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## Homework Statement

Determine Plank's constant by measuring threshold voltage for a range of different colored LED's. The experiment involves increasing the voltage across a diode until a current starts to flow. You then measure the voltage across the LED at this time (Vo - threshold voltage). The theory goes that the threshold voltage is the voltage needed to give the electrons the same energy as a photon that is emitted. This is the bit I don't quite get.

## Homework Equations

E = hc/lambda = eVo (E=energy of photon, h = plank's constant, c = speed of light, lambda = wavelength of light, e = charge of electron, Vo = threshold voltage)

## The Attempt at a Solution

I have done this so that the threshold voltage is recorded when a current of 0.01mA flows. However, at this point, despite a current flowing, the LED does not emit photons of enough intensity to visually see them. If you increase the voltage, after the current increases by a few mA, the LED's shine. I don't understand why only the threshold voltage is equal to the same amount of energy as the photons for that LED? When you increase the voltage and it shines brightly surely the voltage is still transferred directly to the photons? What is the significance of the threshold voltage and is it supposed to be the voltage at which the led first shines or the instant a current starts to flow (i.e. 0.01A)?

Thanks for any guidance given.