Summary
A zinc plate is charged with electrons using a plastic rod rubbed with fur. Longwave UV light does not have enough energy to discharge the electrons. Shortwave UV light discharges the electrons, demonstrating the photoelectric effect.
Hazards
UV light is damaging to tissue, use care with the lamp, protect skin and eyes.
Materials
- Two U.V. lamps
- Amber or plastic rod and fur
- Electroscope
- Zinc plate (freshly scoured)
- 220 grit or finer aluminum oxide sandpaper
- Optional: red 3M scouring pad, new
Procedure
- Scour the charge/discharge surface of the zinc plate with a new, clean piece of sandpaper, removing the dull layer of zinc oxide and exposing a fresh, shiny metal surface. Polish with the red scouring pad and alcohol. Avoid touching the freshly polished surface.
- Place the shiny zinc on the electroscope.
- Charge the electroscope with the plastic rod rubbed with fur. This may take several passes with the charged rod.
- Try to discharge the oscilliscope with a flashlight.
- Use the carbon arc lamp mounted above the scope to discharge. Keep it off to the side, so that ash from the lap does not fall on the zinc.
Hints: This demo doesn’t work consistently. A few tricks to help with your success:
- Scour and polish the zinc surface within a half hour or so of the demo.
- Keep finger oils off the surface (avoid touching the surface).
- Steel wool has an oil coating to protect it from rusting. For this reason, steel wool is not recommended as a scouring material for this demo.
- The UV lamp appears to need about 10 minutes to warm-up for the demo to work predictably.
Discussion
When light is shined on certain metals, electrons may be knocked off the metal. This is called the photoelectric effect. By assuming that light is quantized, Einstein was able to explain the photoelectric effect. For each metal there is a characteristic minimum frequency, the threshold frequency, below which the photoelectric effect does not occur. For example, a red light of any brightness can shine on potassium for hours and no photoelectrons will be released. There is not enough energy in red light to knock off electrons. As soon as yellow light shines on the potassium the photoelectric effect begins. Yellow light has a higher frequency and therefore it has higher energy such that electrons can escape from potassium metal. In this demonstration zinc is placed on an electroscope and then charged. When UV light of a long wavelength and low frequency is shined on the zinc nothing happens. There is not enough energy provided to discharge electrons. When the short wavelength, high frequency UV light is shined on the zinc, the electrons are discharged.