Understanding Physics: Waves: Edexcel IGCSE Science Double Award Explained

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.

Worked example: A sound wave has a frequency of 440 Hz and a wavelength of 0.77 m. Calculate its speed.

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.

Exam tip: Questions on reflection always involve the normal line. If the question provides a diagram without a normal, draw one yourself before measuring or calculating angles. The angle of incidence and the angle of reflection are both measured from the normal, not from the surface. Confusing the two is one of the most common errors on wave questions.

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.

Common mistake: Students sometimes state that "all electromagnetic radiation is dangerous." This is incorrect. The risk depends on the type, the intensity and the duration of exposure. Radio waves at normal levels are considered safe. The edexcel igcse science double award explained examination expects precise and proportionate statements about hazards.

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).

Self-check: Can you state the wave equation and use it to calculate wavelength given speed and frequency? Can you list the electromagnetic spectrum in order from longest to shortest wavelength? Can you explain why total internal reflection occurs only when the angle of incidence exceeds the critical angle? If so, you have a thorough command of this section.

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.