Photoelectric Effect - Charging an Electroscope with Light

Light from a mercury lamp is used to charge an electroscope.

Discussion

When the mercury lamp is turned on to illuminate the zinc-coated metal sheet attached to the post of the electroscope, the leaves of the electroscope move apart, indicating that light from the mercury lamp has created a charge on the electroscope and metal sheet. Bringing a positively charged glass rod near the electroscope makes the leaves move farther apart; bringing a negatively charged hard rubber rod near makes the leaves collapse. This demonstrates that the charge on the electroscope is positive.

To explain the photoelectric effect, Einstein modified Planck's quantum hypothesis by postulating that light consisted of particle-like packets of energy, now called photons. The energy of a photon is proportional to the frequency of the light; and so high frequency, short wavelength light consists of photons that have more energy than photons of low frequency, long wavelength light. The explanation for what we see in this demonstration is that when an electron absorbs a photon with sufficient energy, it can escape from the metal, leaving the metal sheet and electroscope post and foil leaves with a positive charge.