With LEDs, we can measure the voltage at which the LED first
emits lights, 
. This voltage
is related to the energy gap by
 |
(5) |
where e is the charge on the electron. Thus, the frequency of the brightest light emitted by the LED and the threshold voltage are related by:
 |
(6) |
Solving for Planck’s constant,
 |
(7) |
If e is considered a constant is
in Volts, and f is in 1/seconds. The units of Planck’s constant
are electron volts x seconds (eV·s).
