When a glass rod and silk are used to charge a zinc plate, ultraviolet light does not discharge plate.
A glass plate is used to stop the UV light to show the difference with the plate and without.
Demonstration Instructions: Transfer a negative charge to the zinc plate. Now illuminate the zinc plate with the arc light and note the rapid discharge. The relatively high energy photons which make up the ultraviolet portion of the arc's emission cause the release of electrons from the zinc plate. Thus if the electroscope is negative initially, it loses it's excess electrons. If it is charged positively,it cannot be discharged by electron emission.