E X P E R I M E N T I
Keele University Physics/Astrophysics Laboratory
School of Physical and Geographical Sciences Experimental Scripts
68
Photoelectric Effect
1. Introduction
It has been observed that sufficiently short wavelength light incident on certain metal surfaces
emit electrons. This effect is known as the photoelectric effect which led to the postulation that light
consists of a flux of particles called photons with energy of hf (f is the frequency of the light and h is
Plank’s constant) by Albert Einstein in 1905. Crucial observations of the photoelectric effect which led
to the above postulation are:
a) Number of electrons emitted was proportional to the intensity of the incident light;
b) There was no emission of electrons below a critical frequency of the light irrespective of the
intensity of the light or the exposure time.
Classical physics was able to explain observation (a) but failed to explain observation (b). However,
Albert Einstein’s postulation that light consist of photons was able to explain the photoelectric effect.
The maximum kinetic energy of the electron released from the metal surface by a photon with energy
of hf is given by:
hfmv2
max
2
1
(1)
where
is the work function of the metal, m is the mass of the electron and vmax is the maximum
velocity of the ejected electron.
Planck's constant h can be determined by exposing a photocell to monochromatic light and measuring
the negative potential (Vo ) required to halt the ejected electrons with the maximum kinetic energy
collected by the anode. i.e.
o
eVmv
2
max
2
1
A schematic diagram of the experimental set up is shown in figure 1.
Figure 1: A schematic representation of such an experiment. F - Monochromatic filter, A - annular
anode and K - potassium surface (From Leybold Physics Leaflets).
