Pearson International GCSE Physics 3: Waves

Waves transfer energy and information through reflection, refraction, diffraction and superposition.

The same wave model applies across water waves, sound and electromagnetic radiation, but the oscillating quantity and need for a medium differ. Track speed, frequency, wavelength, direction and amplitude separately so that a change in one is not automatically assigned to all the others.

Main ideas

• Distinguish transverse and longitudinal waves and relate speed, frequency and wavelength.
• Interpret wave diagrams and oscilloscope traces.
• Apply reflection and refraction rules, refractive index and total internal reflection.
• Construct ray diagrams for mirrors and lenses.
• Order electromagnetic waves and explain uses and hazards.
• Relate sound pitch to frequency and loudness to amplitude.

Wave quantities and representations

Amplitude is maximum displacement from equilibrium and relates to the energy carried by the wave. Wavelength is the shortest distance between points in the same phase, such as crest to next crest. Frequency is the number of complete oscillations passing a point each second, while period is time for one oscillation.

Wave speed equals frequency multiplied by wavelength. Use consistent units: frequency in hertz, wavelength in metres and speed in metres per second. The relationship describes the propagation of the pattern, not the speed of individual particles in the medium.

A displacement-distance graph is a snapshot along the wave and can show wavelength. A displacement-time or oscilloscope trace is recorded at one position and can show period. The horizontal spacing of peaks therefore represents different quantities depending on the axis.

Transverse and longitudinal waves

In a transverse wave, oscillations are perpendicular to energy transfer. Electromagnetic waves are transverse. In a longitudinal wave, oscillations are parallel to energy transfer, forming compressions and rarefactions; sound in air is longitudinal.

Particles in a mechanical wave oscillate around equilibrium rather than travelling with the wave over long distances. Energy and information propagate through interactions between neighboring particles. Electromagnetic waves can travel through vacuum and do not require particles as a medium.

Reflection, refraction and diffraction

For reflection, angle of incidence equals angle of reflection. Both angles are measured from the normal, not from the surface. Incident ray, reflected ray and normal lie in the same plane under the model used.

Refraction occurs when wave speed changes at a boundary. Frequency remains fixed by the source, so wavelength changes with speed. A ray bends toward the normal when it slows and away from the normal when it speeds up, except at normal incidence where direction does not change.

Diffraction is spreading when waves pass through a gap or around an obstacle. It is most noticeable when gap size is comparable to wavelength. Greater wavelength relative to the opening produces more spreading; frequency is not changed merely by passing through the gap.

Refractive index and total internal reflection

Refractive index relates light speed in vacuum to speed in a material and also links angles through the specified refraction relationship. A larger refractive index means lower light speed in the medium.

Total internal reflection occurs only when light travels from a higher-index medium toward a lower-index medium and the incidence angle exceeds the critical angle. At the critical angle, the refracted ray travels along the boundary. Optical fibres guide signals by repeated total internal reflection, but real systems also involve absorption, dispersion and signal loss.

Mirrors, lenses and images

A plane mirror forms a virtual, upright, laterally inverted image the same size as the object and the same perpendicular distance behind the mirror. Construct rays using the law of reflection and extend them backward with dashed lines to locate a virtual image.

A converging lens can form real or virtual images depending on object position relative to focal length. Use principal rays accurately: a ray parallel to the principal axis refracts through the principal focus, a ray through the optical centre continues approximately straight, and a ray through the near focus emerges parallel. A real image is formed where actual rays meet and can be projected; a virtual image is found where backward extensions appear to meet.

Electromagnetic spectrum

All electromagnetic waves travel at the same speed in vacuum and are transverse. In order of increasing frequency, the spectrum runs from radio waves through microwaves, infrared, visible, ultraviolet, X-rays and gamma rays. As frequency increases, wavelength decreases in vacuum.

Uses depend on interaction with matter, wavelength, penetration and energy. Radio and microwaves support communication; infrared is associated with heating and sensing; visible light supports imaging and fibre communication; ultraviolet can cause fluorescence; X-rays penetrate soft tissue more than bone; gamma radiation is highly penetrating and ionising.

Hazard explanations should name the interaction and biological effect. Higher-frequency ultraviolet, X-rays and gamma rays can damage cells and DNA through ionisation or related processes. Intensity, exposure time, shielding and distance affect risk; naming a region alone is not a complete risk assessment.

Sound and ultrasound

Sound requires a medium. Frequency determines pitch, while amplitude relates to loudness. Waveform affects quality or timbre. Sound speed depends on the medium and conditions, not on pitch under the ordinary model used here.

Echo calculations use the total outward-and-return distance. Ultrasound has frequency above the upper limit of human hearing and can be used for imaging, ranging and testing materials. Boundary echoes occur because part of the wave is reflected when properties change.

Worked example

Light of frequency 5.0 times 10 to the power 14 hertz travels from air into glass at 2.0 times 10 to the power 8 metres per second. Frequency remains 5.0 times 10 to the power 14 hertz because the source fixes it and wavefronts must remain continuous across the boundary. Wavelength in the glass is speed divided by frequency, giving 4.0 times 10 to the power minus 7 metres. Because the light slows on entering the glass, a non-normal ray bends toward the normal. The refractive index relative to vacuum is approximately 3.0 times 10 to the power 8 divided by 2.0 times 10 to the power 8, giving 1.5. A wrong answer that lowers frequency would break the boundary continuity and double-count the speed change.

Common mistakes

• Measuring optical angles from the surface.
• Changing frequency when a wave crosses a boundary.
• Assuming electromagnetic waves require a medium.
• Reading wavelength from a time trace without first using period and wave speed.
• Stating that total internal reflection occurs whenever light meets a boundary, without the direction and critical-angle conditions.

Assessment guidance

Identify graph axes before reading wavelength, period or amplitude. Show unit conversions before using the wave equation. In ray diagrams, draw a normal, measure optical angles from it and use a ruler for rays. Refraction explanations should preserve frequency and link changed speed to changed wavelength and direction. For total internal reflection, state both the higher-to-lower index direction and incidence angle above critical. Spectrum answers need a named wave, property-linked use and mechanism-linked hazard. Echo calculations must include the return journey. Paper 2 may assess all Paper 1 content plus additional statements.

Check yourself

Explain why a wave refracts when its speed changes at a boundary but its frequency does not.

Then distinguish a distance graph from a time trace, state the two conditions for total internal reflection, and explain why an echo distance calculation divides the total travelled distance by two. Order the electromagnetic spectrum by frequency.

Official specification and next topic

This chapter follows Topic 3, Waves, in Pearson Edexcel International GCSE Physics 4PH1. Wave and optical measurement methods belong to the separate practical hub.

• Official Pearson 4PH1 specification

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