Modern Physics > Quantum Effects > Photoelectric Effect DCS# 7A10.30
PHOTOELECTRIC EFFECT

APPARATUS
 
pasco photodiode circuit box 214-03-A
mercury lamp 214-03-A
filter set 214-03-A
neutral density filter 214-03-A
digital multimeter 214-02-B6
laptop 206

DESCRIPTION Plot stopping voltage vs frequency for the four visible lines of mercury + one uv line.  Planck's constant can be obtained from the slope.  Use the neutral density filter to show that intensity doesn't affect the stopping potential.

 NOTES
The frequencies of the visible lines in the mercury spectrum are:
yellow
 5.187 x 1014 Hz
green
 5.490 x 1014 Hz
blue
 6.879 x 1014 Hz
violet
 7.409 x 1014 Hz
ultraviolet
 8.202 x 1014 Hz
The uv appears blue on the fluorescent white mask.

When light of sufficient energy shines on the photodiode, electrons are freed and move toward the cathode and the holes toward the anode.  Ideally the charge should build up until the photodiode voltage reaches the stopping potential Vs = hf, however charge leaks off through the finite impedance of the amplifier and the measured voltage levels off when the charging and discharging rates are equal.  The lower the intensity of the light, the more slowly the photodiode charges, and the smaller the steady state voltage.  Thus when the neutral density filters are used, the measured stopping potential will decrease somewhat with intensity.
 Press the zero button between readings to discharge the photodiode.
REFERENCES Pasco h/e apparatus instruction manual.

AJP 46, 133-135 (1978).