1. YOGESH PANDEY
CLASS XII C
DAYAWATI MODI ACADEMY

2. DUAL NATURE OF RADIATION
AND MATTER

3.  Wave–particle duality postulates that
all particles exhibit both wave and particle properties.
A central concept of quantum mechanics, this duality
addresses the inability of classical concepts like
"particle" and "wave" to fully describe the behavior of
quantum-scale objects. Standard interpretations of
quantum mechanics explain this paradox as a
fundamental property of the Universe, while
alternative interpretations explain the duality as an
emergent, second-order consequence of various
limitations of the observer. This treatment focuses on
explaining the behavior from the perspective of the
widely used Copenhagen interpretation, in which
wave–particle duality is one aspect of the concept
of complementarity, that a phenomenon can be
viewed in one way or in another, but not both

4.  The idea of duality originated in a debate over the
nature of light and matter that dates back to the 17th
century, when competing theories of light were
proposed by Christiaan Huygens and Isaac Newton:
light was thought either to consist of waves (Huygens)
or of particles (Newton). Through the work of Max
Planck, Albert Einstein, Louis de Broglie,Arthur
Compton, Niels Bohr, and many others, current
scientific theory holds that all particles also have a
wave nature (and vice versa).[1] This phenomenon
has been verified not only for elementary particles,
but also for compound particles like atoms and even
molecules. For macroscopic particles, because of
their extremely small wavelengths, wave properties
usually cannot be detected

5.  Aristotle was one of the first to publicly hypothesize about the nature of light,
proposing that light is a disturbance in the element air (that is, it is a wave-like
phenomenon). On the other hand, Democritus—the original atomist—argued that
all things in the universe, including light, are composed of indivisible sub-
components (light being some form of solar atom).[3] At the beginning of the 11th
Century, the Arabic scientist Alhazen wrote the first comprehensive treatise on
optics; describing refraction, reflection, and the operation of a pinhole lens via
rays of light traveling from the point of emission to the eye. He asserted that
these rays were composed of particles of light. In 1630, René
Descartes popularized and accredited in the West the opposing wave description
in his treatise on light, showing that the behavior of light could be re-created by
modeling wave-like disturbances in a universal medium ("plenum"). Beginning in
1670 and progressing over three decades, Isaac Newton developed and
championed his corpuscular hypothesis, arguing that the perfectly straight lines
of reflection demonstrated light's particle nature; only particles could travel in
such straight lines. He explained refraction by positing that particles of light
accelerated laterally upon entering a denser medium. Around the same time,
Newton's contemporaries Robert Hooke and Christian Huygens—and
later Augustin-Jean Fresnel—mathematically refined the wave viewpoint,
showing that if light traveled at different speeds in different media (such as water
and air), refraction could be easily explained as the medium-dependent
propagation of light waves. The resulting Huygens–Fresnel principle was
extremely successful at reproducing light's behavior and, subsequently supported
byThomas Young's discovery of double-slit interference, was the beginning of the
end for the particle light camp

6.  ELECTRON EMISSION
 DIFFERENT METHODS OF ELECTRON
EMISSION
 PHOTOELECTRIC EFFECT
 EXPERIMENTAL STUDY OF PHOTOELECTRIC
EFFECT
 EINSTEIN’S PHOTOELECTRIC EFFECT OR
ENERGY QUANTUM OF RADIATION
 PARTICLE NATURE OF LIGHT
 DAVISSON AND GERMAR EXPERIMENT

7. 
The liberation of electrons from the surface of a metal is
known as Electron Emission. If a piece of metal is
investigated at room temperature, the random motion of
the electrons will be shown in Fig. However, these
electrons are free to the extent that they may transfer from
one atom to another within the metal but they cannot leave
the metal surface to provide electron mission. It is because
the free electrons that start at the surface of metal find
behind them positive nuclei pulling them back and none
pulling forward. Thus at the surface of the metal , a free
electron encounters forces that prevent it to leave the
metal. In other words, the metallic surface offer a barrier to
free electrons, their kinetic energy increases and is known
as surface barrier.
 However, if sufficient energy is given to the free
electrons, their kinetic energy increases and thus the
electrons will cross over the surface barrier to leave the
metal.

8. 
Work function (W0): The minimum energy required by an
electron to just escape (i.e. with zero velocity) from metal's
surface is called Work function (W0) of the metal. The work
function of pure metals varies (roughly) from 2eV to 6eV. Its
value depends upon the nature of the metal, its purity and the
conditions of the surface. We selected those metals for electron
emission which have low work function.

9.  The electron emission from the surface of a metal is possible only if
sufficient addition energy (equal to work function of the sources such as
heat energy, energy stored in electric field, light energy or kinetic energy
of the electric charges bombarding the metal surface. Accordingly; there
are following four principal method of obtaining electron emission from
 (I) Thermionic emission: In this method, the metal is heated to a
sufficient temperature (about 2500oC) to enable the free electrons to
leave the metal surface. The number of electrons emitted depends upon
the temperature. The higher the temperature, the greater is the emission
of electrons. This type of emission is employed in vacuu
 (II) Field emission: In this method, a strong electric field (i.e. a high
positive voltage) is applied at the metal surface which pulls the free
electrons out of the metal because of the attraction of positive field. The
strong the electric field, the greater is the electron emission.m tubes.the
surface of a metal:

10.  (III) Photoelectric emission: In this method, the
energy of light falling upon the metal surface is
transferred to the free electrons within the metal to
enable them to leave the surface. The greater the
intensity of light beam falling on the metal surface, the
greater is the photoelectric emission. Photoelectric
emission is utilized in photo tubes which from the
basis of television and sound films.
 (IV) Secondary emission: In this method, a high
velocity beam of electrons or other out. The intensity
of secondary emission depends upon the emitter
material, mass and energy of bombarding particles.m
the basis of television and sound films.

11.  In the photoelectric effect, electrons are emitted from matter (metals
and non-metallic solids, liquids or gases) as a consequence of their
absorption of energy from electromagnetic radiation of very
short wavelength and high frequency, such as ultraviolet radiation.
Electrons emitted in this manner may be referred to as
photoelectrons.[1][2] First observed by Heinrich Hertz in 1887,[] the
phenomenon is also known as the Hertz effect,[ although the latter term
has fallen out of general use. Hertz observed and then showed
that electrodes illuminated with ultraviolet light create electric
sparks more easily.
 The photoelectric effect requires photons with energies from a
few electronvolts to over 1 MeV in high atomic number elements. Study
of the photoelectric effect led to important steps in understanding the
quantum nature of light and electrons and influenced the formation of
the concept of wave–particle duality.[1] Other phenomena where light
affects the movement of electric charges include the photoconductive
effect (also known as photoconductivity or photoresistivity),
the photovoltaic effect, and the photoelectrochemical effect. It also led
to Max Planck's discovery of quanta (e=hv) which links frequency with
photon energy. Quanta is also known asPlanck constant.

12.  Emission mechanism
 The photons of a light beam have a characteristic energy proportional to
the frequency of the light. In the photoemission process, if an electron
within some material absorbs the energy of one photon and acquires
more energy than the work function (the electron binding energy) of the
material, it is ejected. If the photon energy is too low, the electron is
unable to escape the material. Increasing the intensity of the light beam
increases the number of photons in the light beam, and thus increases
the number of electrons excited, but does not increase the energy that
each electron possesses. The energy of the emitted electrons does not
depend on the intensity of the incoming light, but only on the energy or
frequency of the individual photons. It is an interaction between the
incident photon and the outermost electron.
 Electrons can absorb energy from photons when irradiated, but they
usually follow an "all or nothing" principle. All of the energy from one
photon must be absorbed and used to liberate one electron from atomic
binding, or else the energy is re-emitted. If the photon energy is
absorbed, some of the energy liberates the electron from the atom, and
the rest contributes to the electron's kinetic energy as a free particle

13.  The theory of the photoelectric effect must explain the experimental observations
of the emission of electrons from an illuminated metal surface.
 For a given metal, there exists a certain minimum frequency of incident radiation
below which no photoelectrons are emitted. This frequency is called the threshold
frequency. Increasing the frequency of the incident beam, keeping the number of
incident photons fixed (this would result in a proportionate increase in energy)
increases the maximum kinetic energy of the photoelectrons emitted. Thus the
stopping voltage increases. The number of electrons also changes because the
probability that each photon results in an emitted electron is a function of photon
energy. If the intensity of the incident radiation is increased, there is no effect on
the kinetic energies of the photoelectrons.
 Above the threshold frequency, the maximum kinetic energy of the emitted
photoelectron depends on the frequency of the incident light, but is independent
of the intensity of the incident light so long as the latter is not too high [9]
 For a given metal and frequency of incident radiation, the rate at which
photoelectrons are ejected is directly proportional to the intensity of the incident
light. Increase in intensity of incident beam (keeping the frequency fixed)
increases the magnitude of the photoelectric current, though stopping voltage
remains the same.
 The time lag between the incidence of radiation and the emission of a
photoelectron is very small, less than 10−9 second.
 The direction of distribution of emitted electrons peaks in the direction of
polarization (the direction of the electric field) of the incident light, if it is linearly
polarized

14.  Stopping potential
 The relation between current and applied voltage illustrates the nature of the
photoelectric effect. For discussion, a light source illuminates a plate P, and
another plate electrode Q collects any emitted electrons. We vary the potential
between P and Q and measure the current flowing in the external circuit between
the two plates.
 If the frequency and the intensity of the incident radiation are fixed, the
photoelectric current increases gradually with an increase in positive potential on
collector electrode until all the photoelectrons emitted are collected. The
photoelectric current attains a saturation value and does not increase further for
any increase in the positive potential. The saturation current depends on the
intensity of illumination, but not its wavelength.
 If we apply a negative potential to plate Q with respect to plate P and gradually
increase it, the photoelectric current decreases until it is zero, at a certain
negative potential on plate Q. The minimum negative potential given to plate Q at
which the photoelectric current becomes zero is called stopping potential or cut
off potential.[13]
 i. For the given frequency of incident radiation, the stopping potential is
independent of its intensity.
 ii. For a given frequency of the incident radiation, the stopping potential Vo is
related to the maximum kinetic energy of the photoelectron that is just stopped
from reaching plate Q. If is the mass and is the maximum velocity of
photoelectron emitted

18. 
EINSTEIN'S PHOTOELECTRIC EQUATION
 According to Plank's quantum theory, light is emitted from a
source in the forms of bundles of energy called photons. Energy
of each photon is .
Einstein made use of this theory to explain how photo electric
emission takes place. According to Einstein, when photons of
energy fall on a metal surface, they transfer their energy to the
electrons of metal. When the energy of photon is larger than the
minimum energy required by the electrons to leave the metal
surface, the emission of electrons take place instantaneously.
He proposed that an electron absorbs one whole photon or
none. The chance that an electron may absorb more then one
electron is negligible because the number of photons is much
lower than the electron. After absorbing the photon, an electron
either leaves the surface or dissipates its energy within the metal
in such a short interval that it has almost no chance to absorb
second photon. An increase in intensity of light source simply
increases the number of photon and the number of photo
electrons but no increase in the energy of photo electron.
However, increase in frequency increases the energy of photons
and photo electrons.

19.  Light as a particle
 The only thing that interferes with my learning is my education. -- Albert
Einstein
 Radioactivity is random, but do the laws of physics exhibit randomness
in other contexts besides radioactivity? Yes. Radioactive decay was just
a good playpen to get us started with concepts of randomness, because
all atoms of a given isotope are identical. By stocking the playpen with
an unlimited supply of identical atom-toys, nature helped us to realize
that their future behavior could be different regardless of their original
identicality. We are now ready to leave the playpen, and see how
randomness fits into the structure of physics at the most fundamental
level.
 The laws of physics describe light and matter, and the quantum
revolution rewrote both descriptions. Radioactivity was a good example
of matter's behaving in a way that was inconsistent with classical
physics, but if we want to get under the hood and understand how
nonclassical things happen, it will be easier to focus on light rather than
matter. A radioactive atom such as uranium-235 is after all an extremely
complex system, consisting of 92 protons, 143 neutrons, and 92
electrons. Light, however, can be a simple sine wave.
 However successful the classical wave theory of light had been ---
allowing

20. The Davisson–Germer experiment was a physics experiment conducted
by American physicists Clinton Davisson and Lester Germer in 1927,
which confirmed the de Broglie hypothesis. This hypothesis advanced
by Louis de Broglie in 1924 says that particles of matter such as
electrons have wave like properties. The experiment not only played a
major role in verifying the de Broglie hypothesis and demonstrated the
wave-particle duality, but also was an important historical development
in the establishment of quantum mechanics and of the Schrödinger
equation

21.  Davisson and Germer's actual objective was to study the surface of a piece of nickel by
directing a beam of electrons at the surface and observing how many electrons bounced off
at various angles. They expected that for electrons even the smoothest crystal surface would
be too rough and so the electron beam would experience diffuse reflection.[5]
 The experiment consisted of firing an electron beam from an electron gun directed to a piece
of nickel crystal at normal incidence (i.e. perpendicular to the surface of the crystal). The
experiment included an electron gun consisting of a heated filament that released thermally
excited electrons, which were then accelerated through a potential difference giving them a
certain amount of kinetic energy towards the nickel crystal. To avoid collisions of the
electrons with other molecules on their way towards the surface, the experiment was
conducted in a vacuum chamber. To measure the number of electrons that were scattered at
different angles, an electron detector that could be moved on an arc path about the crystal
was used. The detector was designed to accept only elastically scattered electrons.
 During the experiment an accident occurred and air entered the chamber, producing an oxide
film on the nickel surface. To remove the oxide, Davisson and Germer heated the specimen
in a high temperature oven, not knowing that this affected the formerly polycrystalline
structure of the nickel to form large single crystal areas with crystal planes continuous over
the width of the electron beam.[5]
 When they started the experiment again and the electrons hit the surface, they were
scattered by atoms which originated from crystal planes inside the nickel crystal. As Max von
Laue proved in 1912 the crystal structure serves as a type of three dimensional diffraction
grating. The angles of maxim