Waves carry energy from one place to another without transferring matter. This section of the Edexcel IGCSE Science Double Award covers wave properties, the electromagnetic spectrum, and the behaviour of light and sound.
The edexcel igcse science double award physics: waves section of the 4SD0 specification encompasses four topics: units, properties of waves, the electromagnetic spectrum, and light and sound. Proficiency in this area requires a solid grasp of wave terminology, the ability to apply the wave equation, and a clear understanding of how different types of electromagnetic radiation are used and the hazards they present. These edexcel igcse science double award revision notes address each topic with the thoroughness the examination warrants.
Units for waves
| Quantity | Symbol | Unit | Unit symbol |
|---|---|---|---|
| Wavelength | λ | metre | m |
| Frequency | f | hertz | Hz |
| Wave speed | v | metres per second | m/s |
| Amplitude | A | metre | m |
| Time period | T | second | s |
The relationship between frequency and time period is: f = 1 / T. A wave with a frequency of 50 Hz has a time period of 1/50 = 0.02 seconds. Ensure that frequency is in hertz and the time period is in seconds before applying this equation.
Properties of waves
A wave is a disturbance that transfers energy from one point to another without a net transfer of matter. There are two fundamental types:
- Transverse waves: The oscillations are perpendicular (at right angles) to the direction of energy transfer. Examples include light, all electromagnetic waves, water waves and waves on a string.
- Longitudinal waves: The oscillations are parallel to the direction of energy transfer. Sound is the principal example. Longitudinal waves consist of compressions (regions of high pressure where particles are close together) and rarefactions (regions of low pressure where particles are spread apart).
Key wave properties:
- Wavelength (λ): The distance from one point on a wave to the same point on the next wave (e.g. crest to crest, or compression to compression).
- Amplitude: The maximum displacement from the rest position. Larger amplitude means greater energy.
- Frequency: The number of complete waves passing a point per second, measured in hertz (Hz).
The wave equation relates these quantities: v = fλ (wave speed = frequency x wavelength). This equation applies to all waves without exception.
v = fλ = 440 x 0.77 = 338.8 m/s
This is consistent with the speed of sound in air (approximately 340 m/s), which provides a useful check on your answer.
Reflection, refraction and diffraction: These three behaviours are exhibited by all waves.
Reflection occurs when a wave bounces off a surface. The angle of incidence equals the angle of reflection, both measured from the normal (an imaginary line perpendicular to the surface at the point of incidence). This law applies to all reflecting surfaces.
Refraction occurs when a wave changes speed as it passes from one medium to another. If the wave enters a denser medium (e.g. light passing from air into glass), it slows down and bends towards the normal. If it enters a less dense medium, it speeds up and bends away from the normal. A wave that hits the boundary at 90 degrees (along the normal) changes speed but does not change direction.
Diffraction occurs when a wave passes through a gap or around an obstacle and spreads out. The effect is most significant when the gap width is similar to the wavelength of the wave. Wider gaps produce less diffraction; narrower gaps (relative to wavelength) produce more.
When drawing wave diagrams, ensure you clearly mark the rest position (the equilibrium line). Amplitude is measured from the rest position to the peak, not from trough to peak. The trough-to-peak distance is twice the amplitude. This distinction matters because the exam regularly tests whether you can read amplitude correctly from a diagram.
The electromagnetic spectrum
The electromagnetic (EM) spectrum is a continuous range of transverse waves that all travel at the speed of light in a vacuum (approximately 3 x 108 m/s). The igcse 4sd0 physics: waves specification requires you to know the order, uses and hazards of each type.
| Type | Wavelength (order) | Uses | Hazards |
|---|---|---|---|
| Radio waves | Longest | Broadcasting, communication | Generally safe at normal levels |
| Microwaves | ↓ | Cooking, satellite communication, mobile phones | Can heat body tissue internally |
| Infrared | ↓ | Remote controls, thermal imaging, heating | Can cause skin burns |
| Visible light | ↓ | Vision, photography, optical fibres | Intense light can damage the retina |
| Ultraviolet | ↓ | Sterilisation, fluorescent lamps, security marking | Skin cancer, premature aging, eye damage |
| X-rays | ↓ | Medical imaging, airport security | Cell damage, cancer with prolonged exposure |
| Gamma rays | Shortest | Sterilising medical equipment, cancer treatment, food irradiation | Cell damage, cancer, most penetrating |
As you move from radio waves to gamma rays, wavelength decreases, frequency increases, and energy per photon increases. The hazards become more serious because higher-energy radiation can ionise atoms in living tissue, damaging cells and DNA.
Light and sound
Light is a transverse electromagnetic wave. When it passes from one medium to another, it can be reflected, refracted or, in certain circumstances, undergo total internal reflection.
Total internal reflection occurs when light travelling in a denser medium (such as glass) hits the boundary with a less dense medium (such as air) at an angle greater than the critical angle. Instead of refracting through the boundary, all of the light is reflected back inside the denser medium. This principle is the basis of optical fibres, which carry data as pulses of light over long distances with minimal signal loss. Medical endoscopes also rely on total internal reflection.
The edexcel igcse science double award notes on light should include the distinction between a real image and a virtual image. A real image can be projected onto a screen (formed by converging light). A virtual image cannot be projected; it appears to come from a point behind the mirror or lens (as with the image you see in a plane mirror).
Sound is a longitudinal wave produced by vibrating objects. It requires a medium to travel through and cannot travel through a vacuum. Sound travels fastest in solids (particles are closest together), slower in liquids, and slowest in gases. The speed of sound in air is approximately 340 m/s.
The pitch of a sound is determined by its frequency: higher frequency means higher pitch. The loudness is determined by the amplitude: larger amplitude means louder sound. Human hearing ranges from approximately 20 Hz to 20,000 Hz. Sound above 20,000 Hz is called ultrasound. Ultrasound has several practical applications, including medical imaging (prenatal scans), industrial flaw detection, and sonar for measuring ocean depth or locating objects underwater.
Echoes are reflections of sound waves. When sound hits a hard, flat surface, it bounces back. The time delay between the original sound and the echo can be used to calculate distance or verify the speed of sound.
To measure the speed of sound, one method involves timing an echo. If a person stands a known distance from a large flat wall, claps, and measures the time for the echo to return, the speed of sound = 2 x distance / time (the factor of 2 accounts for the sound travelling to the wall and back).
Preparing for the exam
The physics: waves edexcel igcse practice questions in the edexcel igcse science double award tend to combine definitions with calculations and diagram interpretation. You may be asked to label a wave diagram, apply the wave equation, or explain the use and hazard of a particular type of electromagnetic radiation in a real-world context.
When you study these edexcel igcse science double award notes, make sure you can draw and label a transverse wave (showing wavelength, amplitude, crest and trough) and describe a longitudinal wave (identifying compressions and rarefactions). Practice the wave equation until rearranging it feels automatic. Pay close attention to unit conversions: wavelengths are sometimes given in nanometres or centimetres rather than metres, and forgetting to convert is a reliable source of lost marks. Work through edexcel igcse science double award revision notes and past paper questions on the Green Bridge CBT platform for edexcel igcse science double award practice questions to consolidate your understanding and identify any remaining gaps before the exam.
Edexcel IGCSE Science Double Award revision notes on waves: wave properties, the electromagnetic spectrum, light and sound.
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