Required Practicals / OCR A / Practical 11
11 A2 PAG 11

Planck's constant using LEDs (PAG 11)

Determine Planck's constant by measuring the threshold voltage of LEDs of known emission wavelength.

Apparatus

  • LEDs of at least four colours with known peak emission wavelengths
  • Variable power supply (0-5 V)
  • Voltmeter and milliammeter
  • Connecting leads

Safety

  • Do not look directly into LEDs, especially blue or UV.
  • Keep voltages below 5 V.

Method

  1. Connect each LED in series with the milliammeter; voltmeter across the LED.
  2. Increase voltage slowly; record V and I at small increments.
  3. Determine threshold voltage $V_{th}$ where current just begins to flow (or use the tangent-intercept method on the I-V graph).
  4. Repeat for all LED colours. Look up emission wavelength lambda.
  5. Plot $V_{th}$ vs $c/\lambda$ (frequency f): gradient $= h/e$.

Key Variables

Independent Frequency f of LED emission
Dependent Threshold voltage $V_{th}$
Controlled Temperature; Consistent criterion for threshold across all LEDs

Analysis and Results

  • $hf = eV_{th}$, so $V_{th} = (h/e)f$.
  • Plot $V_{th}$ vs f: straight line through origin, gradient $= h/e$.
  • $h = \text{gradient} \times e = \text{gradient} \times 1.6 \times 10^{-19}$ J s.

Common Errors

  • Using the wavelength of the LED casing colour rather than the manufacturer's emission peak.
  • Inconsistent definition of threshold voltage between different LEDs.
  • Not plotting a straight line (need at least four different coloured LEDs).

Exam-style questions on this practical. Click Show mark scheme to reveal the answer after attempting each question.

Q1 4 marks

LEDs of wavelengths 660 nm (red) and 470 nm (blue) have threshold voltages of 1.88 V and 2.64 V respectively. Use both sets of data to estimate Planck's constant.

Q2 2 marks

Explain why using more LED colours gives a more reliable value of h.