### Lecture 27 Slides

```No office hours tomorrow

I also procrastinate . . .
Why don’t I see an
interference pattern
From two light bulbs?
Coherence
(temporal and spatial)
Polarization
Interference of BEC
Phase Shift on Reflection of Light
1.0
Low n to high n
0.5
4
2
2
4
180 degree
phase shift
0.5
1.0
1.0
0.5
High n to low n
4
2
2
4
no phase shift
0.5
1.0
A soap film in air is illuminated with light near normal
incidence (angles exaggerated).
n=1
n=1.3
n=1
A soap bubble has an index of n=1.3. A series of colorful
fringes are seen in the reflection from the lower part
of the soap film where it has gradually greater
thickness. What is the minimum thickness of the film
that will produce a bright green fringe (λ=530nm)
when looking from above?
We see thin film interference all
the time. Why no thick film
intereference?
Group problem: What is the minimum thickness of a
glycerin film (n=1.47) on which light of wavelength 600 nm
shines that results in constructive interference of the
reflected light? Assume the film is surrounded front and
back by air.
A.
B.
C.
D.
E.
72 nm
102 nm
150 nm
204 nm
300 nm
Discussion question: An oil film on water reflects
different colors more or less brightly owing to
interference, depending on the film thickness. All
the colors reflect brightly when the film is really thin
(i.e., for films much smaller than visible
wavelengths, white light reflects relatively well).
From this information, we can tell that the index of
refraction of the oil is
Less than that of water.
B. The same as that of water.
C. Greater than that of water.
A.
The two reflected rays must be in-phase
with each other when they reflect
because the second one does not travel
significantly further than the other.
The n of oil is higher than air so there is a
phase shift of 180.
Therefore the n of water must also cause a
phase shift so noil < n water

A soap bubble has an index of n=1.3. The
bubble is thinnest near the top, and there is no
reflection from that region where it is very thin.
A series of colorful fringes are seen in the
reflection from the lower part of the soap film
where it has gradually greater thickness. How
thick is the film at the first bright green fringe
(λ=530nm) near the top?

A soap film has an index of n=1.3. The bubble
is thinnest near the top, and there is reflection
from that region where it is very thin. A series
of colorful fringes are seen in the reflection from
the lower part of the soap film where it has
gradually greater thickness. How thick is the
film at the first bright green fringe (λ=530nm)
near the top?
MOSIAH 18:8
And it came to pass that he said unto them:
Behold, here are the waters of Mormon (for
thus were they called) and now, as ye are
desirous to come into the fold of God, and
to be called his people, and are willing to
bear one another’s burdens, that they may
be light;
Practical Uses of Interference
Active Sound Control
 Nature uses it . . .

?v=jeUd_ittNns
Practical Uses of Interference
Active Sound Control
 Nature uses it . . .
 Anti-reflective coatings on lenses

Practical Uses of Interference
Active Sound Control
 Nature uses it . . .
 Anti-reflective coatings on lenses
 Extremely reflective mirrors
 Selectively reflective mirrors
 Interferometry

Michelson Interferometer
Michelson Interferometer
Michelson Interferometer
How much do I need to move a
mirror?
Cesium Interferometer
f2
f3
p/2
f1
p/2
p
N orm alized signal
1
0
-1
-1 0
-5
0
5
R o ta tio n ra te (x 1 0
10
-5
) ra d /s e c
15
20
Ion Interferometer
```