Whenever two portions of the same light arrive at the eye

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Whenever two portions of the same light arrive at
the eye by different routes, either exactly or
very nearly in the same direction, the light
becomes most when the difference of the routes
is any multiple of a certain length, and the
least intense in the intermediate state of
interfering portions and this length is
different for light of different colour. T.
Young from a paper to the Royal Society in 1802
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Interference phenomena ? Pathlength difference
? P
dsinq
Along the center line, it is obvious that the
distances to two sources are identical. ?
r1-r2 0 and constructive int.
r1 - r2 dsinq ml Constructive Int.
(2m 1)l/2 Destructive Int.
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ml (0, l, 2l, ) Constructive
Ds d(x/h)
(m1)l/2 (l/2, 3l/2,) Destructive
d(x/h) ml ? x m(h/d) l for constructive
int.
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Coherent light sources by splitting
Huygens principle Each point on a wavefront
acts as a new Source of identical waves.
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Interference in thin films
Half reflecting planes
2x ml constructive
(2m1)l/2 destructive
For an arbitrary angle q
Ds 2x/cosq
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x
n
lf
Difference in two routes Ds 2x
m constructive
Wavelength in the film (not in air)
v c/n f lf c f l
lf l/n
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Diffraction Grating
provides much clearer and sharper interference
pattern and a practical device for resolving
spectra.
Dr dsinq ml ? Constructive
q
d
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632.8 nm red beam of a helium-neon laser through
a 600 lines/mm diffraction grating
(1 mm/ 600) sin a (n1)(632.8 nm) Then sin a
distance between spots / distance to screen.
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HITT A laser through a grid of slits, 0.0625 mm
each, 0.25 mm separations between their centers.
This picture was taken in the teaching labs of
the Ben Gurion University Physics Department
Find wavelength of the light (in the few hundred
nm range)?
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X ray Crystallography

• Q How do you determine the atomic structure of a
  crystal? A interference patterns

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Panel A, peaks vs. angle (ignore panel B, that
has to do with their determination of the
structure) material pyrite (FeS2) X-ray
diffraction Nature Materials 5, 647 - 652 (2006)