Answer
(a) Excess electrons can be ejected from the metal by a photon of ultraviolet light.
(b) If a photon of ultraviolet light does not have enough energy, the photon will not be able to eject bound electrons from the neutral electroscope.
Work Step by Step
(a) If the electroscope is negatively charged, then there are excess electrons on the electroscope. Each of these electrons can be ejected from the metal by a photon, as long as the photon has enough energy to eject the electron. If the photons of ultraviolet light have enough energy, then they are able to eject the excess electrons, thus leaving the electroscope with a neutral charge.
That is, a negatively charged electroscope can be discharged by shining ultraviolet light on it.
(b) If the electroscope is uncharged, then there are no excess electrons. Although there are still electrons, these electrons are bound in atoms or molecules, and thus would require a higher amount of energy to eject them from the electroscope. If a photon of ultraviolet light does not have enough energy, the photon will not be able to eject these bound electrons from the neutral electroscope.
