Explain why radio waves and gamma rays travel at the same speed in a vacuum but have very different frequencies.

AQA GCSE Physics (8463) — 4.6.2 Electromagnetic waves · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Radio waves and gamma rays are both part of the electromagnetic spectrum, yet their frequencies differ enormously.

Model answer (4 marks)

All electromagnetic waves travel at the same speed in a vacuum – the speed of light, 3×10⁸ m/s.
Because the speed is constant, wavelength and frequency are inversely related (c=λν).
Gamma rays have a much shorter wavelength than radio waves.
Therefore gamma rays have a much higher frequency than radio waves, giving them greater energy.

Examiner tips

State the constant speed of light first, then use the relationship c=λν to link wavelength and frequency.
Mention that shorter wavelength → higher frequency → higher energy.
Keep the answer concise and use the exact terminology (speed of light, wavelength, frequency, energy).

Common mistakes

Saying the waves travel at different speeds; they do not. Forgetting to explain the inverse relationship between wavelength and frequency. Using vague terms like ‘fast’ instead of the speed of light.”

Mark scheme (4 marks)

All electromagnetic waves travel at the same speed in a vacuum (the speed of light / 3 × 10⁸ m/s)
Because speed is constant, wavelength and frequency are inversely related (as wavelength decreases, frequency increases)
Gamma rays have a much shorter wavelength than radio waves
Therefore gamma rays have a much higher frequency than radio waves (and higher frequency means greater energy)

Key terms in this question

vacuum