An LED is a device that emits light of a particular frequency when the voltage applied across the LED reaches a certain value. This effect can be used to measure Planck’s constant.
In the circuit shown below the switch is closed and the potentiometer is adjusted until the LED just emits light. When this occurs the voltmeter reads 2.5 V. The uncertainty in the voltage reading is 0.2 V.

The LED operates in such a way that the electrons can give up the energy that they gain from the battery by emitting a photon. Assuming that all the energy gained by an electron is transferred to a photon and that the LED used in this experiment emits light of wavelength 480 nm calculate a value for Planck’s constant, together with an estimate of its uncertainty.

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