Determining Planck’s constant with LEDs—what could possibly go wrong?

Light emitting diodes have been used to determine Planck's constant in introductory physics laboratories. One common method relies on the energy of the light emitted by the LED and its relation to the energy gap in the solid of which the diode is composed. However, there could be a problem with the data that are collected for this experiment. For some LEDs the energy of the light emitted from the LED can be quite different from the gap energy. If these LEDs are used for the experiment, the results will give different results for Planck's constant.