Biomedical Engineering Reference
In-Depth Information
8.2
Photoelectric Effect
The ejection of electrons from a surface as a result of light absorption is called the
photoelectric effect. The arrangement in Fig. 8.1 can be used to study this process
experimentally. Monochromatic light passes into an evacuated glass tube through
a quartz window (which allows ultraviolet light to be used) and strikes an electrode
1 causing photoelectrons to be ejected. Electrode 1 can be made of a metal to be
studied or have its surface covered with such a metal. The current
I
that flows
during illumination can be measured as a function of the variable potential dif-
ference
V
21
applied between the two electrodes, 1 and 2, of the tube. Curves (a)
and (b) represent data obtained at two different intensities of the incident light.
With the surface illuminated, there will be some current even with
V
21
= 0
.When
V
21
is made positive and increased, the efficiency of collecting photoelectrons at
electrode 2 increases; the current rises to a plateau when all of the electrons are be-
Fig. 8.1
Experiment on photoelectric effect. With electrode 1
illuminated with monochromatic light of constant intensity, the
current
I
is measured as a function of the potential difference
V
21
between electrodes 2 and 1. Curves (a) and (b) represent
data at two different intensities of the incident light.
