BS Physics (Foundation) Series

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Module # 43
Dual Nature of Light & Polarization of Light Waves
Dual Nature of Light
The study of properties and nature of light has been an active
field of research since Newton's time and even before. As regards
the nature of light, there were two "schools of thoughts" active
from the beginning of the seventeenth century. One was that of
Newton who proposed the corpuscular nature of light and the
other was that of Huygens who believed that light was a wave of
some sort.
Most of the scientists accepted Newton's corpuscular nature of
light as it could successfully explain experimental facts such as
reflection and refraction concerning the properties of light.
However, Huygens showed that reflection and refraction could be
explained assuming the wave nature of light. Thus, the theory did
not receive immediate attention and acceptance for several
reasons. All the waves known at that time (sound waves, water
waves etc.) travelled through some medium, while; light could
travel to us from the Sun through the vacuum. Furthermore, it was
argued that if light were some form of wave motion, then, the
waves could bend around obstacles, and we should be able to

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see around corners. It is now known that the bending of light
around the edges of an object does indeed occur.
According to corpuscular theory of light, the light travels with
greater speed in glass or in liquids than in air which is against the
observed speed of light. Additional developments during the
nineteenth century led to the general acceptance of the wave
nature of light.
In 1890 (at the end of classical physics period 1700-1890)
physicists thought that the wave theory of light had solved all the
problems pertaining to light. However, in the beginning of 20th
century, experimental phenomena, such as (1) the black body
radiation, (2) the photoelectric effect, (3) Compton scattering and
other were discussed which could not be explained by the wave
theory.
Presently, we hold the view that light possesses dual nature. On
one hand, the classical theory is adequate to explain propagation
of light interference and diffraction phenomena. While, on the
other hand, the phenomena like photo electric effect, Compton
effect, etc. involving the interaction of light with matter are best
explained by quantum theory.
Thus, we can conclude that the interference, diffraction and
polarization of light can only be explained by wave theory of light

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i.e. in these phenomena, light shows wave nature, while,
photoelectric effect and Compton effect can only be explained by
quantum theory of light i.e. in these phenomena, light behaves
like particles.
So, we have seen that in some phenomena light shows wave
nature, and, in some other phenomena, it behaves like particles.
Therefore, we can say that light has (or possesses) dual nature.
It should be noted here that it never behaves as both at the same
time, but, sometimes behaves as particles and sometimes as
waves depending on how we look at it.
Maxwell's Electromagnetic Theory of Light
According to electromagnetic theory, a light wave consists of a
periodic vibration of electric field vector accompanied by the
magnetic field vector at right angles to each other.
In 1865, James Clerk Maxwell showed that light waves are
electromagnetic in nature. These waves have high frequency and
shorter wavelength. They consist of an oscillating electric field
and an oscillating magnetic field; both are perpendicular to each
other and have the same frequency and phase.

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The above Figure illustrates the electric and magnetic fields
distribution at a particular instant. Notice that both E and H are
mutually perpendicular to each other and also perpendicular to
the direction of propagation of the waves which is along the x-axis
when the electric field E is shown in the vertical plane and the
magnetic field H is shown in the horizontal plane. The various
optical phenomena such as refraction, polarization, interference
and diffraction can be explained by the electromagnetic theory.
Salient Features of Electromagnetic Theory
(1) Light consists of electromagnetic waves.
(2) A hypothetical medium ether helps the light waves to
propagate from one place to another.
(3) The ether lies everywhere in the universe, therefore, light
waves travel through vacuum.
(4) The phenomena like interference, diffraction, polarization,
reflection and refraction of light etc. are obeyed by
electromagnetic waves.
X

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Thus, according to electromagnetic theory, light and radio waves
are the same in nature i.e., electric and magnetic field mutually
perpendicular to each other travelling at a speed of 3 x 108
m/s
through space. Only the frequencies are different.
Later on, it was proved experimentally by Michelson and Morley
that there is nothing existing like ether. Electromagnetic waves
require no material medium for their propagation.
In 1887, Hertz confirmed experimentally Maxwell's
electromagnetic wave theory of light.
The electromagnetic wave theory, however, failed to account for
the phenomenon of photoelectric emission (the emission of
electrons from a metal whose surface is exposed to light). This
difficulty was overcome by Albert Einstein who in 1905 suggested
that light is carried from one place to another in wave packets
called quantum or photons, each having a definite energy and
momentum.
Corpuscular Theory of Light
According to Newton (the chief architect of corpuscular theory),
light consists of stream of tiny (or minute) particles (corpuscles)
which on emission from the source of light (i.e. a luminous body)
travel along straight line with great speed. When these particles
enter the eye, they create sensation of sight on interacting with

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the retina of eye. This is known as the Corpuscular Theory of
Light. This theory explains the linear motion of light and
phenomenon of reflection and refraction. According to this theory,
it was (not being a universal truth) assumed that velocity of light in
denser media (like water, glass etc.) was greater than that in air.
This theory could not explain the properties of light like
interference, diffraction and polarization.
Salient Features of Corpuscular Theory
(1) Light consists of tiny (or minute) particles.
(2) These minute particles of light travel in a straight line with
great speed.
(3) Light particles create the sensation of sight on interacting
with the retina of eye.
(4) This theory explains the formation of shadows and
propagation along a straight path.
(5) It also explains that reflection of light particles from a surface
takes place in the same manner as the reflection of a rubber ball
from a hard surface.
(6) According to Newton, the velocity of light in a denser
medium like water will be greater than that in air. French physicist

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Foucault, in 1950, showed that the last assumption was wrong.
Newton's theory was, therefore, abandoned.
Huygen's Principle
It has two parts:
(1) Every point on a wave front can be considered as a source
of secondary spherical wave front.
(2) The new position of the wave front after a time t can be
found by drawing a plane tangential to the secondary wavelets.
OR
Every point on a wave front may be considered as a source that
produces secondary wavelets. The wavelets propagate in the
forward direction with a speed equal to the speed of the wave
motion.
Huygen’s Wave Theory of Light
In 1678, a Dutch physicist and astronomer Christian Huygens, a
contemporary (of the same age or period) of Newton, proposed a
new theory of light known as Wave Theory of Light in opposition
to (or contrast with) Newton’s Corpuscular Theory of Light.
According to this theory, light travels in space in the form of
waves from the source of light (i.e. a luminous body). The
phenomena of reflection and refraction can also be explained on

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the basis of wave theory of light. Newton did not believe in the
wave theory of light. His main objection was that light waves are
not diffracted like sound waves. Although Francisco Grimaldi
discovered diffraction of light, yet, Newton (the chief architect of
corpuscular theory) did not accept this idea. This objection was
removed later on when the diffraction was observed by actual
experiment with small obstacles. Most of the Scientists did not
accept his corpuscular theory for more than a century. Some
scientists continued to believe in his theory due to Newton's great
influence as a Scientist.
In 1801, Thomas Young proved experimentally the phenomena of
interference of light on the basis of wave theory of light. Several
years later, a French Physicist August Fresnel, performed a
number of detailed experiments dealing with interference and
diffraction of light.
In 1850, Jean Foucault proved experimentally, on the basis of
wave theory, that light travels more slowly in glass or liquids (i.e.
denser medium) than in air (rarer medium). It means that velocity
of light in liquids is less than that in air. This experimental result
gave a decisive death blow to Newton's corpuscular theory.
Additional developments, during the nineteenth century, led to the
general acceptance of the wave nature of light.

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Salient Features of Wave Theory
(1) Light travels in the form of waves (i.e. train of waves).
(2) According to this theory, speed of light in optically denser
media like glass and water should be lower than the speed of light
in air.
(3) There should be a medium through which light waves can
move. Thus, for this purpose, the whole space in the universe
was assumed to be filled with hypothetical medium called ether.
This medium was considered to be necessary for the generation
and propagation of light.
Newton's towering personality did not allow Huygen’s theory to
gain acceptance for over one hundred years. But, in the later half
of 19th century, Huygen’s theory got wide acceptance.
Difference between Newton’s and Huygen’s Theory of Light
(1) According to Newton, light consists of minute (or tiny)
particles, while, according to Huygen, light consists of waves.
(2) Newton proposed that velocity of light in denser medium is
larger than that in rare medium, while, Huygen proposed that
velocity of light in denser medium is lower than that in rarer
medium.

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Emission of Light (Photons) by Atoms
All the matter is composed of small particles called atoms. The
atom consists of three particles i.e. Neutron, Proton and Electron.
According to Bohr, electrons move round the nucleus only in
certain discrete orbits without radiating energy. The energy of an
electron in each orbit is well defined. The orbit closest to the
nucleus corresponds to the lowest energy. When atoms are
excited, electrons jump from orbits of lower energy to orbits of
higher energy. However, they cannot remain in excited state for
long and when the electrons jump back to orbits corresponding to
lower energy, then, they emit energy in the form of photon.
That is, the return of excited atoms to their stable state results in
the emission of light whose frequency depends on the atom.
The energy possessed by this photon precisely equals the
difference of energy between the two orbits. The frequency of
photon is proportional to the energy difference. Thus, the
emission of photon is in accordance with the quantum theory. The
frequencies of radiation emitted together form a spectrum called
the bright line spectrum. Each atom of an element has its own
characteristic spectrum.
Monochromatic Light

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Light of a single frequency is called monochromatic light, for
example; yellow light obtained from a sodium lamp looks nearly
monochromatic, but, in fact, it is a mixture of two frequencies.
Plane Polarized Light
If all the vibrations of transverse waves are confined to a single
plane of vibration which contains the direction of propagation of
wave, then, the wave is said to be plane polarized.
OR
The beam of light in which all vibrations are confined to one plane
of vibration is called plane polarized light or polarization.
Explanation
Tourmaline and many other materials have their internal
molecular structure such that by their interactions, the electric
vibrations are confined in a particular plane and are executed
parallel to only one direction. Therefore, when light passes
through such a crystal, it becomes plane polarized.
When a beam of ordinary light falls on two tourmaline crystals (the
tourmaline crystals used here can be replaced by Polaroid disks)
with their crystallographic axes (analogous or similar to slits)
parallel, then, the beam is transmitted. If, however, one of the

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crystals is rotated with respect to the other, then, the intensity of
the emergent beam decreases (i.e. the emergent beam becomes
dimmer and dimmer) and ultimately the light is totally cut off when
the axes of the two crystals become perpendicular to each other
as shown in the figure below.
Fig: Light passing through a Polaroid sheet P is plane-polarized.
The second sheet of Polaroid A works as an analyzer. Light may
or may not pass through it, depending upon its orientation
whether it is parallel or perpendicular to the first crystal.
On rotating further, the light re-appears and becomes brightest
when the axes are again parallel.
It follows from the above discussion that when light passes
through the first tourmaline crystal, then, one component of the
wave is absorbed and the other component vibrating parallel to
the crystallographic axis is transmitted.
No Light

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The emerging beam differs from the incident light in the sense
that all the vibrations are in one plane. Such a beam of light is
called plane polarized light. If it falls on a second crystal,
vibrations can only pass if they are parallel to the transmission
direction of the crystal as shown in the figure above.
Polarization of Light Waves
Ordinary light (un-polarized) consists of large number of waves,
each wave has its own plane of vibration. The process of
confining the beam of light to one plane of vibration is called
polarization. The waves are transverse waves if the particles of
the medium vibrate perpendicular to the direction of motion of the
wave. The waves are longitudinal waves if the particles of the
medium vibrate parallel to the direction of motion of wave.
The experiments on interference and diffraction show clearly that
light is a form of wave motion. These effects, however, do not tell
us about the type of wave motion i.e. whether the light waves are
longitudinal or transverse. In connection with this settlement, the
phenomenon of polarization has helped to establish beyond doubt
that light waves are transverse waves. Malus and Brewster
discovered Polarization of Light.

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Polarizer
When we use polaroid disc to polarize light, it is called polarizer.
The polarizer is used to control head-light glare in the night driving
and to find the concentration of sugar in sugar solution.
Polaroid
A polaroid is a transparent plastic sheet in which special needle-
like crystals of heropathite have been embedded and oriented.
This sheet allows light to pass through it only if the electric vector
is vibrating in a specific direction (i.e. parallel to the axis of
transmission of the polaroid). Polaroid is used as a polarizer and
analyzer in liquid crystal displays (LCD) used in watches and
calculators, etc.
Applications of Polarized Light
There are several technical and scientific applications of
polarization of light.
(1) Determination of the concentration (or percentage) of
optically active substance such as sugar in blood, urine etc. is one
of these applications.
(2) Curtainless window is one of the simplest applications of
polaroids. In this case, an outer polarizing disc is fixed in position

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and an inner one may be rotated to adjust the amount of light to
be admitted through it into the room.
(3) It is used to control the head light glaze in night driving of the
motor cars. It is also used in making non-glaring spectacles.
(4) Suitably oriented polarising discs which serve as sky filters
are placed in front of the camera lens to enhance the effect of the
sky and clouds in photography.
(5) Plane polarized light is also used for checking the quality of
crystals.
Dispersion of Light
The separation of white light into its component colors by a prism
is called dispersion.
Dispersion
The phenomenon of spreading out or splitting up of light into its
constituent colors is known as dispersion of light.
When light passes from one medium to another, it deviates from
its original path. The angle of deviation of light depends upon
wave length of light.

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Experiment
The fact that sunlight consists of different colors was first
investigated by Newton with the help of following experiment.
Fig: Dispersion of Sunlight through Prism
A beam of sunlight was allowed to enter a dark room through a
hole in the window. A prism was placed in the path of light and a
band of different colors was observed on the wall. This band of
colors is known as spectrum and this phenomenon of spreading
out light into its constituent colors is known as dispersion of light.
We know that the amount of refraction of the waves depends on
their frequencies or wave lengths. This experiment shows that
sunlight is a combination of various waves. When they pass
through a glass prism, refraction takes place. The waves of higher
frequencies bend more than those of lower frequencies. Thus,
different waves with different frequencies and wave lengths in the
visible region would separate in the shape of a spectrum of

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colors. These colors are violet, indigo, blue, green, yellow, orange
and red. The shortest wave length visible to human eye is violet
(0.4 m) and its deviation is the greatest. The longest wave length
visible to human eye is red (0.7 m) and its deviation is the least.
When Newton placed a second prism in the path of waves coming
from the first prism; white light was obtained in place of a
spectrum. The second prism was placed in such a way that its
base was opposite to the base of the first prism that produced the
spectrum. This is because the second prism refracts light in the
opposite direction so the waves recombine to form white light.
Thus, in this way, Newton proved that white light was made up of
different colors.
Fig: Dispersion & Recombination of White Light