Interference

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Transcript Interference

Wave Optics
The physics of interference, diffraction,
thin films and more...
Wave Superposition and
Interference
• What happens when waves meet?
• Path Difference - Phase Difference:
illustrate this with the following applet
r
  2
r
Double and Multi-Slit
Interfence
• Simple Case: Assume each slit acts as
a single source
• The Grating Equation for Antinodes:
n  d sin
where d = separation between slits
Interfence Patterns
• Use the following applet to help explore
the different interference patterns
produced by single, double and multiple
slits...
Single Slit Diffraction...
• A single slit can interfere with itself!
simulate this
• Equation for Nodes: n  W sin 
Poisson’s Spot - a curious tale!
• By the early 19th century the wave model
was becoming accepted and the French
Physicist Fresnel had worked out an
elaborate mathematical theory of light.
However, the famous physicist Poisson
claimed to have found a fatal flaw in the
theory...
The Physics of Thin Films
• A Phase difference can occur because
of…
– path differences for different combining
wavetrains
– phase shifts introduced under certain
reflections
– go to applet thin film
Path Differences to Phase Shifts...
How to calculate
phase shift:
•
•
•
•
Find path difference
Divide by wavelength of light IN THE FILM
Add any additional phase shifts produced by reflections.
If the phase shift is:
– 0.5, 1.5, 2.5, …. “n.5” then destructive interference will
result
– 0,1,2,3,… n then constructive interference will result
Structural Colour in Plants and
Animals
• The iridescent sheen of a bluebird wing or the
extravagant colours of many butterflies and moths
arise - not from chemistry (pigments) but from
physics (interference effects)!!
The Physics of Structural
Colour
• Light reflects from
slightly different
places along the
surface. An optical
path difference is
introduced
Some colours work - some
don’t...
• The optical path
difference  is just
the extra distance
that one ray travels
wrt to an adjacent
one. It’s easy to see
that this leads to:
  2l / cos  n
for constructive
interference
Application… Lens Coating
• Why do some lens
have a purple
colour?
• Use the applet thin
film to help
understand the
power of the
“quarter wave”!
So - if it’s a wave - What Kind
of Wave?
• Known since antiquity - some crystals
(calcite) can cause light to refract into two
distinct paths when passing through the
crystal (1669 - Bartholinus)
• Early 19th century, light under glancing
collisions can exhibit a “new property” polarization
• Light is a transverse wave