Library · Experiment
Find Planck's constant with LEDs
Different coloured LEDs switch on at different voltages. A multimeter and a little maths reveal the number at the heart of quantum physics.
- When
- Late Nov – mid Dec · Your first fundamental constant
- Time
- 2 evenings
- Cost
- ~£15 (multimeter + LED kit)
- Difficulty
- 2 of 3
Part of The Road Map (Your first fundamental constant) · Quantum from your kitchen (Light is also a particle)
The question
Light comes in packets called photons, with energy \(E = hf\). Can I measure \(h\), Planck's constant, on my kitchen table?
Learn just enough first
- Read about photons and the photoelectric effect
- Look up how an LED works: one electron crossing the junction makes one photon
Kit
- A cheap digital multimeter
- At least four LEDs of different colours with known wavelengths (red ~630 nm, yellow ~590 nm, green ~525 nm, blue ~470 nm — check the seller's specs)
- A 9 V battery and battery clip
- A 10 kΩ potentiometer and a 1 kΩ resistor
- A small breadboard and jumper wires
My prediction
Will blue LEDs need more voltage or less than red ones to switch on? Why?
Method
- Wire battery → potentiometer → 1 kΩ resistor → LED, with the multimeter measuring the voltage across the LED.
- In a dark room, slowly turn up the voltage until the LED just starts to glow. Record that voltage.
- Repeat three times per LED and take the average.
Safety: Always keep the 1 kΩ resistor in the circuit — without it the LED will burn out.
Data
| Colour | Wavelength \(\lambda\) (nm) | Turn-on voltage \(V\) (V) | \(h\) estimate (J·s) |
|---|---|---|---|
| Red | |||
| Yellow | |||
| Green | |||
| Blue |
Result
Each electron gains energy \(eV\) and gives it up as one photon of energy \(hc/\lambda\), so:
with \(e = 1.602 \times 10^{-19}\) C and \(c = 2.998 \times 10^{8}\) m/s. Better still, plot \(V\) against \(1/\lambda\): the slope of the straight line is \(hc/e\). The accepted value is \(6.626 \times 10^{-34}\) J·s — getting within 10–20% is a great result.
Explain it to a friend
Bluer light carries more energy per photon, so blue LEDs need a bigger push to switch on. Measuring that push for several colours lets you work out the conversion rate between a photon's colour and its energy. That number, Planck's constant, is the foundation of quantum physics.
New questions
- Why isn't the "just glowing" point perfectly sharp?
- How does my error compare between the straight-line method and single LEDs?