel, forming compressions and rarefactions. Examples: sound, waves on a spring.
TEST YOUR UNDERSTANDING
Q1. What does a wave transfer?
Q2. State two differences between transverse and longitudinal waves.
2. Wave Properties and the Equation v = fλ
- Amplitude (A) – maximum displacement from rest position.
- Wavelength (λ) – distance between two successive crests (or compressions).
- Frequency (f) – number of complete waves per second (Hz).
- Period (T) – time for one complete oscillation; T = 1/f.
- Wave speed (v) – distance travelled per unit time.
TEST YOUR UNDERSTANDING
Q3. A wave has a period of 0.25 s. Find its frequency.
Q4. A wave of frequency 50 Hz has wavelength 4 m. Find its speed.
3. Reflection, Refraction, Diffraction and Interference
Reflection
A wave bouncing back from a barrier. Laws: (1) the incident ray, reflected ray and normal lie in the same plane; (2) angle of incidence = angle of reflection. An echo is sound reflected from a distant obstacle. Distance to obstacle = (speed × time) ÷ 2.
Refraction
Change in direction when a wave’s speed changes on entering a new medium. Frequency is fixed by the source and does not change. When water waves enter shallow water, speed and wavelength decrease; the waves bend towards the normal.
Diffraction
Spreading of a wave through a gap or around an obstacle. Greatest when the gap is about equal to the wavelength. Sound diffracts around corners; light does not noticeably because its wavelength is very short.
Interference
When two waves meet, the resultant displacement is the sum of the individual displacements (principle of superposition). Constructive: crest meets crest → larger amplitude. Destructive: crest meets trough → smaller amplitude or silence. Beats: beat frequency = |f₁ − f₂|.
TEST YOUR UNDERSTANDING
Q5. Which quantity does not change when a wave is refracted?
Q6. Two notes of 440 Hz and 436 Hz sound together. What is the beat frequency?
4. Applications in Tanzania
- Communication: radio, TV, mobile phones and mobile money all use electromagnetic waves.
- Music and culture: ngoma drums, marimba and zeze produce sound by vibrating skins, bars and strings.
- Water waves: waves on Lake Victoria, Lake Tanganyika and the Indian Ocean affect fishermen, ferries and beaches.
- Earthquake waves: tremors along the East African Rift are detected by seismometers.
- Health: X-rays and ultrasound scans are used in hospitals; ultrasound also locates fish and measures water depth (sonar).
Chapter Summary
- A wave transfers energy without transferring matter.
- Mechanical waves need a medium; electromagnetic waves do not.
- Transverse waves have crests and troughs; longitudinal waves have compressions and rarefactions.
- Key quantities: amplitude, wavelength (λ), frequency (f), period (T = 1/f), wave speed (v = fλ).
- Reflection: i = r. Echoes are used to measure distance.
- Refraction: speed and wavelength change; frequency stays the same.
- Diffraction: spreading around gaps; greatest when gap ≈ λ.
- Interference: constructive (crest + crest) or destructive (crest + trough); beats = |f₁ − f₂|.
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Continue in the appEnd-of-Topic Questions
Attempt all questions in your exercise book before checking the suggested answers. (New Syllabus 2027). Speed of sound in air = 340 m/s; speed of light = 3 × 10⁸ m/s unless stated.
- Section A: Multiple choice. (i) A wave transfers: A. matter only B. energy C. mass D. electric charge. (ii) Which is NOT a mechanical wave? A. Sound B. Water wave C. Light D. Wave on a rope. (iii) In a longitudinal wave the particles vibrate: A. perpendicular to the direction of travel B. parallel to the direction of travel C. in circles D. not at all. (iv) The distance between two successive crests is the: A. amplitude B. period C. wavelength D. frequency. (v) A wave has a period of 0.02 s. Its frequency is: A. 0.02 Hz B. 5 Hz C. 50 Hz D. 500 Hz. (5 marks)
- Define a wave. (2 marks)
- State two differences between transverse and longitudinal waves. (4 marks)
- A wave has a period of 0.25 s. Find its frequency. (2 marks)
- A wave of frequency 50 Hz has wavelength 4 m. Find its speed. (2 marks)
- State the laws of reflection of waves. (2 marks)
- Explain why the wavelength decreases when water waves enter shallow water, although the frequency is constant. (3 marks)
- Explain why you can hear a person speaking around a corner but cannot see the person. (3 marks)
- Two notes of 440 Hz and 436 Hz are sounded together. Find the beat frequency. (2 marks)
- An FM station broadcasts at 90 MHz. Find the wavelength of the waves. (3 marks)
- A boy stands 99 m from a mountain, claps and hears an echo 0.6 s later. Calculate the speed of sound. (3 marks)
- A ship’s sonar signal returns from the sea bed after 1.2 s. Sound travels at 1 500 m/s in sea water. Find the depth. (3 marks)
Suggested Answers
- (i) B. (ii) C. (iii) B. (iv) C. (v) C.
- A progressive disturbance that transfers energy without transferring matter / without the medium moving along.
- Transverse: vibration perpendicular to travel, crests and troughs. Longitudinal: vibration parallel to travel, compressions and rarefactions. (Or: longitudinal can travel through gases; transverse cannot travel through the inside of a gas.)
- f = 1 ÷ 0.25 = 4 Hz.
- v = 50 × 4 = 200 m/s.
- (1) Incident ray, reflected ray and normal lie in the same plane. (2) Angle of incidence = angle of reflection.
- Speed decreases in shallow water. Since f is fixed by the source, λ = v ÷ f decreases.
- Sound has a long wavelength comparable to the corner/doorway so it diffracts; light has a very short wavelength and does not diffract noticeably.
- 440 − 436 = 4 Hz.
- λ = (3 × 10⁸) ÷ (9 × 10⁷) = 3.33 m.
- v = 2d ÷ t = (2 × 99) ÷ 0.6 = 330 m/s.
- d = vt ÷ 2 = 1 500 × 1.2 ÷ 2 = 900 m.
These are suggested answers. Credit is also given to correct points expressed in your own words.
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