With a piece of glass between the mercury lamp and the metal sheet, the electroscope leaves do not move apart. When the glass is removed, the leaves move apart, as in the previous demonstration where there was not a piece of glass between the lamp and metal sheet. Light passing through the glass does not eject electrons from the metal plate, because short wavelength light is responsible for the photoelectric effect and short wavelength light is absorbed by the glass.

To explain the photoelectric effect, Einstein modified Planck's quantum hypothesis by postulating that light consisted of particle-like packets of energy, called quanta (singular quantum). The energy of a photon is proportional to the frequency of the light. High frequency, short wavelength light consists of photons that have more energy than photons of low frequency, long wavelength light. Photons of lower frequency, longer wavelength light that passes through glass do not have enough energy to eject electrons.