Required Practicals/Edexcel/Practical 15
15A2CP15

Determining Planck's constant using LEDs (CP15)

Determine Planck's constant by measuring the threshold voltage of LEDs of different wavelengths.

Apparatus

  • LEDs of at least four colours (red, amber, green, blue) with known wavelengths
  • Variable power supply (0-5 V)
  • Voltmeter and milliammeter
  • Diffraction grating (to verify LED wavelengths if needed)
  • Connecting leads

Safety

  • Do not stare directly into a lit LED, particularly blue or UV LEDs.
  • Keep voltages below 5 V.

Method

  1. Connect an LED in series with the milliammeter and variable power supply. Connect the voltmeter across the LED.
  2. Slowly increase voltage from zero; record V and I at small intervals near the turn-on region.
  3. Determine the threshold voltage $V_{th}$ as the voltage at which the LED just begins to emit light noticeably (I begins to rise rapidly).
  4. Repeat for each LED colour. Look up or measure the peak emission wavelength lambda for each LED.
  5. Plot $V_{th}$ against $c/\lambda = f$: straight line through origin, gradient $= h/e$, so $h = $ gradient $\times e$.

Key Variables

Independent
Frequency f of LED emission (determined by wavelength)
Dependent
Threshold voltage $V_{th}$
Controlled
Temperature; Method of determining $V_{th}$ (consistent criterion for all LEDs)

Analysis and Results

  • At threshold: photon energy $= $ electrical energy supplied: $hf = eV_{th}$.
  • So $V_{th} = (h/e)f$. Plot $V_{th}$ vs f: gradient $= h/e$.
  • Multiply gradient by $e = 1.6 \times 10^{-19}$ C to obtain Planck's constant h.
  • Accepted value: $h = 6.63 \times 10^{-34}$ J s.

Common Errors

  • Using the wavelength of visible light passing through the LED casing rather than the emission peak wavelength.
  • Defining threshold voltage inconsistently between LEDs (use the same criterion, e.g. I = 1 mA, for all).
  • Not extrapolating the linear part of the I-V curve back to zero current to find threshold voltage more precisely.

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

Q14 marks
An LED emits light of wavelength 520 nm and has a threshold voltage of 2.39 V. Use these data to calculate Planck's constant.
Q23 marks
Explain why the threshold voltage of a blue LED is higher than that of a red LED.