Interactive Virtual Lab: LEDs and Determination of Planck's Constant

Students build the circuit, move equipment, connect wires, measure current and voltage, collect a spectrum, and calculate Planck's constant from LED threshold voltage and wavelength.

Experiment procedure

Goal: investigate the I-U characteristic of LEDs with different wavelengths and use them to estimate h.

  1. Drag a colored LED chip into the LED holder.
  2. Click terminals to connect wires. Build a series circuit with power supply, LED, resistor, and ammeter.
  3. Connect the voltmeter across the LED.
  4. Turn the power supply knob slowly and record paired values of I and U.
  5. Fit the upper linear part of the graph to obtain the threshold voltage U₀.
  6. Calibrate the Vernier probes and run an automatic measurement.
  7. Move the spectrometer fiber tip near the glowing LED and collect the emission spectrum.
  8. Use h = U₀eλ/c and compare the average result.

Interactive checklist

The checklist updates automatically. Click a pending step to see what to do next.

Click one terminal, then another terminal to connect a wire. The 100 Ω resistor is now on the right side of the breadboard.
LED parts shelf
DC power supply 0-6.5 V
0.00 V
drag knob or scroll
−+
Breadboard
100 Ω Resistor series
drag into circuit
LED holder red
anode +cathode −
Ammeter series
0.00 mA
Voltmeter across LED
connect V
Current Probe Logger Pro
not calibrated
Differential Voltage Probe Logger Pro
not calibrated
USB650 Spectrometer Intensity
Move fiber tip close to LED. Avoid saturation.
red
orange
yellow
green
blue
Selected LEDred
Supply voltage0.00 V
Current0.00 mA
LED voltage0.00 V
Brightnessoff
Measure
Graph
Spectrum
Results

Measurement table

Use manual readings or the simulated Logger Pro automatic collection.

#I (mA)U LED (V)V supply

I-U characteristic

Collect data first.

Spectrometer

Move the fiber tip to the LED before collecting.

No spectrum collected yet.

Calculate Planck's constant

Formula: h = U₀eλ/c, with λ in metres.

LEDU₀λh ×10⁻³⁴Error

Average h: not enough data