CAT NO: DIDACTICNPS0021
PHOTOELECTRIC EFFECT This equipment enables students to repeat the experiment for determining the quantum theory of radiation. It can
demonstrate that the energy of a photoelectron depends on
the wavelength of impact radiation, not on its intensity.
In this experiment the photocathode is irradiated by a source
of monochromatic radiation, and a voltage applied to tube will
oppose the energy emitted by photoelectrons. The voltage
required to stop the current flow is proportional to the energy
Plotting the voltage enabling to stop the current flow, versus
the reciprocal of radiation wavelength will lead to a straight line
whose slope can be used to calculate Planck’s constant.
Measuring very faint photocurrents will lead to accurate results.
Consequently the amplifier will be positioned at a distance of
only few centimetres from photodiode.
This will avoid the introduction of foreign voltages and the
lowest detectable photocurrent will be equal to approximately:
5 x 10-10 A.
This apparatus includes three filters that allow spectral
separation. The use of a small source of monochromatic
mercury light is recommended. A tungsten lamp can also be
used although the results are not so good as those coming
from monochromatic source.
• Phototube : mounted inside the amplifier
• Amplifier: it consists of a FET Op Amp input followed by a
second operational amplifier driving the measuring panel ;
internal power supply units
• Amplifier gain: 3 x 107
• minimum current sensitivity: 5 x 1010 (spectral)
• Separation ensured by the red, green and blue filters
• The best results can be obtained with monochromatic
sources such as mercury arc lamps and He-Ne laser
• Dimensions : 9 x 171 x 10 (h) cm
The low-voltage mercury arc lamp of 7 W is a perfect source of
monochromatic radiation for photoelectric effect. It is provided
with its own power supply and is mounted on a desk-type
support. It also includes another support for keeping the lamp
in its position when it is used with photoelectric effect. Its
envelope is of quartz and it is provided with a shield for stray