Determine Fundamental Constants with LEDs and a Multimeter

There are (probably) less than two dozen fundemental constants that define the physics of our universe. Determining the value of them might seem like the sort of thing for large, …read more

May 18, 2025 - 00:22

Determine Fundamental Constants with LEDs and a Multimeter

There are (probably) less than two dozen fundemental constants that define the physics of our universe. Determining the value of them might seem like the sort of thing for large, well funded University labs, but many can be determined to reasonable accuracy on the benchtop, as [Marb’s Lab] proves with this experiment to find the value of Planck’s Constant.

[Marv’s Lab] setup is on a nice PCB that uses a rotary switch to select between 5 LEDs of different wavelengths, with banana plugs for the multi-meter so he can perform a linear regression on the relation between energy and frequency to find the constant. He’s also thoughtfully put connectors in place for current measurement, so the volt-current relationship of the LEDs can be characterized in a second experiment. Overall, this is a piece of kit that would not be out of place in any high school or undergraduate physics lab.

To use this to determine Planck’s constant, you need to use Planck’s relation for the energy of a photon:

E = hf

Get some Energies (E), get some energies (f), and bam! You can generate a value for h, Planck’s constant. The energies? Well, that’s a very easy measurement, but it requires some understanding of how LEDs work. [Marb] is simply measuring the voltage needed to just barely light the LED of a given frequency. (For frequency, he’s relying on the LED datasheets.) That translates to the energy of the photon because it corresponds to the energy (in electron volts) required to jump electrons over the bandgap of the semiconductor in the LED– that’s how the light is generated. Those photons will have the energy of the gap, in theory.

In practice, the LEDs do not emit perfectly monochromatic light; there’s a normal distribution centered on the color they’re “supposed” to be, but it is fairly tight. That’s probably why is able to [Marv] get to within 5% of the canonical value, which is better than we’d expect.

This isn’t the first time we’ve determined plank’s constant; it’s quite possible to get to much higher accuracy. The last time we featured this particular technique, the error was 11%.