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INTERFERENCE FILTER
INTERFERENCE FILTER
• Arrangement for filtering
monochromatic beam from
incident white light is called
interference filter.
• When a narrow beam of
white light is incident on the
filter, it undergoes multiple
reflections in the dielectric, it
loses a major part of its
intensity and finally emerges
from the filter as a very
narrow band with one or two
maxima.
• By reducing the thickness,
corresponding wavelength
can be filtered.
ANTIREFLECTION COATING
• Antireflection coating is a type of optical
coating applied to the surface of lenses and other
optical elements to reduce reflection.
• The intensity of the transmitted beam gets
reduce after each reflection.
• Such loss of intensity can be reduced to a large
extend by using antireflective coating.
• Eg:- calcium fluoride, magnesium fluoride, etc.
• Refractive index of such material must
be in between that of air and glass.
ANTIREFLECTION COATING
• .
ANTIREFLECTION COATING
• When a white light is
incident on the film, a
part of it reflected from
the upper surface and the
lower surface.
• Phase diff. π occurs in
both cases.
• Choosing the thickness of
the film such a way that
the 2 beams produce
destructive interference.
ANTIREFLECTION COATING
• Different λ can be
eliminated by choosing
the thickness of the
film corresponding to
that λ.
• Usually yellow-green
region λ is select,
which is very much
sensitive to eye.
POLARIZATION
EM Waves
• Light is an electromagnetic
wave. EM waves are
transverse. Thus, the
electrical field can vibrate
in any direction
perpendicular to the
direction of propagation.
Most light sources
(candles, incandescent
light bulbs etc..) emit light
that is unpolarized – the
electric field has all
possible directions of
vibrations
POLARITY
• In the case of transverse
wave, a number of
directions a particle can
execute a periodic motion
perpendicular to the
direction of propagation.
• So two similar waves may
differ from each other
because of their different
directions of vibration.
• THE DEPENDANCE OF THE
PROPERTIES OF A WAVE
WITH THE DIRECTION OF
PROPAGATION is called
POLARITY.
Polarization
Polarization
• Wave having a characteristic of polarity is called
POLARIZED WAVE.
• Transforming unpolarized light into polarized
light.
• Restricting electric field vector E in a particular
plane so that the vibration occur in a single plane.
• Plane of vibration: Plane containing the direction
of propagation and the plane of electric field
vibration.
• Plane of polarization: Plane perpendicular to the
plane of vibration.
Polarization
Types of polarization
Types of polarization
• Linear polarization: Electric field vector oscillate
along straight line in one plane.
• Circular polarization: Circularly polarized light
consists of two perpendicular electromagnetic
plane waves of equal amplitude and 90°
difference in phase.
• Elliptically polarized: Elliptically polarized light
consists of two perpendicular waves of unequal
amplitude which differ in phase by 90°.
Methods of achieving polarization
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Reflection
Scattering
Dichroism
Birefringence
Reflection
• Unpolarized light can be undergo polarization by
reflection of non metallic surfaces like snow,
glass.
• The reflected light is partially polarized and the
degree of polarization depends on the angle of
incidence.
• When angle of incidence equal to the angle of
polarization, the degree of polarization is high.
• Incident angle is such that the angle between the
reflected and refracted ray is 90 degree. Such an
incident angle is polarized angle or Brewster's
angle.
• Reflected ray is linearly polarized parallel to the
reflecting surface.
MALUS LAW
• According to Malus,
when completely plane
polarized light is
incident on the
analyzer, the intensity I
of the light transmitted
by the analyzer is
directly proportional to
the square of the cosine
of angle between the
transmission axes of the
analyzer and the
polarizer.
MALUS LAW
• A
Amplitude of incident radiation
• Θ
Angle between analyzer and polarizer
• The component of amplitude II to the optic
axis = A cosθ
• Transmitted Intensity I = ?
• When θ = 0 or 180, I=?
• When θ = 90, I =?
Brewster’s law
Brewster’s law
• Brewster's found that there is a relation
between the angle of incidence and the
refractive index.
• When light is incident at polarizing angle:
• Tan (i) = Refractive index of the material
• Then i = ?
• If the angle of incidence is not exactly the
Brewster's angle then the reflected ray will be
only partially polarized.
Brewster’s law- Proof
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According to Brewster's law, µ = tan i
Tan i = (sin i / cos i) = µ
Snells law, µ = ?
i+ r = 90
Pile of plates
Scattering
• When light strikes the atoms of a material, it
will often set the electrons of those atoms
into vibration. The vibrating electrons then
produce their own electromagnetic wave that
is radiated outward in all directions.
• This newly generated wave strikes
neighboring atoms, forcing their electrons into
vibrations at the same original frequency.
Scattering
• This absorption and reemission of light waves
causes the light to be scattered about the
medium. (This process of scattering
contributes to the blueness of our skies)
• This scattered light is partially polarized
Dichroism
• Polarization by selective
absorption
• Eg: Tourmaline crystal
Birefringence
Birefringence
• Polarization due to double refraction
• Crystals like calcite
• If an object is viewed through a double
refracting crystal two images are seen
• The ray corresponds two both images are
polarized.
Calcite crystal
Calcite crystal
• Doubly refracting crystal
• Rhombohedron – 6 faces.
• 2 opposite corners A & H blunt corners
( 3obtuse angles meet)
• The direction of line passing through this
corners are called optic axis.
• Ray incident in the direction of optic axis there
is no double refraction.
Huygens's theory of double refraction
• A point source of monochromatic light in a
doubly refracting crystal is origin of two
wavefronts.
• O-Ray travel with same velocity in all direction so
the corresponding wavefront is spherical.
• E-Ray travel with different velocity in different
direction so the corresponding wavefront is
elliptical.
• Along the direction of the optic axis both rays
travel with same speed.
Types of Crystals
• Depending on the refractive index of O-Ray
and E-Ray, crystals are classified into two.
• POSITIVE CRYSTAL & NEGATIVE CRYSTAL
Positive crystal
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O-Ray lies outside the surface of E-Ray
Velocity of O-Ray is constant.
At optic axis velocities are same
μ ordinary < μ Extraordinary
VE-Ray Varies, maximum along optic axis
VE-Ray = VO-Ray.
• VE-Ray Minimum perpendicular to the direction
f optic axis.
• Eg: quartz,ice
Negative Crystal
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E-Ray lies outside the surface of O-Ray
Velocity of O-Ray is constant.
At optic axis velocities are same
μ ordinary > μ Extraordinary
VE-Ray Varies, minimum along optic axis
VE-Ray = VO-Ray.
• VE-Ray Maximum perpendicular to the
direction f optic axis.
• Eg : tourmaline
NICOL PRISM
NICOL PRISM
NICOL PRISM
• Invented by William Nicol 1828.
• It is a device to produce and analyze plane polarized
light.
• Calcite crystal whose length is 3 times that of its
breadth is used.
• By grinding and polishing the angles of small faces
are 680 and 1120 .
• This crystal is then cut into 2 pieces , then these two
pieces are made optically flat by polishing and joined
together using a transparent material called CANADA
BALSAM.
TOTAL INTERNAL REFLECTION
• When a ray travelling
fro denser medium to
rarer medium at an
angle of incidence
greater than the critical
angle total internal
reflection takes place in
the boundary between
two media.
NICOL PRISM
• The refractive index of canada balsam is
μ= 1.55
• When the rays split into two then,
μ o-ray = 1.658
μ e-ray = 1.486
• Ordinary ray strikes on the canada balsam layer
with an angle greater than the critical angle and
then O-Ray undergo total internal reflection.
• The reflected rays absorbed by the black paint
coated on the sides of the crystal.
NICOL PRISM
• E-Ray strikes on the canada balsam layer at an
angle less than the critical angle and it will be
coming out through the other end of the
crystal .
Quarter wave plate
• Quarterwave plate is a
device which produces
path difference of λ/4
and a phase difference
of π/2 between two
rays.
• Q.wave plate is used to
produce circularly
polarized and elliptically
polarized light.
Half wave plate
• Doubly refracting
uniaxial crystal
• The thickness of the
half wave plate is such
that it introduces a path
difference of λ/2 and
phase difference of π
between two rays.
• Eg: mica, quartz
Polaroids
• It is an optical device based on selective
absorption to produce plane polarized light for
commercial purpose.
• When unpolarized light is transmitted through
a Polaroid, it emerges with one-half the
intensity and with vibrations in a single plane;
it emerges as polarized light.
• Parallel position intensity transmitted and
perpendicular position absorbed.
Uses of polaroids
• Polaroids are used in the laboratory to produce and analyze
plane polarized light.
• Polaroids are widely used as polarizing sun glasses.
• They are used to eliminate the head light glare in motor cars.
• They are used to improve colour contrasts in old oil paintings.
• polaroid films are used to produce 3-D moving pictures.
• They are used as glass windows in trains and aeroplanes to
control the intensity of light. In aeroplane one polaroid is fixed
outside the window while the other is fitted inside which can
be rotated. The intensity of light can be adjusted by rotating
the inner polaroid.
• In calculators and watches, letters and numbers are formed by
liquid crystal display(LCD) through polarization of light.
• Polarization is also used to study size and shape of
molecules.
Applications of polarized light
• 3-D movies:
Polarization is used for 3D movies, in which the
images intended for each
eye are projected by
using two different
projectors.
Polarized 3-D glasses with
suitable polarized filters
ensure that each receives
the intended image.
Applications of polarized light
• Plane polarized light
reduces glare.
In foreign countries the
wind screen in front of
the driver is polarizer.
Hence it reduces glare and
thus accidents can be
minimized.