Quick answer: Light is absorbed and emitted in photons, each carrying energy E = hf. This AP Physics 2 lab on photoelectric effect gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Can a small LED teach you about the quantum nature of light? You will find the voltage at which LEDs of different colours begin to glow and use the pattern to estimate the Planck constant.
Level: AP Physics 2 (algebra-based) · Curriculum: AP P2 Unit 15
Aim
Find the Planck constant by measuring the voltage at which LEDs of different colours just begin to glow, and plotting that voltage against the reciprocal of the wavelength.
Equipment
- five LEDs: red, orange, yellow, green and blue, with known wavelengths
- variable DC supply (0–5 V) or a battery pack with a potentiometer
- protective resistor of 330 Ω
- digital voltmeter and microammeter
- dark tube or card cylinder to shade each LED
- connecting leads
Safety
Never exceed the current rating of the LED and keep the 330 Ω resistor in the circuit. Do not look directly at bright blue LEDs from close range. Switch the supply off before changing the LED.
Method
- Build the circuit shown, with the voltmeter across the LED and the resistor in series.
- Place the dark tube over the first LED and start with the supply at 0 V.
- Raise the voltage slowly until the LED first glows faintly in the tube and record the voltmeter reading.
- Repeat three times for the same LED, then change to the next colour.
Results
Sample measurements by Mei in Singapore:
| LED | Wavelength (nm) | Turn-on voltage (V) |
|---|---|---|
| red | 635 | 1.91 |
| orange | 605 | 2.02 |
| yellow | 590 | 2.08 |
| green | 525 | 2.31 |
| blue | 470 | 2.59 |
Analysis questions
Question 1 Reciprocal of wavelength
Calculate 1/λ in µm⁻¹ for each LED (use λ in µm).Question 2 Plot the graph
Plot turn-on voltage V (vertical) against 1/λ (horizontal) and draw a line of best fit.Question 3 Gradient
Calculate the gradient of the line using the red and blue LEDs. Give the unit.Question 4 Planck constant
Use the gradient to calculate h. Use e = 1.60 × 10⁻¹⁹ C and c = 3.00 × 10⁸ m/s.Question 5 Percentage difference
Express the gap between your result and the accepted constant, 6.63 × 10⁻³⁴ J·s, as a percentage of the accepted value.Question 6 Meaning of the intercept
State the expected intercept on the voltage axis and compare it with the line.Question 7 Predict a violet LED
Predict the turn-on voltage of a violet LED of wavelength 405 nm, and suggest one source of uncertainty in the experiment.
