Topic 5: Waves and Particle Nature of Light — Edexcel A-Level Physics
Test yourself on Topic 5: Waves and Particle Nature of Light with PEARSON EDEXCEL A-Level practice questions.
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Topic 5: Waves and Particle Nature of Light explained
A longitudinal wave travels through a medium as a series of compressions and rarefactions.
Read the full explanation
In a sound wave in air, molecules oscillate back and forth along the direction in which the wave travels, so the displacement of molecules is parallel to the propagation direction. Where molecules bunch together, the pressure and density rise: a compression. Where they spread apart, the pressure and density fall: a rarefaction. A pressure–distance graph therefore shows peaks at compressions and troughs at rarefactions, and it is shifted by a quarter of a wavelength relative to a displacement–distance graph. The wave transfers energy, not matter, because each molecule only vibrates about a fixed mean position. The amplitude is the maximum displacement of a molecule from equilibrium, and the wavelength is the distance between successive compressions or between successive rarefactions.
Your focus
- Describe a longitudinal wave in terms of regions of compression and rarefaction.
- Explain how pressure variation relates to the displacement of molecules in the medium.
- Distinguish between energy transfer and the oscillation of molecules about fixed mean positions.
Topic 5: Waves and Particle Nature of Light exam tips
Marking Points
- States that in a longitudinal wave the oscillations of the medium are parallel to the direction of energy transfer (propagation).
- Identifies a compression as a region of raised pressure and density where molecules are closer together.
- Identifies a rarefaction as a region of reduced pressure and density where molecules are further apart.
- Explains that molecules oscillate about fixed mean positions, so energy is transferred without net transfer of matter.
- Relates the pressure variation to the displacement of molecules, recognising that pressure maxima correspond to maximum compression rather than maximum displacement.
- Defines amplitude as the maximum displacement of a molecule from its equilibrium position and wavelength as the distance between successive compressions (or rarefactions).
Examiner Tips
- 💡Sketch a pressure–distance graph and label compressions at the peaks and rarefactions at the troughs to anchor your reasoning.
- 💡When asked about energy transfer, state explicitly that energy moves but the medium's molecules do not have a net drift.
- 💡Use the words parallel and perpendicular deliberately to distinguish longitudinal from transverse waves in a multiple-choice option.
Common Mistakes
- Thinking that molecules travel along with the wave: correct this by stressing that each molecule oscillates about a mean position and only energy is transferred.
- Confusing compressions with troughs of a displacement graph: correct this by noting that a compression is a pressure maximum, where molecules are closest together.
- Claiming that pressure is greatest where molecular displacement is greatest: correct this by explaining that pressure peaks where molecules are most compressed, a quarter of a wavelength from maximum displacement.
- Describing a longitudinal wave as having oscillations perpendicular to propagation: correct this by stating that the oscillations are parallel to the direction of travel.