The Maxwell’s equations of electromagnetism and Hertz experiments on
the generation and detection of electromagnetic waves in 1887 strongly
established the wave nature of light. Towards the same period at the end
of 19th century, experimental investigations on conduction of electricity
(electric discharge) through gases at low pressure in a discharge tube led
to many historic discoveries. The discovery of X-rays by Roentgen in 1895,
and of electron by J. J. Thomson in 1897, were important milestones in
the understanding of atomic structure. It was found that at sufficiently
low pressure of about 0.001 mm of mercury column, a discharge took
place between the two electrodes on applying the electric field to the gas
in the discharge tube. A fluorescent glow appeared on the glass opposite
to cathode. The colour of glow of the glass depended on the type of glass,
it being yellowish-green for soda glass. The cause of this fluorescence
was attributed to the radiation which appeared to be coming from the
cathode. These cathode rays were discovered, in 1870, by William
Crookes who later, in 1879, suggested that these rays consisted of streams
of fast moving negatively charged particles. The British physicist
J. J. Thomson (1856-1940) confirmed this hypothesis. By applying
mutually perpendicular electric and magnetic fields across the discharge
tube, J. J. Thomson was the first to determine experimentally the speed
and the specific charge [charge to mass ratio (e/m)] of the cathode ray
Chapter Eleven
Dual Nature of Radiation
and Matter
particles. They were found to travel with speeds ranging from about 0.1
to 0.2 times the speed of light (3 ×10
m/s). The presently accepted value
of e/m is 1.76 × 10
C/kg. Further, the value of e/m was found to be
independent of the nature of the material/metal used as the cathode
(emitter), or the gas introduced in the discharge tube. This observation
suggested the universality of the cathode ray particles.
Around the same time, in 1887, it was found that certain metals, when
irradiated by ultraviolet light, emitted negatively charged particles having
small speeds. Also, certain metals when heated to a high temperature were
found to emit negatively charged particles. The value of e/m of these particles
was found to be the same as that for cathode ray particles. These
observations thus established that all these particles, although produced
under different conditions, were identical in nature. J. J. Thomson, in 1897,
named these particles as electrons, and suggested that they were
fundamental, universal constituents of matter. For his epoch-making
discovery of electron, through his theoretical and experimental
investigations on conduction of electricity by gasses, he was awarded the
Nobel Prize in Physics in 1906. In 1913, the American physicist R. A.
Millikan (1868-1953) performed the pioneering oil-drop experiment for
the precise measurement of the charge on an electron. He found that the
charge on an oil-droplet was always an integral multiple of an elementary
charge, 1.602 × 10
C. Millikan’s experiment established that electric
charge is quantised. From the values of charge (e) and specific charge
(e/m), the mass (m) of the electron could be determined.
We know that metals have free electrons (negatively charged particles) that
are responsible for their conductivity. However, the free electrons cannot
normally escape out of the metal surface. If an electron attempts to come
out of the metal, the metal surface acquires a positive charge and pulls the
electron back to the metal. The free electron is thus held inside the metal
surface by the attractive forces of the ions. Consequently, the electron can
come out of the metal surface only if it has got sufficient energy to overcome
the attractive pull. A certain minimum amount of energy is required to be
given to an electron to pull it out from the surface of the metal. This
minimum energy required by an electron to escape from the metal surface
is called the work function of the metal. It is generally denoted by
measured in eV (electron volt). One electron volt is the energy gained by an
electron when it has been accelerated by a potential difference of 1 volt, so
that 1 eV = 1.602 ×10
This unit of energy is commonly used in atomic and nuclear physics.
The work function (
) depends on the properties of the metal and the
nature of its surface. The values of work function of some metals are
given in Table 11.1. These values are approximate as they are very
sensitive to surface impurities.
Note from Table 11.1 that the work function of platinum is the highest
= 5.65 eV) while it is the lowest (
= 2.14 eV) for caesium.
The minimum energy required for the electron emission from the metal
surface can be supplied to the free electrons by any one of the following
physical processes:
(i) Thermionic emission: By suitably heating, sufficient thermal energy
can be imparted to the free electrons to enable them to come out of the
(ii) Field emission: By applying a very strong electric field (of the order of
V m
) to a metal, electrons can be pulled out of the metal, as in a
spark plug.
(iii) Photoelectric emission: When light of suitable frequency illuminates
a metal surface, electrons are emitted from the metal surface. These
photo(light)-generated electrons are called photoelectrons.
11.3.1 Hertz’s observations
The phenomenon of photoelectric emission was discovered in 1887 by
Heinrich Hertz (1857-1894), during his electromagnetic wave experiments.
In his experimental investigation on the production of electromagnetic
waves by means of a spark discharge, Hertz observed that high voltage
sparks across the detector loop were enhanced when the emitter plate
was illuminated by ultraviolet light from an arc lamp.
Light shining on the metal surface somehow facilitated the escape of
free, charged particles which we now know as electrons. When light falls
on a metal surface, some electrons near the surface absorb enough energy
from the incident radiation to overcome the attraction of the positive ions
in the material of the surface. After gaining sufficient energy from the
incident light, the electrons escape from the surface of the metal into the
surrounding space.
11.3.2 Hallwachs’ and Lenard’s observations
Wilhelm Hallwachs and Philipp Lenard investigated the phenomenon of
photoelectric emission in detail during 1886-1902.
Lenard (1862-1947) observed that when ultraviolet radiations were
allowed to fall on the emitter plate of an evacuated glass tube enclosing
two electrodes (metal plates), current flows in the circuit (Fig. 11.1). As
soon as the ultraviolet radiations were stopped, the current flow also
Metal Work function Metal Work function
Cs 2.14 Al 4.28
K 2.30 Hg 4.49
Na 2.75 Cu 4.65
Ca 3.20 Ag 4.70
Mo 4.17 Ni 5.15
Pb 4.25 Pt 5.65
Dual Nature of Radiation
and Matter
stopped. These observations indicate that when ultraviolet radiations fall
on the emitter plate C, electrons are ejected from it which are attracted
towards the positive, collector plate A by the electric field. The electrons
flow through the evacuated glass tube, resulting in the current flow. Thus,
light falling on the surface of the emitter causes current in the external
circuit. Hallwachs and Lenard studied how this photo current varied with
collector plate poten