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Chapter 24
The Wave Nature of Light
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24.1 Waves Versus Particles; HuygensPrinciple and Diffraction
Huygens principle:
Every point on a wavefront acts as a pointsource; the wavefrontas it develops is
tangent to their envelope
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Examples of lights wave nature
Light can exhibit behavior specific to bothwaves and particles (later in course)
Need to know examples of each
Interference Youngs Double Slit ExperimentThe Visible Spectrum and Dispersion
Diffraction by a Single Slit
Diffraction by a Grating of slits
Interference by Thin Films
Refraction next unit 3
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24.1 Waves Versus Particles; HuygensPrinciple and Diffraction
Huygens principle is consistent with diffraction:
Diffraction occurs with water waves but not withstreams of particles.
We will examine 3 types of diffraction through
double slit
single slit
diffraction grating 4
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single slit diffraction
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24.3 Interference Youngs Double-SlitExperiment
If light is a wave, interference effects will be seen,where one part of wavefront can interact with another part.
The interference pattern that resulted was proof of wave
not particle nature of light
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If light is a wave,there should be aninterference patternwhich is a set of alternating brightand dark lines onthe screen.
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Formation of an interference patternThe interference occursbecause each point on the
screen is not the samedistance from both slits.Depending on the pathlength difference, the wavecan interfere constructively(bright spot) or destructively (dark spot).
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24.3 Interference Youngs Double-SlitExperiment
We can use geometry to find the conditions for constructive and destructive interference:
(24-2a)
(24-2b)
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sin tan
sin
x
L
xd d m
L
U U
U P
} !
! !
A ngle U is a very small angle so:
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m is the number of the order of the higher fringesfrom the bright central maximum
Since the position of the maxima (except the centralone) depends on wavelength, the first- and higher-order
fringes contain a spectrum of colors.11
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monochromatic light
white light
Youngs experiment
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24.4 The Visible Spectrum and Dispersion
Wavelengths of visible light: 400 nm to 750 nm
Shorter wavelengths are ultraviolet; longer areinfrared
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24.4 The Visible Spectrum and Dispersion
The various wavelengths of visible light refractdifferent amounts another example of the wavenature of light
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24.4 The Visible Spectrum and Dispersion
Actual rainbows are created by dispersion in tinydrops of water.
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I set My bow in the cloud, and it shall be for a sign of a covenant between Meand the earth. Genesis 9:13
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24.5 Diffraction by a Single Slit or Disk
Light will also diffract through a single slit or around an obstacle.
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24.5 Diffraction by a Single Slit or Disk
The resulting pattern of light and dark stripes iscalled a diffraction pattern.
P attern has a wide, very bright
central maximum with weakhigher order fringes.
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1 8single slit diffraction
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The minima of the single-slit diffraction patternoccur when
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cant use sin U = tan U because U is large
use tan U = x/L to get width of maxima
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24.6 Diffraction Grating
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diffraction gratings
sin 1, 2,3..
1/
m d m
d N lines cm N
P U! !
! !
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24.6 Diffraction Grating
The maxima of the diffraction pattern aredefined by
(24-4)
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Interference pattern from diffraction gratings
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23diffraction with laser
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diffraction with white light