®
Model No. AP-8209
Photoelectric Effect Apparatus
6
Principle of the Experiment
When incident light shines on the cathode (K), photoelectrons can be
emitted and transferred to the anode (A). This constitutes a
photocurrent. By changing the voltage between the anode and cathode,
and measuring the photocurrent, you can determine the characteristic
current-voltage curves of the photoelectric tube.
The basic facts of the photoelectric effect experiments are as follows:
• For a given frequency (color) of light, if the voltage between the cathode
and anode,
V
AK
, is equal to the stopping potential,
V
, the photocurrent is
zero.
• When the voltage between the cathode and anode is greater than the
stopping voltage, the photocurrent will increase quickly and eventually
reach saturation. The saturated current is proportional to the intensity of
the incident light. See Figure 2.
• Light of different frequencies (colors) have different stopping potentials.
See Figure 3
• The slope of a plot of stopping potential versus frequency is the value of
the ratio, h/e. See Figure 1.
• The photoelectric effect is almost instantaneous. Once the light shines on
the cathode, photoelectrons will be emitted in less than a nanosecond.
Basic Setup
Install the Mercury Lamp in the Mercury Light Source
Enclosure
• Use a phillips head screwdriver to remove the four small screws that hold the back
plate onto the Mercury Light Source enclosure.
• Use a small flat-blade screwdriver to pry the back panel off of the enclosure.
Note
: Do not touch the glass envelope of the mercury lamp. Oil and moisture
from the skin may diminish the lamp’s performance. Use gloves, a clean cloth, or
a paper towel to handle the mercury lamp.
• Screw the mercury lamp into the socket inside the enclosure.
• Replace the back panel and screws on the enclosure.
V
A
A
K
Anode
Cathode
Ammeter
Voltmeter
V
AK
I
V
AK
Intensity 1
Intensity 2
Stopping
Potential
Figure 2: Current vs. Intensity
I
V
AK
Frequency 2
Frequency 1
V
stop
1
V
stop
2
Figure 3: Current vs. Frequency