2 nd & 3 th n.u.t.s. workshops gulu university naples federico ii university
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2 nd & 3 th N.U.T.S. Workshops Gulu University Naples FEDERICO II University. 6 – Interference. Soap Bubbles … and Oil Spot. What is producing so nice c o l o u r s ?. 2nd & 3th NUTS Workshop ( Jan 2010). Other Examples of Nice Coulours …. 3. 6- Interference. - PowerPoint PPT PresentationTRANSCRIPT
6- Interference
2nd & 3th NUTS Workshop ( Jan 2010)
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Soap Bubbles … and Oil Spot
What is producing so nice colours ?
6- Interference
2nd & 3th NUTS Workshop ( Jan 2010)
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It’s just a Phase Difference Pattern!
or a Thin Film Interference Constructive and destructive interference of
light waves is the reason why thin films, such as soap bubbles, show colorful patterns. Light waves reflecting off the top surface of a film interfere with the waves reflecting from the
bottom surface. To obtain a nice colored pattern, the thickness of the film has to be of
the order of the wavelength of light.
Variable thickness of the film give variable wavelength (colour) of the refracted light
constructive interference
What is Interference?
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2nd & 3th NUTS Workshop ( Jan 2010)
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Combined Waveform
wave 1
wave 2
If two waves (same wavelength and frequency) are in phase, both wave crests and troughs align. Constructive interference results increase in the wave amplitude, for
light a brightening of the waveform in that location.
If the two waves are out of phase, then the crests will align with the troughs. Destructive Interference results, a decrease in the amplitude of the combined wave, for
light a dimming of the waveform at that location.
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SUPERPOSITION of 2 or more Waves in the same region
ONLY UNDER SPECIFIC CONDITIONS
Interference:
Conditions to Have Interference
In the simplest case: Superposition of periodic waves
with same frequency. The waves’ sources oscillate in
phase. i.e. synchronously, or with phase difference constant and known (COHERENT SOURCES)
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(longitudinal and transversal waves)
Interference for Coherent Sources
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Young’s ExperimentThe double-slit experiment, performed by the
English scientist T. Young in 1801, is an attempt to resolve the question of whether light was composed of
particles (Newton's "corpuscular" theory), or rather consisted of waves. The Interference Patterns
observed in the experiment seemed to discredit the corpuscular theory; the wave theory of light remained
well accepted until early 20th century.The original
drawing by T. Young to
illustrate its experiments.
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Double-slit Experiment: Schema
To have a constructive interference along the θ direction the path length difference between the wavefronts coming from the two
apertures have to be an integer number of wavelengths: d sin θ= mλ
plane waveforms
to focus on the screen
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YOUNG Ex conditions for MAX and MIN Intensity
BRIGHT FRINGE :
DARK FRINGE :
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Geometry of N-slits Interference
d = spacing between two slits
L = screen distance from the plane of the slits
N = total number of slits = angle between the
direction of incoming beam and the considered out coming one
= wavelength of the incident light
λ
rj
a
x
x′
L
θ
d1
2
j
N
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N-slits Interference: the Solution for I
2
0 sinsin
sinsin
)(I
d
Nd
I
Interference of red laser light
2 slits
5 slits
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Double-slit Maxima Location
2
0 /sinsin
/sin2sin)(I
d
dI
Maxima when denominator = 0
dnn
d
sin
sinn is the fringe order
- n is a positive o negative integer- there is a nmax (nmax= max integer ≤d/λ) - total number of fringes =2 nmax+1 (from -nmax to +nmax )
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2nd & 3th NUTS Workshop ( Jan 2010)
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5-slits Versus Double Slit
2
0 /sinsin
/sin5sin)(I
d
dI
Maxima 5-slit when denominator = 0
dnn
d
sin
sinsame as 2-slit!!!
only the fringe width is narrower with respect to 2-slit
(the fringe width is proportional to the numerator period!)
Interference of red laser light
2 slits
5 slits