Photoelectric Effect

light quantization - color/energy relation - work function

What it shows:
A direct observation that the photoelectric effect is color (ie. frequency) dependant and not intensity dependant. We discharge an electroscope using UV radiation after all attempts to discharge it under long wavelength light has failed.

How it works:
A negative charge is put onto a Braun electroscope (figure 1) fitted with a thick zinc plate using an ebonite rod and fur; the deviation of the arm from the vertical being an indication of the charge. Next we hit it with light from a 1000W filament lamp whose radiation is primarily towards the red end of the spectrum; it has no effect on the electroscope despite its intensity. Then approach the electroscope with a small UV source; the electroscope discharges.

figure 1. Braun Electroscope

Setting it up:
Prepare the Braun electroscope by making sure the zinc is clean; give it a quick polish with steel wool. The arm needs to be correctly balanced so that it rests vertically but will swing about 30° when charged. Connect the bulb through a variac. The UV source is a small dental style Pen-ray UV source (UV Products, San Gabriel, CA). The zinc plate purchased after great searching from a naval/shipping supply, has to have one face very smooth and clean. The charge is provided by an ebonite rod and fur; make sure this is dry (use a heat lamp).

Comments:
The 1000W bulb is BRIGHT and the UV source tiny - a graphic illustration that the photoelectric effect is intensity independent. Show also that the electroscope won’t discharge when a positive charge (supplied by a glass rod and silk) is placed on it. Rating ***

References:
1. Physics Demonstration Experiments ed. H. F. Meiners (AAPT 1970) vol.II p.836