OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Apply the relationships between frequency and wavelength across the…
Full specification wording
The electromagnetic spectrum: apply the relationships between frequency and wavelength across the electromagnetic spectrum
Across the electromagnetic spectrum, frequency and wavelength are inversely related: as frequency increases, wavelength decreases, and vice versa.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 3 of 9
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Across the electromagnetic spectrum, frequency and wavelength are inversely related: as frequency increases, wavelength decreases, and vice versa. This follows from the wave equation v = f × λ, where for EM waves in a vacuum v is the speed of light, about 3.0 × 10⁸ m/s. So radio waves have long wavelengths and low frequencies, while gamma rays have short wavelengths and high frequencies. To apply the relationship, rearrange as λ = v ÷ f or f = v ÷ λ and substitute values, keeping units consistent (frequency in hertz, wavelength in metres). For example, a wave of frequency 1.0 × 10⁹ Hz has wavelength λ = (3.0 × 10⁸) ÷ (1.0 × 10⁹) = 0.30 m. Ordering the spectrum by increasing frequency gives decreasing wavelength.
What earns the marks
- States that frequency and wavelength are inversely related across the EM spectrum.
- Uses the wave equation v = f × λ to relate frequency and wavelength.
- Rearranges the equation correctly to find wavelength or frequency.
- Substitutes values with consistent units and calculates a correct result.
- Orders EM waves by increasing frequency and decreasing wavelength.
Common mistakes and how to put them right
Treats frequency and wavelength as directly proportional.
Put it right: State that they are inversely related.
Uses the wrong speed for EM waves in a vacuum.
Put it right: Use about 3.0 × 10⁸ m/s.
Mixes units such as kilohertz and metres without converting.
Put it right: Convert to hertz and metres first.
Exam tips
- Write the wave equation before substituting numbers.
- Check that frequency is in hertz and wavelength in metres before calculating.
- Use the inverse relationship to sanity-check whether your answer is larger or smaller than expected.
Try the lesson
The start of the The electromagnetic spectrum: apply the relationships between frequency and wavelength across the electromagnetic spectrum lesson, made from checked teaching notes for this specification point.
Have a go first
Across the electromagnetic spectrum, how does the wavelength of a wave change as its frequency increases?
Listen
Across the electromagnetic spectrum, frequency and wavelength are inversely related: as frequency increases, wavelength decreases, and vice versa. This follows from the wave equation v = f × λ, where for EM waves in a vacuum v is the speed of light, about 3.0 × 10⁸ m/s. So radio waves have long wavelengths and low frequencies, while gamma rays have short wavelengths and high frequencies. To apply the relationship, rearrange as λ = v ÷ f or f = v ÷ λ and substitute values, keeping units consistent (frequency in hertz, wavelength in metres). For example, a wave of frequency 1.0 × 10⁹ Hz has wavelength λ = (3.0 × 10⁸) ÷ (1.0 × 10⁹) = 0.30 m. Ordering the spectrum by increasing frequency gives decreasing wavelength. A strong answer earns marks for these points. States that frequency and wavelength are inversely related across the EM spectrum. Uses the wave equation v = f × λ to relate frequency and wavelength. Rearranges the equation correctly to find wavelength or frequency. Substitutes values with consistent units and calculates a correct result. Exam tip: Write the wave equation before substituting numbers.
See it
Key words
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Wave equation
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Inverse relationship
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Speed of light
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Ordering the EM spectrum
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See every definition
- Wave equation
- v = f × λ relates frequency and wavelength, where for EM waves in a vacuum v is the speed of light.
- Inverse relationship
- Across the electromagnetic spectrum, as frequency increases, wavelength decreases, and vice versa.
- Speed of light
- For EM waves in a vacuum, v is about 3.0 × 10⁸ m/s.
- Ordering the EM spectrum
- Ordering the spectrum by increasing frequency gives decreasing wavelength.
Label the diagram
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