Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
Recall that all electromagnetic waves are transverse, that they travel at…
Full specification wording
Topic 5 – Light and the electromagnetic spectrum: 5.7 Recall that all electromagnetic waves are transverse, that they travel at the same speed in a vacuum
Electromagnetic (EM) waves form a continuous family: radio, microwave, infrared, visible, ultraviolet, X-ray and gamma.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 5 of 19
Study notes
From the checked teaching notes for this specification point.
The idea
Electromagnetic (EM) waves form a continuous family: radio, microwave, infrared, visible, ultraviolet, X-ray and gamma. Two facts apply to every member. First, each is transverse: the oscillating electric and magnetic fields vibrate at right angles to the direction in which the wave travels, so no medium is needed and the wave can cross empty space. Second, in a vacuum all EM waves travel at the same speed, the speed of light, about 3 × 10⁸ m/s. That shared speed is why light from the Sun and radio signals both cross space at the same rate, even though their wavelengths and frequencies differ. A question may ask you to pick the true statement about direction of vibration or about speed in a vacuum; check that the option keeps both ideas correct and does not claim that only visible light is transverse or that speed in a vacuum depends on wavelength.
What earns the marks
- States that electromagnetic waves are transverse, with oscillations at right angles to the direction of energy transfer.
- States that all electromagnetic waves travel at the same speed in a vacuum, approximately 3 × 10⁸ m/s.
- Recognises that the shared vacuum speed applies across the whole spectrum, not only to visible light.
- Rejects the idea that speed in a vacuum varies with wavelength or frequency for different EM waves.
Common mistakes and how to put them right
Claims that only some electromagnetic waves are transverse.
Put it right by stating that every EM wave is transverse.
Assumes that longer-wavelength waves such as radio travel faster in a vacuum.
Put it right by stating that all EM waves share the same vacuum speed.
Describes EM oscillations as along the direction of travel.
Put it right by stating that the oscillations are at right angles to the direction of travel.
Exam tips
- Read each option for two separate claims: direction of vibration and speed in a vacuum; a wrong option often gets one right and one wrong.
- Remember the shared vacuum speed is about 3 × 10⁸ m/s, so an option giving a different speed for radio than for light is wrong.
- Link 'transverse' to 'at right angles to the direction of travel' rather than to any particular wavelength.
Try the lesson
The start of the Topic 5 – Light and the electromagnetic spectrum: 5.7 Recall that all electromagnetic waves are transverse, that they travel at the same speed in a vacuum lesson, made from checked teaching notes for this specification point.
Have a go first
In which direction do the oscillations of an electromagnetic wave occur relative to its direction of travel?
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