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
- Connect each LED in series with the milliammeter; voltmeter across the LED.
- Increase voltage slowly; record V and I at small increments.
- Determine threshold voltage $V_{th}$ where current just begins to flow (or use the tangent-intercept method on the I-V graph).
- Repeat for all LED colours. Look up emission wavelength lambda.
- 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.
Q14 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.
Q22 marks
Explain why using more LED colours gives a more reliable value of h.