1. Build & operate the circuit
Interactive simulation following the supplied reference topology: the resistor output (Rout) feeds both the LED anode and voltmeter positive terminal, while the LED cathode and voltmeter negative terminal share the ground/negative return. All electrical connections are continuous when the key is closed; the only intentional break is the key when it is open. Operate the rheostat slowly: the LED turns ON at its model knee voltage.
Reference circuit diagram (provided)
Use this supplied schematic as the wiring reference. Complete the connections on the interactive circuit below.
Before you begin: Connect the circuit terminals
Build the circuit directly on the diagram: Rout branches to the LED anode (A) and the voltmeter positive (+); the LED cathode (K) and voltmeter negative (−) both connect to the common ground/negative return. Click a gold source terminal, drag its wire to the correct blue destination terminal, then release. You can also click the source and then click the destination. Complete all nine connections and select Validate Circuit to unlock the experiment.
● Gold = wire source ● Blue = destination. Terminal labels appear beside the points.
2. Observation & calculation
| LED | λ (nm) | 1/λ (m⁻¹) | Vk (V) | Vλ (V·m) |
|---|
Typical LED operating ranges
| LED | Typical wavelength | Typical threshold voltage |
|---|---|---|
| Red | 630–650 nm | 1.7–1.9 V |
| Yellow | 585–595 nm | 2.0–2.1 V |
| Green | 520–540 nm | 2.1–2.4 V |
| Blue | 460–475 nm | 2.5–2.7 V |
The table above gives the typical LED wavelength and threshold-voltage ranges supplied for this simulation. One representative wavelength and knee voltage are used for each selected LED, and the LED begins to glow exactly at that knee voltage.
3. Graph: Knee voltage V versus 1/λ
The graph remains hidden until you press Plot Graph. At least two recorded LED readings are required. The best-fit straight line is calculated using ordinary least squares, including a fitted intercept.
The graph uses a centimetre-style graph-paper grid with equal-sized small squares. The scale for each axis is displayed at the top-right of the plotted graph. The slope is demonstrated using a large triangle on the fitted line (points A, B and C are not experimental data points), with slope = AB/BC.
4. Procedure for Simulation
- Click and drag from each gold source terminal to its required blue destination on the circuit diagram. Complete all nine wires, click Validate Circuit, then click Close Key (equivalent to inserting the key).
- Select the required LED from the LED selector.
- Operate the graphical rheostat contact slowly.
- Observe the voltmeter. When the LED just begins to glow, the displayed voltage is the knee voltage Vk.
- Click Record Knee Voltage and repeat for all four LEDs.
- Calculate Vλ for each LED and obtain the average.
- Determine Planck's constant using h = (e/c) × average(Vλ).
- Use the graph of V versus 1/λ and determine h from the slope.
5. Result
This section summarizes the experimental Planck constant obtained by the direct calculation method and the least-squares graphical method. Values are populated from the readings recorded by the student.