NCERT Textbook: Dual Nature of Wave & Radiation | Physics Class 12 - NEET PDF Download

 Page 1


Physics
274
11.1  INTRODUCTION
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
Chapter Eleven
DUAL NATURE OF
RADIATION AND
MATTER
2024-25
Page 2


Physics
274
11.1  INTRODUCTION
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
Chapter Eleven
DUAL NATURE OF
RADIATION AND
MATTER
2024-25
275
Dual Nature of Radiation
and Matter
and the specific charge [charge to mass ratio (e/m)] of the cathode ray
particles. They were found to travel with speeds ranging from about 0.1
to 0.2 times the speed of light (3 ×10
8 
m/s). The presently accepted value
of e/m is 1.76 × 10
11 
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
–19
 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.
11.2  ELECTRON EMISSION
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 f
0 
and
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
–19
 J.
This unit of energy is commonly used in atomic and nuclear physics.
The work function (f
0
)  depends on the properties of the metal and the
nature of its surface.
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
metal.
2024-25
Page 3


Physics
274
11.1  INTRODUCTION
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
Chapter Eleven
DUAL NATURE OF
RADIATION AND
MATTER
2024-25
275
Dual Nature of Radiation
and Matter
and the specific charge [charge to mass ratio (e/m)] of the cathode ray
particles. They were found to travel with speeds ranging from about 0.1
to 0.2 times the speed of light (3 ×10
8 
m/s). The presently accepted value
of e/m is 1.76 × 10
11 
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
–19
 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.
11.2  ELECTRON EMISSION
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 f
0 
and
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
–19
 J.
This unit of energy is commonly used in atomic and nuclear physics.
The work function (f
0
)  depends on the properties of the metal and the
nature of its surface.
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
metal.
2024-25
Physics
276
(ii) Field emission: By applying a very strong electric field (of the order of
10
8
 V m
–1
) 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  PHOTOELECTRIC EFFECT
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
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 potential, and with frequency and intensity of incident light.
Hallwachs, in 1888, undertook the study further and connected a
negatively charged zinc plate to an electroscope. He observed that the
zinc plate lost its charge when it was illuminated by ultraviolet light.
Further, the uncharged zinc plate became positively charged when it was
irradiated by ultraviolet light. Positive charge on a  positively charged
zinc plate was found to be further enhanced when it was illuminated by
ultraviolet light. From these observations he concluded that negatively
charged particles were emitted from the zinc plate under the action of
ultraviolet light.
After the discovery of the electron in 1897, it became evident that the
incident light causes electrons to be emitted from the emitter plate. Due
2024-25
Page 4


Physics
274
11.1  INTRODUCTION
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
Chapter Eleven
DUAL NATURE OF
RADIATION AND
MATTER
2024-25
275
Dual Nature of Radiation
and Matter
and the specific charge [charge to mass ratio (e/m)] of the cathode ray
particles. They were found to travel with speeds ranging from about 0.1
to 0.2 times the speed of light (3 ×10
8 
m/s). The presently accepted value
of e/m is 1.76 × 10
11 
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
–19
 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.
11.2  ELECTRON EMISSION
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 f
0 
and
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
–19
 J.
This unit of energy is commonly used in atomic and nuclear physics.
The work function (f
0
)  depends on the properties of the metal and the
nature of its surface.
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
metal.
2024-25
Physics
276
(ii) Field emission: By applying a very strong electric field (of the order of
10
8
 V m
–1
) 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  PHOTOELECTRIC EFFECT
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
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 potential, and with frequency and intensity of incident light.
Hallwachs, in 1888, undertook the study further and connected a
negatively charged zinc plate to an electroscope. He observed that the
zinc plate lost its charge when it was illuminated by ultraviolet light.
Further, the uncharged zinc plate became positively charged when it was
irradiated by ultraviolet light. Positive charge on a  positively charged
zinc plate was found to be further enhanced when it was illuminated by
ultraviolet light. From these observations he concluded that negatively
charged particles were emitted from the zinc plate under the action of
ultraviolet light.
After the discovery of the electron in 1897, it became evident that the
incident light causes electrons to be emitted from the emitter plate. Due
2024-25
277
Dual Nature of Radiation
and Matter
to negative charge, the emitted electrons are pushed towards the collector
plate by the electric field. Hallwachs and  Lenard also observed that when
ultraviolet light fell on the emitter plate, no electrons were emitted at all
when the frequency of the incident light was smaller than a certain
minimum value, called the threshold frequency. This minimum frequency
depends on the nature of the material of the emitter plate.
It was found that certain metals like zinc, cadmium, magnesium, etc.,
responded only to ultraviolet light, having short wavelength, to cause
electron emission from the surface. However, some alkali metals such as
lithium, sodium, potassium, caesium and rubidium were sensitive
even to visible light. All these photosensitive substances emit electrons
when they are illuminated by light. After the discovery of electrons, these
electrons were termed as photoelectrons. The phenomenon is called
photoelectric effect.
11.4 EXPERIMENTAL STUDY OF PHOTOELECTRIC
EFFECT
Figure 11.1 depicts a  schematic view of the arrangement used for the
experimental study of the photoelectric effect. It consists of an evacuated
glass/quartz tube having a thin photosensitive plate C and another metal
plate A. Monochromatic light from the source S of sufficiently short
wavelength passes through the window W and falls on the photosensitive
plate C  (emitter). A transparent quartz window is sealed on to the glass
tube, which permits ultraviolet radiation to pass through it and irradiate
the photosensitive plate C. The electrons are emitted by the plate C and
are collected by the plate A (collector), by the electric field created by the
battery. The battery maintains the potential difference between the plates
C and A, that can be varied. The polarity of the plates C and A can be
reversed by a commutator. Thus, the plate A can be maintained at a desired
positive or negative potential with respect to emitter C.
When the collector plate A is positive with respect to the
emitter plate C, the electrons are attracted to it. The
emission of electrons causes flow of electric current in
the circuit. The potential difference between the emitter
and collector plates is measured by a voltmeter (V)
whereas the resulting photo current flowing in the circuit
is measured by a microammeter (mA). The photoelectric
current can be increased or decreased by varying the
potential of collector plate A with respect to the emitter
plate C. The intensity and frequency of the incident light
can be varied, as can the potential difference V between
the emitter C and the collector A.
We can use the experimental arrangement of Fig.
11.1 to study the variation of photocurrent with (a)
intensity of radiation, (b) frequency of incident radiation,
(c) the potential difference between the plates A and C,
and (d) the nature of the material of plate C. Light of
different frequencies can be used by putting appropriate
coloured filter or coloured glass in the path of light falling
FIGURE 11.1 Experimental
arrangement for  study of
photoelectric effect.
2024-25
Page 5


Physics
274
11.1  INTRODUCTION
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
Chapter Eleven
DUAL NATURE OF
RADIATION AND
MATTER
2024-25
275
Dual Nature of Radiation
and Matter
and the specific charge [charge to mass ratio (e/m)] of the cathode ray
particles. They were found to travel with speeds ranging from about 0.1
to 0.2 times the speed of light (3 ×10
8 
m/s). The presently accepted value
of e/m is 1.76 × 10
11 
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
–19
 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.
11.2  ELECTRON EMISSION
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 f
0 
and
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
–19
 J.
This unit of energy is commonly used in atomic and nuclear physics.
The work function (f
0
)  depends on the properties of the metal and the
nature of its surface.
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
metal.
2024-25
Physics
276
(ii) Field emission: By applying a very strong electric field (of the order of
10
8
 V m
–1
) 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  PHOTOELECTRIC EFFECT
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
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 potential, and with frequency and intensity of incident light.
Hallwachs, in 1888, undertook the study further and connected a
negatively charged zinc plate to an electroscope. He observed that the
zinc plate lost its charge when it was illuminated by ultraviolet light.
Further, the uncharged zinc plate became positively charged when it was
irradiated by ultraviolet light. Positive charge on a  positively charged
zinc plate was found to be further enhanced when it was illuminated by
ultraviolet light. From these observations he concluded that negatively
charged particles were emitted from the zinc plate under the action of
ultraviolet light.
After the discovery of the electron in 1897, it became evident that the
incident light causes electrons to be emitted from the emitter plate. Due
2024-25
277
Dual Nature of Radiation
and Matter
to negative charge, the emitted electrons are pushed towards the collector
plate by the electric field. Hallwachs and  Lenard also observed that when
ultraviolet light fell on the emitter plate, no electrons were emitted at all
when the frequency of the incident light was smaller than a certain
minimum value, called the threshold frequency. This minimum frequency
depends on the nature of the material of the emitter plate.
It was found that certain metals like zinc, cadmium, magnesium, etc.,
responded only to ultraviolet light, having short wavelength, to cause
electron emission from the surface. However, some alkali metals such as
lithium, sodium, potassium, caesium and rubidium were sensitive
even to visible light. All these photosensitive substances emit electrons
when they are illuminated by light. After the discovery of electrons, these
electrons were termed as photoelectrons. The phenomenon is called
photoelectric effect.
11.4 EXPERIMENTAL STUDY OF PHOTOELECTRIC
EFFECT
Figure 11.1 depicts a  schematic view of the arrangement used for the
experimental study of the photoelectric effect. It consists of an evacuated
glass/quartz tube having a thin photosensitive plate C and another metal
plate A. Monochromatic light from the source S of sufficiently short
wavelength passes through the window W and falls on the photosensitive
plate C  (emitter). A transparent quartz window is sealed on to the glass
tube, which permits ultraviolet radiation to pass through it and irradiate
the photosensitive plate C. The electrons are emitted by the plate C and
are collected by the plate A (collector), by the electric field created by the
battery. The battery maintains the potential difference between the plates
C and A, that can be varied. The polarity of the plates C and A can be
reversed by a commutator. Thus, the plate A can be maintained at a desired
positive or negative potential with respect to emitter C.
When the collector plate A is positive with respect to the
emitter plate C, the electrons are attracted to it. The
emission of electrons causes flow of electric current in
the circuit. The potential difference between the emitter
and collector plates is measured by a voltmeter (V)
whereas the resulting photo current flowing in the circuit
is measured by a microammeter (mA). The photoelectric
current can be increased or decreased by varying the
potential of collector plate A with respect to the emitter
plate C. The intensity and frequency of the incident light
can be varied, as can the potential difference V between
the emitter C and the collector A.
We can use the experimental arrangement of Fig.
11.1 to study the variation of photocurrent with (a)
intensity of radiation, (b) frequency of incident radiation,
(c) the potential difference between the plates A and C,
and (d) the nature of the material of plate C. Light of
different frequencies can be used by putting appropriate
coloured filter or coloured glass in the path of light falling
FIGURE 11.1 Experimental
arrangement for  study of
photoelectric effect.
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on the emitter C. The intensity of light is varied by changing
the distance of the light source from the emitter.
11.4.1  Effect of intensity of light on photocurrent
The collector A is maintained at a positive potential with
respect to emitter C so that electrons ejected from C are
attracted towards collector A. Keeping the frequency of the
incident radiation and the potential fixed, the intensity of
light is varied and the resulting photoelectric current is
measured each time. It is found that the photocurrent
increases linearly with intensity of incident light as shown
graphically in Fig. 11.2. The photocurrent is directly
proportional to the number of photoelectrons emitted per
second. This implies that the number of photoelectrons
emitted per second is directly proportional to the intensity
of incident radiation.
11.4.2  Effect of potential on photoelectric current
We first keep the plate A at some positive potential with respect to the
plate C and illuminate the plate C with light of fixed frequency n  and fixed
intensity I
1
. We next vary the positive potential of plate A gradually and
measure the resulting photocurrent each time. It is found that the
photoelectric current increases with increase in positive (accelerating)
potential. At some stage, for a certain positive potential of plate A, all the
emitted electrons are  collected by the plate A and the photoelectric current
becomes maximum or  saturates. If we increase the accelerating potential
of plate A further, the photocurrent does not increase. This maximum
value of the photoelectric current is called  saturation current. Saturation
current corresponds to the case when all the photoelectrons emitted by
the emitter plate C reach the collector plate A.
We now apply a negative (retarding)
potential to the plate A with respect to the
plate C and make it increasingly negative
gradually. When the polarity is reversed,
the electrons are repelled and only the
sufficiently energetic electrons are able to
reach the collector A. The photocurrent
is found to decrease rapidly until it drops
to zero at a certain sharply defined,
critical value of the negative potential V
0
on the plate A. For a particular frequency
of incident radiation, the minimum
negative (retarding) potential V
0
 given to
the plate A for which the photocurrent
stops or becomes zero is called the cut-
off or stopping potential.
The interpretation of the observation
in terms of photoelectrons is
straightforward. All the photoelectrons
emitted from the metal do not have the
FIGURE 11.2 Variation of
Photoelectric current with
intensity of light.
FIGURE 11.3 Variation of  photocurrent with
collector plate potential for different
intensity of incident radiation.
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