# Waves

> CIE A-Level Physics · CIE 9702 A-Level Physics
> Source: https://www.owlsprep.com/study/cie-9702-u7-overview/
> Weight: n/a

This unit introduces core concepts of wave motion, the fundamental behavior explaining light, sound, and many other physical phenomena. You will learn key properties, calculations, and applications that form the foundation for later wave topics.

**Prerequisites:** Basic understanding of energy, frequency, and wavelength from IGCSE/GCSE Physics

## Learning objectives

- Describe core properties of progressive, transverse, and longitudinal waves
- Calculate key wave parameters and use the wave equation to solve problems
- Order regions of the electromagnetic spectrum by wavelength, frequency, and energy
- Explain polarisation and its common practical applications

## Unit at a Glance

This unit builds from the basic definition of wave motion up to specialized properties like polarisation. We start by defining progressive waves and distinguishing between the two main types of waves, then learn how to quantify waves using standard parameters.

Next we cover the universal wave equation, the electromagnetic spectrum, wave intensity, and end with polarisation. All concepts in this unit are prerequisite for understanding superposition, interference, and standing waves in the next unit.

This unit is split into the following sequential sub-topics:
- [Progressive waves](https://www.owlsprep.com/study/cie-9702-u7-progressive-waves/) — Learn what a progressive wave is and how it transfers energy without transferring matter.
- [Transverse and longitudinal waves](https://www.owlsprep.com/study/cie-9702-u7-transverse-and-longitudinal-waves/) — Distinguish between the two main classes of waves with common examples of each.
- [Wave parameters](https://www.owlsprep.com/study/cie-9702-u7-wave-parameters/) — Define and calculate key properties: displacement, amplitude, wavelength, period, frequency, and phase difference.
- [Wave equation](https://www.owlsprep.com/study/cie-9702-u7-wave-equation/) — Use the wave equation $v = f\lambda$ to solve for unknown wave speed, frequency, or wavelength.
- [Electromagnetic spectrum](https://www.owlsprep.com/study/cie-9702-u7-electromagnetic-spectrum/) — Order EM spectrum regions by wavelength, frequency, and energy, and list common uses for each.
- [Wave intensity](https://www.owlsprep.com/study/cie-9702-u7-wave-intensity/) — Relate wave intensity to amplitude and distance from the source using the inverse square law.
- [Polarisation](https://www.owlsprep.com/study/cie-9702-u7-polarisation/) — Explain polarisation of transverse waves, Malus's law, and practical applications.

## Common pitfalls

- **Wrong:** Mixing up which wave properties affect intensity
  - Why it fails: Many students incorrectly assume higher frequency equals higher intensity, ignoring the dependence on amplitude.
  - Correct: Remember intensity is proportional to the square of amplitude, and proportional to the square of frequency for a given amplitude.
- **Wrong:** Forgetting all EM waves travel at the same speed in a vacuum
  - Why it fails: Students often assume higher frequency EM waves travel faster than lower frequency ones.
  - Correct: All electromagnetic waves travel at $c = 3 \times 10^8$ m/s in a vacuum, regardless of frequency or wavelength.
- **Wrong:** Thinking longitudinal waves can be polarised
  - Why it fails: Polarisation requires vibration perpendicular to the direction of travel, which does not occur in longitudinal waves.
  - Correct: Only transverse waves can be polarised; longitudinal waves cannot be polarised.

## Cheatsheet

| Concept | Formula / Key Fact |
| --- | --- |
| Wave speed relationship | $v = f\lambda$ |
| Frequency and period conversion | $f = 1/T$ |
| Intensity vs amplitude | $I \propto A^2$ |
| Inverse square law for intensity | $I \propto 1/r^2$ |
| Speed of EM waves in vacuum | $c = 3.0 \times 10^8$ m/s |
| Malus's law for polarisation | $I = I_0 \cos^2\theta$ |
| Polarisation rule | Only transverse waves can be polarised |
| Progressive wave property | Energy is transferred, matter is not displaced |

## What's next

Start with the first sub-topic of this unit to build your foundational understanding of wave motion. Once you complete all sub-topics here, you will be ready to move on to the next unit on superposition, which directly builds on all the wave concepts you will learn in this unit.

- [Progressive waves](https://www.owlsprep.com/study/cie-9702-u7-progressive-waves/)
- [Unit 8: Superposition Overview](https://www.owlsprep.com/study/cie-9702-u8-overview/)
- [Transverse and Longitudinal Waves](https://www.owlsprep.com/study/cie-9702-u7-transverse-and-longitudinal-waves/)

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