METHOD :
Part A :
In this experiment, the light source is a mercury vapor lamp. Mercury vapor lamps have a
characteristic line spectrum with lines of the following wavelengths (only lines in the visible part
of the spectrum are given):
Color
yellow
green
blue
violet #1
violet #2
Light of one line (wavelength
Wavelength/nm (Pasco)
577.0
546.1
435.8
404.7
365.0
l ) of the spectrum will be directed on the cathode of a vacuum
tube. This means the cathode is bombarded with photons, each of which has an energy hv =
hc
l
When the energy of a photon hv is equal or greater than the work function w an electron will be
emitted from the cathode. w is the minimum energy needed to release an electron from the
metal of which the cathode is made. More photons will release more electrons with the same
energy.
When an electron is emitted it will reach the anode of the vacuum tube (if the anode is charged
positive relative to the cathode). The anode is called collector plate. When electrons reach the
anode a current I (called photocurrent, measured in ampere A) can be measured. The collector
plate (anode) can also be charged negative (or less positive relative to the cathode) by the
operator and the negative potential V can be measured. When the anode becomes less positive
(more negative) the current I will decrease. When the negative potential V is such that the
current I is just zero, no electrons reach the collector plate and this V is then called stopping
potential.
At this point the kinetic energy of the photoelectrons must be equal to V e:
Ek = V e
V = stopping potential (measured in volt, V)
(3)
; e = charge of one electron = - 1.602176 ×10
1J =1V ×C
-19
C
(4)
The stopping potential V will be measured for each of the lines in the Hg spectrum. This means
the operator will measure the maximum kinetic energy of the electrons emitted from the
cathode for each of the different frequencies of the mercury spectrum. With this the equation
for the photoelectric effect becomes:
Ek = Ve = hv - w
(5)
h
w
V = ve
e
(6)
This is the equation of a straight line :
y = mx + b
with:
y=V
and
x= v
The slope of the line is equal to h/e and the y-intercept is equal to
A2
–w/e