Photoelectric Effect - Blocked by Glass

Short wavelength, higher energy UV light is absorbed by glass, but longer wavelength, lower energy light is transmitted.

Discussion

With a piece of glass between the mercury lamp and the metal sheet, the electroscope leaves do not move apart. Light that has passed through glass does not eject electrons from the metal plate, because the short wavelength photons with sufficient energy to eject electrons from the metal have been absorbed by the glass. When the glass is removed, the leaves become charged and move apart, as in the previous demonstration.