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Diffraction of light when two fingers brought close together infront of a light source
Diffraction by razor blade when illuminated by intense blue light
“light is never known to follow
crooked passages nor to
bend into the shadow”.
Sir Isaac Newton (1642-1727)
“Any deviation of light rays from
rectilinear path which is neither
reflection nor refraction known
as diffraction.’’
Arnold Johannes Wilhelm Sommerfeld
(1868-1951)
Diffraction of Sound
Radio waves diffract around mountains.
When the wavelength is km long, a mountain peak diffract the wave.
Another effect that occurs is scattering – role of diffraction is not obvious.
Huygens’s Principle
“Every point in a propagating wavefront serves as the source
of spherical secondary wavelets, such that the wavefront at some
later time is the envelope of these wavelets.”
Huygens-Fresnel Principle
Every unobstructed point of a wavefront, at a given instant, serves as a
source of spherical secondary wavelets, The amplitude of the optical
field at any point beyond is the superposition of all these wavelets.
http://www.walter-fendt.de/ph11e/huygenspr.htm
Reflection and Refraction of waves© 2006 Walter Fendt
Christiaan Huygens(1629-1695)
Augustin Fresnel(1788-1827)
More refinement by Kirchhoff and Sommerfeld
Gustav Robert Kirchhoff(1824-1887)
Arnold Johannes Wilhelm Sommerfeld(1868-1951)
Classical model of diffraction
wavefrontobstacle
screen
On obstacle, the electron oscillator vibrating and reemittting at source frequency
Incident field and field of all vibrating electrons superpose in such away that there is zero field beyond the obstacle.
Assume the mutual interaction between the oscillators are essentiallynegligible.
Classical model of diffraction
Diffraction of a wave by a slit
Whether waves in water or electromagnetic radiation in air, passage through a slit yields a diffraction pattern that will appear more dramatic as the size of the slit approaches the wavelength of the wave.
Narrower the slit, the wider the pattern
A A AB B B
l>ABl<AB
Fraunhofer and Fresnel Diffraction
Joseph von Fraunhofer (1787-1826) Augustin Jean Fresnel
(1788 - 1827)
Fraunhofer vs. Fresnel diffraction
• In Fraunhofer diffraction, both incident and diffracted waves may be considered to be plane (i.e. both S and P are a large distance away)
• If either S or P are close enough that wavefront curvature is not negligible, then we have Fresnel diffraction
P
SS
P
•Fraunhofer limit diffraction
2ad
• If aperture (obstacle) has a width a
•Fresnel limit diffraction
2ad
d is the smaller of the two distances from S and S and P
•Fresnel diffraction pattern does change in shape as we move further away from the object (until, of course, we are so far away that the Fraunhofer condition is satisfied).
•The surface of calculation
http://www.rodenburg.org/theory/y1200.html
Fraunhofer or far field diffraction
Fresnel or near field diffraction
Fresnel –Fraunhofer Diffraction
Far from
the slit
zClose to the slit
Incident plane wave
1exp3exp
2expexpexp
niaia
iaiaaiAA
Superposition of N Oscillators
21exp
2sin
2sin
exp1exp1
nin
a
iinaA
2sin
2sin
n
A
21 n
2
0 2
sin ( / 2)*
sin / 2
nI AA I
2/cos4 2
I 1
0 0For
20
0
IIN
IN
IN
0 200 400 600
0.0
0.2
0.4
0.6
0.8
1.0
Inte
nsi
ty
Theta
2)]*10[sin( 2)][sin(
0 200 400 600
0
20
40
60
80
100
Inte
nsi
ty
Theta
2)]*10[sin(
2)][sin(
1. Optics Author: Eugene Hecht Class no. 535 HEC/O Central library IIT KGP