OCR · A-Level · Physics

    Course version: H556 · Version from Sept 2015

    Wave motion: (c) the equation f = 1/T

    The equation f = 1/T links frequency f, measured in hertz (Hz), to period T, measured in seconds (s).

    Checked against the OCR H556 specification (opens in a new tab) on .

    • Point 4 of 11
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    The equation f = 1/T links frequency f, measured in hertz (Hz), to period T, measured in seconds (s). Frequency is the number of complete oscillations per second, while period is the time for one complete oscillation, so each is the reciprocal of the other. If a wave has a period of 0.25 s, then f = 1/0.25 = 4.0 Hz. Rearranged, T = 1/f. Always convert a period given in milliseconds to seconds before substituting: 20 ms becomes 0.020 s, giving f = 1/0.020 = 50 Hz. The equation applies to any periodic wave, including sound, light and waves on a string, and connects directly to oscilloscope measurements where the period is read from the trace.

    What earns the marks

    • Frequency is the number of complete oscillations per unit time, measured in hertz.
    • Period is the time for one complete oscillation, measured in seconds.
    • Frequency and period are reciprocals, so f = 1/T and T = 1/f.
    • Substitute the period in seconds to obtain frequency in hertz.
    • Convert periods given in milliseconds or microseconds to seconds before calculating.

    Common mistakes and how to put them right

    • Substituting a period in milliseconds directly into f = 1/T.

      Put it right: Convert to seconds first, for example 20 ms becomes 0.020 s.

    • Writing f = T instead of the reciprocal relationship.

      Put it right: Frequency and period are reciprocals, so f = 1/T.

    • Giving frequency the unit seconds.

      Put it right: Frequency is measured in hertz, where 1 Hz = 1 s⁻¹.

    • Confusing period with wavelength.

      Put it right: Period is a time in seconds; wavelength is a distance in metres.

    Exam tips

    • Check the unit of the period and convert to seconds before dividing.
    • Use T = 1/f when a question gives frequency and asks for period.
    • Sanity-check the answer: a short period must give a high frequency.

    Try the lesson

    The start of the Wave motion: (c) the equation f = 1/T lesson, made from checked teaching notes for this specification point.

    Listen

    The equation f = 1/T links frequency f, measured in hertz (Hz), to period T, measured in seconds (s). Frequency is the number of complete oscillations per second, while period is the time for one complete oscillation, so each is the reciprocal of the other. If a wave has a period of 0.25 s, then f = 1/0.25 = 4.0 Hz. Rearranged, T = 1/f. Always convert a period given in milliseconds to seconds before substituting: 20 ms becomes 0.020 s, giving f = 1/0.020 = 50 Hz. The equation applies to any periodic wave, including sound, light and waves on a string, and connects directly to oscilloscope measurements where the period is read from the trace. A strong answer earns marks for these points. Frequency is the number of complete oscillations per unit time, measured in hertz. Period is the time for one complete oscillation, measured in seconds. Frequency and period are reciprocals, so f = 1/T and T = 1/f. Substitute the period in seconds to obtain frequency in hertz. A common slip: Substituting a period in milliseconds directly into f = 1/T. Instead, convert to seconds first, for example 20 ms becomes 0.020 s. Exam tip: Check the unit of the period and convert to seconds before dividing.

    See it

    A flowchart with six nodes. 'Period T in seconds' leads to 'f = 1/T', which leads to 'Frequency f in hertz'. Separately, 'Period given in milliseconds' leads to 'Convert to seconds', which leads back to 'Period T in seconds'. From 'Frequency f in hertz' an arrow leads to 'T = 1/f', which leads back to 'Period T in seconds', showing the reciprocal relationship in both directions.
    Shows how period in seconds and frequency in hertz are reciprocals via f = 1/T and T = 1/f, with milliseconds converted to seconds first.

    Key words

    Say what each word means before you turn the card.

    Frequency

    Try answering aloud or in your head before turning the card.

    Period

    Try answering aloud or in your head before turning the card.

    Reciprocal relationship

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    See every definition
    Frequency
    The number of complete oscillations per second, measured in hertz (Hz).
    Period
    The time for one complete oscillation, measured in seconds (s).
    Reciprocal relationship
    Frequency and period are reciprocals of each other, so f = 1/T and T = 1/f.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A flowchart with six nodes. 'Period T in seconds' leads to 'f = 1/T', which leads to 'Frequency f in hertz'. Separately, 'Period given in milliseconds' leads to 'Convert to seconds', which leads back to 'Period T in seconds'. From 'Frequency f in hertz' an arrow leads to 'T = 1/f', which leads back to 'Period T in seconds', showing the reciprocal relationship in both directions.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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