Welcome to Waves (CCEA GCSE Single Award Science: Unit 3 Physics)
Welcome to your complete revision guide for Waves! Whether you are listening to your favourite music on your phone, getting an ultrasound scan, or warming up food in a microwave, waves are hard at work around you every single second. Don't worry if physics sometimes feels intimidating—we will break down every concept step-by-step with clear examples, simple memory tricks, and worked calculations so you can walk into your exam with total confidence.
Exam Tip: This topic is assessed in Unit 3: Physics (Foundation and Higher tiers) and practical skills may also appear in Unit 4: Practical Skills (Booklet B).
---1. What is a Wave?
A wave is defined as an oscillation or vibration that transfers energy and information from one place to another without transferring matter.
Everyday Analogy: Think of a "Mexican wave" in a sports stadium. When fans stand up and sit down, the wave travels all the way around the arena. However, the individual people do not move around the stadium—they simply move up and down in their seats! The wave transfers movement and energy, not the people themselves.
The Two Main Types of Waves
In physics, all waves are sorted into two distinct categories based on how their particles vibrate compared to the direction the wave travels:
1. Transverse Waves
In a transverse wave, the oscillations (vibrations) are perpendicular (\(90^\circ\)) to the direction of energy transfer (the direction the wave travels).
* Key features: Transverse waves have high points called peaks (or crests) and low points called troughs.
* Examples: Light waves, all electromagnetic (EM) waves, water ripples/waves, and seismic S-waves.
2. Longitudinal Waves
In a longitudinal wave, the oscillations (vibrations) are parallel to the direction of energy transfer (the direction the wave travels).
* Key features: Rather than peaks and troughs, longitudinal waves have areas where particles are squashed together, called compressions (high pressure), and areas where particles are spread apart, called rarefactions (low pressure).
* Examples: Sound waves, ultrasound waves, and seismic P-waves.
Key Takeaway: Transverse = vibrations at \(90^\circ\) (perpendicular) to wave travel. Longitudinal = vibrations parallel to wave travel.
---2. Key Wave Terminology
Examiners love to ask you to label wave diagrams or define key properties. Here are the four essential terms you must know:
1. Wavelength (\(\lambda\)):
The distance between two successive identical points on a wave (for example, peak to peak, or trough to trough).
* Symbol: \(\lambda\) (the Greek letter lambda)
* Unit: Metres (\(\text{m}\))
2. Amplitude (\(A\)):
The maximum displacement of a particle from its undisturbed (equilibrium or rest) position.
* On a diagram, this is measured from the centre rest line up to a peak, or from the centre rest line down to a trough (never from peak to trough!).
* Unit: Metres (\(\text{m}\))
* What it affects: Amplitude relates directly to the loudness/volume of a sound wave or the intensity/brightness of a light wave.
3. Frequency (\(f\)):
The number of complete waves passing a fixed point per second.
* Unit: Hertz (\(\text{Hz}\))
* Unit Conversion: \(1\text{ kilohertz (kHz)} = 1000\text{ Hz}\)
* What it affects: In sound waves, frequency determines the pitch. A high frequency produces a high-pitched sound; a low frequency produces a low-pitched sound.
4. Wave Speed (\(v\)):
The distance a wave travels per unit of time.
* Unit: Metres per second (\(\text{m/s}\))
Quick Review:
* Pitch depends on frequency.
* Loudness depends on amplitude.
3. Core Wave Formulae and Calculations
Formula 1: The Wave Speed Equation
The relationship between wave speed, frequency, and wavelength is given by:
\(v = f \times \lambda\)
Where:
* \(v =\) Wave speed in metres per second (\(\text{m/s}\))
* \(f =\) Frequency in hertz (\(\text{Hz}\))
* \(\lambda =\) Wavelength in metres (\(\text{m}\))
Rearranging the Formula:
* To find frequency: \(f = \frac{v}{\lambda}\)
* To find wavelength: \(\lambda = \frac{v}{f}\)
Worked Example 1:
A water wave has a frequency of \(5\text{ Hz}\) and a wavelength of \(0.4\text{ m}\). Calculate the speed of the wave.
* Step 1: State the equation: \(v = f \times \lambda\)
* Step 2: Substitute the numbers: \(v = 5\text{ Hz} \times 0.4\text{ m}\)
* Step 3: Calculate the answer: \(v = 2\text{ m/s}\)
Worked Example 2 (With Unit Conversion):
A radio transmitter emits a signal at a frequency of \(200\text{ kHz}\) traveling at a speed of \(3 \times 10^8\text{ m/s}\). Calculate the wavelength of the radio wave.
* Step 1: Convert frequency to standard units (\(\text{Hz}\)): \(200\text{ kHz} = 200 \times 1000 = 200{,}000\text{ Hz}\)
* Step 2: Rearrange the equation: \(\lambda = \frac{v}{f}\)
* Step 3: Substitute values: \(\lambda = \frac{300{,}000{,}000}{200{,}000} = 1500\text{ m}\)
Formula 2: Speed, Distance, Time and Echo Calculations
For any wave travelling at a constant speed, you can use the standard speed equation:
\(\text{Speed } (v) = \frac{\text{Distance } (d)}{\text{Time } (t)}\)
Echo and Sonar Calculations: Watch the Trap!
An echo occurs when a sound or ultrasound wave reflects off a surface and returns to the detector. Because the wave has to travel to the object and back again, it covers double the distance.
To calculate the distance to an object (such as the seabed or a shoal of fish):
\(\text{Distance to object} = \frac{\text{Speed } (v) \times \text{Time } (t)}{2}\)
Worked Example: Sonar Depth Sounding
A ship uses a sonar pulse to measure the depth of the sea. The speed of sound in seawater is \(1500\text{ m/s}\). The echo returns to the ship \(0.8\text{ seconds}\) after being emitted. How deep is the water?
* Step 1: Calculate the total distance travelled by the pulse: \(\text{Total distance} = v \times t = 1500\text{ m/s} \times 0.8\text{ s} = 1200\text{ m}\)
* Step 2: Halve the distance to find the depth: \(\text{Depth} = \frac{1200\text{ m}}{2} = 600\text{ m}\)
Key Takeaway: Always check echo questions carefully! If the time given is for the return trip, divide by 2 to find the one-way distance.
---4. Sound, Human Hearing, and Ultrasound
The Nature of Sound
Sound is a longitudinal, mechanical wave caused by vibrating particles.
* Because sound requires particles to pass on vibrations, sound cannot travel through a vacuum (such as outer space). In space, there are no particles, so it is completely silent!
Human Hearing Range
Healthy human ears can only detect a specific range of frequencies:
* Audible Range of Human Hearing: \(20\text{ Hz}\) to \(20{,}000\text{ Hz}\) (which is the same as \(20\text{ Hz}\) to \(20\text{ kHz}\)).
* Any sound with a frequency below \(20\text{ Hz}\) or above \(20{,}000\text{ Hz}\) cannot be heard by human ears.
What is Ultrasound?
Ultrasound is defined as sound waves with a frequency greater than \(20{,}000\text{ Hz}\) (\(> 20\text{ kHz}\))—above the upper limit of human hearing.
Uses of Ultrasound
1. Medical Imaging (Prenatal Scanning):
* Ultrasound pulses are sent into the body and partially reflect back whenever they meet a boundary between different tissue types or fluids (e.g. between muscle and bone).
* A computer converts these timed echoes into an image of the unborn baby.
* Advantage: Ultrasound is non-ionising, making it completely safe for pregnant mothers and developing foetuses (unlike X-rays, which are ionising and dangerous).
2. Sonar and Depth Sounding:
* Ships and submarines send ultrasound pulses down into the ocean to measure water depth or detect underwater obstacles, shipwrecks, and shoals of fish by timing how long the echo takes to return.
3. Industrial Cleaning and Flaw Detection:
* Cleaning: High-frequency vibrations shake delicate items (like jewellery or engine components) clean in fluid baths.
* Non-destructive flaw detection: Ultrasound waves passing through solid metal beams or aircraft wings reflect early if there is an internal crack or bubble, alerting engineers to hidden faults without destroying the metal.
Key Takeaway: Ultrasound = sound frequencies \(> 20{,}000\text{ Hz}\). It is non-ionising, making it ideal for pregnancy scans and sonar.
---5. The Electromagnetic (EM) Spectrum
General Properties of all EM Waves
All electromagnetic waves share three fundamental features:
1. They are all transverse waves.
2. They all travel at the same speed in a vacuum (\(3 \times 10^8\text{ m/s}\), the speed of light).
3. They can travel through a vacuum (they do not require particles or a physical medium).
Order of the EM Spectrum
The spectrum is a continuous family of waves arranged in order of increasing frequency and decreasing wavelength:
Radio Waves \(\rightarrow\) Microwaves \(\rightarrow\) Infrared (IR) \(\rightarrow\) Visible Light \(\rightarrow\) Ultraviolet (UV) \(\rightarrow\) X-rays \(\rightarrow\) Gamma Rays
Memory Trick: Learn this simple mnemonic to remember the order from lowest frequency to highest frequency:
Raging Martians Invaded Venus Using X-ray Guns
(Radio, Microwaves, Infrared, Visible light, Ultraviolet, X-rays, Gamma rays)
The Visible Light Spectrum
Visible light is the only part of the spectrum detectable by the human eye. Its colours, in order of longest wavelength to shortest wavelength, are:
Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROYGBIV)
* Red: Longest wavelength, lowest frequency.
* Violet: Shortest wavelength, highest frequency.
Uses and Dangers of EM Waves
As you move across the spectrum from radio waves to gamma rays, the waves carry more energy. High-frequency waves (UV, X-rays, Gamma rays) are ionising radiation, meaning they can knock electrons out of atoms, damaging living cells and DNA.
1. Radio Waves
* Uses: Radio and TV broadcasting, wireless communication.
* Dangers: No known significant hazards at normal everyday exposure levels.
2. Microwaves
* Uses: Mobile phones, satellite communications, microwave cooking.
* Dangers: Internal heating of body tissues.
3. Infrared (IR)
* Uses: TV remote controls, thermal imaging cameras, cooking grills, optical fibre communication.
* Dangers: Can cause skin burns from excessive heat.
4. Visible Light
* Uses: Human vision, photography, optical fibre communications.
* Dangers: Very bright light (like staring at the Sun) can damage the retina in the eye.
5. Ultraviolet (UV)
* Uses: Security marking (banknotes and ID cards), tanning sunbeds, fluorescent lamps.
* Dangers: Causes skin cancer, premature skin ageing, and cataracts or eye damage.
6. X-rays
* Uses: Medical imaging of broken bones and teeth, airport baggage security scanners.
* Dangers: Ionising radiation—causes cell damage, cell mutations, and cancer.
7. Gamma Rays
* Uses: Sterilising medical surgical equipment, radiotherapy (destroying cancer cells).
* Dangers: Highly ionising radiation—destroys living cells, damages DNA, causes gene mutations and cancer.
Key Takeaway: All EM waves are transverse and travel at \(3 \times 10^8\text{ m/s}\) in a vacuum. High-frequency waves (UV, X-rays, Gamma) are ionising and can cause mutations and cancer.
---6. Common Exam Mistakes to Avoid
Make sure you don't lose easy marks by checking these five common examiner traps:
1. Forgetting to Halve the Echo: In sonar or echo calculations, the pulse travels to the object and back. If calculating distance to the object, divide your total distance (or time) by 2!
2. Confusing Pitch and Loudness:
* Frequency controls pitch (high frequency = high pitch).
* Amplitude controls loudness (large amplitude = loud sound).
3. Imprecise Wave Definitions: Never just write "transverse waves move up and down". Use the precise CCEA definition: "Oscillations are perpendicular to the direction of energy transfer."
4. Missing Unit Conversions: Watch out for kilohertz (\(\text{kHz}\)). Always multiply by 1000 to convert to hertz (\(\text{Hz}\)) before using \(v = f \times \lambda\).
5. Thinking Sound Can Travel in Space: Sound needs particles to vibrate. It cannot travel through a vacuum. Only electromagnetic waves can travel through a vacuum!