Quick answer: Light comes in photons of energy E = hf. In the photoelectric effect a photon ejects an electron only if hf is at least the work function φ, and the electron then has maximum kinetic energy Kmax = hf − φ. This Physics 11-12 lab on photoelectric effect gives the aim, the equipment, the safety points, a step-by-step method, sample results and 6 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Can a few LEDs and a voltmeter give you Planck’s constant? An LED emits photons of energy about eV when it starts to glow, so its turn-on voltage depends on the colour. You will plot the voltage against 1/λ and find h from the gradient.
Level: Physics grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB E.2
Aim
To estimate Planck’s constant from the minimum voltage at which LEDs of different colours begin to glow.
Equipment
- five LEDs of known wavelengths (about 630, 590, 525, 470 and 405 nm)
- 3 V supply (two 1.5 V cells), a 10 kΩ variable resistor used as a potential divider, and a 470 Ω protective resistor
- digital voltmeter
- short black card tube and a darkened room
- connecting leads
Safety
Never connect an LED without the protective resistor, and never look straight into the violet LED because it emits near-ultraviolet light. Use only the 3 V supply. Keep the cells away from metal objects that could short them.
Method
- Connect the first LED, with the 470 Ω resistor in series, across the output of the potential divider, with the voltmeter in parallel with the LED.
- Look into the end of the tube in the dark. Start at 0 V and raise the voltage slowly until the LED is just visibly glowing.
- Record the voltage at that point as the turn-on voltage. Repeat three times and take the mean.
- Repeat for each of the other four LEDs.
- Calculate 1/λ for each LED in units of 10⁶ m⁻¹.
Results
Mean turn-on voltages:
| Colour | λ / nm | 1/λ / 10⁶ m⁻¹ | Turn-on voltage V / V |
|---|---|---|---|
| red | 630 | 1.59 | 1.90 |
| amber | 590 | 1.69 | 2.03 |
| green | 525 | 1.90 | 2.29 |
| blue | 470 | 2.13 | 2.57 |
| violet | 405 | 2.47 | 2.99 |
Analysis questions
Question 1 Plotting
Plot V on the vertical axis against 1/λ on the horizontal axis and draw the line of best fit.Question 2 Gradient
Determine the gradient of the line in V per 10⁶ m⁻¹.Question 3 Planck’s constant
Use eV = hc/λ to show that the gradient is hc/e, and calculate h.Question 4 Comparison
Compare your value with the accepted value 6.63 × 10⁻³⁴ J s as a percentage difference.Question 5 The intercept
The best-fit line does not pass through the origin. Suggest a reason and give the intercept.Question 6 Evaluation
Identify the main source of uncertainty and suggest two improvements.
