OCR · A-Level · Physics

    Course version: H556 · Version from Sept 2015

    (i) using LEDs and the equation eV = hc/λ to estimate the value of Planck…

    Full specification wording

    Photons: (i) using LEDs and the equation eV = hc/λ to estimate the value of Planck constant h

    In the LED experiment, a light-emitting diode emits a photon when an electron crosses the junction potential difference V. The energy lost by the electron is eV, and this equals the photon energy…

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

    • Point 5 of 6
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    In the LED experiment, a light-emitting diode emits a photon when an electron crosses the junction potential difference V. The energy lost by the electron is eV, and this equals the photon energy hc/λ, giving eV = hc/λ and hence h = eVλ/c. Measure the threshold voltage at which the LED just begins to emit light and the wavelength of the emitted light, then substitute. For example, V = 1.9 V and λ = 6.5 × 10⁻⁷ m give h = (1.60 × 10⁻¹⁹ × 1.9 × 6.5 × 10⁻⁷) ÷ (3.00 × 10⁸) ≈ 6.6 × 10⁻³⁴ J s. In MCQs, check the rearrangement and unit consistency.

    What earns the marks

    • States that the energy lost by an electron crossing the LED junction is eV and equals the emitted photon energy hc/λ.
    • Rearranges eV = hc/λ to h = eVλ/c and substitutes measured values correctly.
    • Uses the threshold voltage at which the LED just begins to emit light as V in the equation.
    • Converts wavelength to metres and uses e = 1.60 × 10⁻¹⁹ C and c = 3.00 × 10⁸ m s⁻¹.
    • Evaluates h to an order of magnitude consistent with 6.63 × 10⁻³⁴ J s and comments on uncertainty in the estimate.

    Common mistakes and how to put them right

    • Using the operating voltage rather than the threshold voltage.

      Put it right: The threshold voltage corresponds to the minimum photon energy, so use that value.

    • Forgetting to convert wavelength from nanometres to metres.

      Put it right: 650 nm must be entered as 6.50 × 10⁻⁷ m.

    • Rearranging incorrectly to h = eVc/λ.

      Put it right: The correct rearrangement is h = eVλ/c.

    Exam tips

    • Write the full equation eV = hc/λ before rearranging, so the algebra is visible and checkable.
    • Keep wavelength in metres and use standard form to avoid power-of-ten errors.
    • Compare your result with the accepted value 6.63 × 10⁻³⁴ J s and comment briefly on the uncertainty of the estimate.

    Try the lesson

    The start of the Photons: (i) using LEDs and the equation eV = hc/λ to estimate the value of Planck constant h lesson, made from checked teaching notes for this specification point.

    Listen

    In the LED experiment, a light-emitting diode emits a photon when an electron crosses the junction potential difference V. The energy lost by the electron is eV, and this equals the photon energy hc/λ, giving eV = hc/λ and hence h = eVλ/c. Measure the threshold voltage at which the LED just begins to emit light and the wavelength of the emitted light, then substitute. For example, V = 1.9 V and λ = 6.5 × 10⁻⁷ m give h = (1.60 × 10⁻¹⁹ × 1.9 × 6.5 × 10⁻⁷) ÷ (3.00 × 10⁸) ≈ 6.6 × 10⁻³⁴ J s. In MCQs, check the rearrangement and unit consistency. A strong answer earns marks for these points. States that the energy lost by an electron crossing the LED junction is eV and equals the emitted photon energy hc/λ. Rearranges eV = hc/λ to h = eVλ/c and substitutes measured values correctly. Uses the threshold voltage at which the LED just begins to emit light as V in the equation. Converts wavelength to metres and uses e = 1.60 × 10⁻¹⁹ C and c = 3.00 × 10⁸ m s⁻¹. Exam tip: Write the full equation eV = hc/λ before rearranging, so the algebra is visible and checkable.

    See it

    A top-down flowchart with eight boxes. The first box says an electron crosses the LED junction at threshold voltage V. An arrow leads to a box saying the energy lost by the electron is eV. An arrow leads to a box saying this equals the emitted photon energy hc/λ. An arrow leads to a box giving the equation eV = hc/λ. An arrow leads to a box saying rearrange to h = eVλ/c. An arrow leads to a box saying measure threshold voltage V and wavelength λ. An arrow leads to a box saying substitute values, wavelength in metres. A final arrow leads to a box saying estimate h and compare with 6.63 × 10⁻³⁴ J s.
    Shows how the LED experiment links electron energy eV to photon energy hc/λ and leads to an estimate of Planck constant h.

    Put it in order

    Put these steps in the order they happen.

    1. 1. Rearrange to h = eVλ/c
    2. 2. Estimate h and compare with 6.63 × 10⁻³⁴ J s
    3. 3. Measure threshold voltage V and wavelength λ
    4. 4. Electron crosses LED junction at threshold voltage V
    5. 5. Energy lost by electron = eV
    6. 6. Substitute values, wavelength in metres
    7. 7. This equals emitted photon energy hc/λ
    8. 8. eV = hc/λ

    Key words

    Say what each word means before you turn the card.

    Threshold voltage

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

    eV = hc/λ

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

    h = eVλ/c

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

    Photon energy

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

    See every definition
    Threshold voltage
    The voltage at which the LED just begins to emit light, corresponding to the minimum photon energy.
    eV = hc/λ
    The equation stating that the energy lost by an electron crossing the LED junction equals the emitted photon energy.
    h = eVλ/c
    The rearrangement of eV = hc/λ used to estimate the value of Planck constant h from measured values.
    Photon energy
    The energy hc/λ of the light emitted by the LED, equal to the energy lost by the electron.

    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 top-down flowchart with eight boxes. The first box says an electron crosses the LED junction at threshold voltage V. An arrow leads to a box saying the energy lost by the electron is eV. An arrow leads to a box saying this equals the emitted photon energy hc/λ. An arrow leads to a box giving the equation eV = hc/λ. An arrow leads to a box saying rearrange to h = eVλ/c. An arrow leads to a box saying measure threshold voltage V and wavelength λ. An arrow leads to a box saying substitute values, wavelength in metres. A final arrow leads to a box saying estimate h and compare with 6.63 × 10⁻³⁴ J s.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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