A gold leaf electroscope with a zinc plate top is charged by briefly connecting it to the negative electrode of a high-voltage supply. Explain how the gold leaf will appear and how the leaf can be caused to drop again.

The gold leaf will diverge from the metal stem. This is because connecting the negative electrode to the zinc plate, causes an influx of electrons to flow into the electroscope. When the electrode is removed, the electroscope is left with an overall negative charge. This causes the part of the electroscope that can move, the gold leaf, to repel from the rest and diverge.To return it, photons with energy above the work function of the zinc is needed to trigger the electrons from the plate to be emitted. For zinc, this corresponds to UV photons or any with a shorter wavelength, even at a low intensity.