properties of light
DESCRIPTION
properties of light: polarization, interference and Huygen's principleTRANSCRIPT
Prepared by:Prepared by:Karen A. AdelanKaren A. Adelan
BSE 3 BSE 3
PROPERTIES OF LIGHTPROPERTIES OF LIGHTInterferenceInterferencePolarizationPolarization
Huygens's PrincipleHuygens's Principle
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Interference of Water WavesInterference of Water WavesAn An interference patterninterference pattern is set up by is set up by water waves leaving two slits at the water waves leaving two slits at the same instant.same instant.
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Young’s ExperimentYoung’s ExperimentIn In Young’s experimentYoung’s experiment, light from a monochromatic , light from a monochromatic source falls on two slits, setting up an source falls on two slits, setting up an interference interference patternpattern analogous to that with water waves. analogous to that with water waves.
Light source S1
S2
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The Superposition PrincipleThe Superposition Principle• The The resultant displacementresultant displacement of two simul- of two simul-
taneous waves (taneous waves (blueblue and and greengreen) is the ) is the algebraic sum of the two displacements.algebraic sum of the two displacements.
The superposition of two coherent light waves The superposition of two coherent light waves results in light and dark fringes on a screen. results in light and dark fringes on a screen.
• The The compositecomposite wave is shown in wave is shown in yellowyellow..
Constructive Constructive InterferenceInterference
Destructive Destructive InterferenceInterference
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Young’s Interference PatternYoung’s Interference Patterns1
s2
s1
s2
s1
s2
Constructive
Constructive
Bright fringe
Bright fringe
Dark fringe
Destructive
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Conditions for Bright FringesConditions for Bright FringesBright fringesBright fringes occur when the difference in path occur when the difference in path pp is is an integral multiple of one wave length an integral multiple of one wave length ..
pp11
pp22
pp33
pp44
Path difference
p = 0, , 2, 3, …
Bright fringes: p = n, n = 0, 1, 2, . . .
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Conditions for Dark FringesConditions for Dark FringesDark fringesDark fringes occur when the difference in path occur when the difference in path pp is an is an odd multiple of one-half of a wave length odd multiple of one-half of a wave length ..
pp11
pp22 2
pp33
pp33
2p n
n n = = oddoddn n = = 1,3,5 …1,3,5 …
Dark fringes: 1, 3, 5, 7, . . .2
p n n
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Analytical Methods for FringesAnalytical Methods for Fringes
x
y
d sin s1
s2
d
p1
p2
Bright fringes: d sin = n, n = 0, 1, 2, 3, . . .
Dark fringes: d sin = n, n = 1, 3, 5, . . .
p = p1 – p2
p = d sin
Path difference Path difference determines light and determines light and dark pattern.dark pattern.
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Analytical Methods (Cont.)Analytical Methods (Cont.)
x
y
d sin s1
s2
d
p1
p2
From geometry, we From geometry, we recall that:recall that:
Bright fringes:
, 0, 1, 2, ...dy
n nx
Dark fringes:
, 1, 3, 5...2
dyn n
x
sin tany
x
So that . . .So that . . .
sindy
dx
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Interference From Single SlitInterference From Single Slit
Pattern ExaggeratedPattern Exaggerated
When monochromatic light strikes a single slit, diffraction from the When monochromatic light strikes a single slit, diffraction from the edges produces an edges produces an interference patterninterference pattern as illustrated. as illustrated.
Relative intensity
The interference results from the fact that not all paths of light The interference results from the fact that not all paths of light travel the same distance some arrive out of phase.travel the same distance some arrive out of phase.
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Single Slit Interference Single Slit Interference PatternPattern
a/2
aa/2
sin2
a
1
2
4
3
5
Each point inside slit Each point inside slit acts as a source. acts as a source.
For rays 1 and 3 and for For rays 1 and 3 and for 2 and 4:2 and 4:
sin2
ap
First dark fringe:First dark fringe:
sin2 2
a sin2 2
a
For every ray there is another ray that differs by this path and For every ray there is another ray that differs by this path and therefore interferes destructively.therefore interferes destructively.
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Single Slit Interference Single Slit Interference PatternPattern
a/2
aa/2
sin2
a
1
2
4
3
5
sin2 2
a
First dark fringe:First dark fringe:
sina
sina
OtherOther dark fringesdark fringes occur for occur for integral multiples of this fraction integral multiples of this fraction /a/a..
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Example 3:Example 3: Monochromatic light shines on a Monochromatic light shines on a single slit of width single slit of width 0.45 mm0.45 mm. On a screen . On a screen 1.5 m1.5 m
away, the first dark fringe is displaced away, the first dark fringe is displaced 2 mm2 mm from the central maximum. What is the from the central maximum. What is the
wavelength of the light?wavelength of the light?
x = 1.5 m y
a = 0.35 mm
= ?
sina
sina
ysin tan ; ;
x
y ya
x a x
(0.002 m)(0.00045 m)
1.50 m = 600 nm
POLARIZATIONPOLARIZATION
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Polarized vs. UnpolarizedPolarized vs. Unpolarized
Unpolarized light:Unpolarized light: light wave which light wave which is vibrating in more than one planeis vibrating in more than one plane
Polarized light:Polarized light: light waves in light waves in which the vibrations occur in a single which the vibrations occur in a single plane. plane.
PolarizationPolarization: The process of : The process of transforming unpolarized light into transforming unpolarized light into polarized light is known aspolarized light is known as
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Polarized Light
Polarized LightVibrations lie on one single plane only.
Unpolarized LightSuperposition of many beams, in the same direction of propagation, but each with random polarization.
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Representation . . . Representation . . .
Unpolarized Polarized
EE
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Representation . . . Representation . . .
Unpolarized Polarized
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Polarization of LightPolarization of Light
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Selective AbsorptionSelective Absorption
Light
Unpolarized
Horizontal Component being Absorbed
Vertical Component being Transmitted
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Polarization by AbsorptionPolarization by AbsorptionPolaroid crystalline materials absorb Polaroid crystalline materials absorb more light in one incident plane than more light in one incident plane than another, so that light progressing another, so that light progressing through the material become more through the material become more and more polarizedand more polarized
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Crossed Polariods can Eliminate Crossed Polariods can Eliminate LightLight
Huygens’ Principle Huygens’ Principle The first person to explain how wave theory can also account for the laws of geometric optics was Christian Huygens in 1670.The principle states that:
Every point on a wave-front may be Every point on a wave-front may be considered a source of secondary spherical considered a source of secondary spherical wavelets which spread out in the forward wavelets which spread out in the forward direction at the speed of light. The new wave-direction at the speed of light. The new wave-front is the tangential surface to all of these front is the tangential surface to all of these secondary wavelets.secondary wavelets.
2222
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Fig 35-17a, p.1108
Huygens’s PrincipleHuygens’s Principle
All points on a wave front act as new sources for the production of spherical secondary waves All points on a wave front act as new sources for the production of spherical secondary waves
k
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Reflection According to HuygensReflection According to Huygens
Side-Side-SideDAA’C ADC1 = 1’
Side-Side-SideDAA’C ADC1 = 1’
Incoming ray Outgoing ray
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Huygens’ wave front constructionHuygens’ wave front construction
Given wavefront at t
Allow wavelets to evolve for time Δt
r = c Δt ≈ λ
New wavefrontConstruct the wave front tangent to the wavelets