Transcript Physics 52 - Heat and Optics Dr. Joseph F. Becker Physics

Fig. 27–10 THIN-FILM INTERFERENCE
When light shines on
a thin film of oil
floating on a layer of
water, two light
waves (#1 and #2),
enter the eye after
reflection and
refraction.
The oil thickness
t = several ln’s
of light in the oil
C 2001 Wiley, Physics Cutnell & Johnson 5th Ed.
l oil = l vac / n
D path = 2t
Fig. 27–11 NOTE:
When a wave on a
rope reflects from
a wall, the wave
undergoes a 180o
phase change.
Light waves can
also undergo a 180o
phase shift upon
reflection
depending on the
reflecting surfaces.
C 2001 Wiley, Physics Cutnell & Johnson 5th Ed.
The 180o phase change depends on:
1. When light travels from a larger to a
smaller index of refractive index,
there is NO phase change upon reflection.
2. When light travels from a smaller to a
larger index of refractive index,
there is a 180o phase change upon
reflection.
Larger to a smaller index NO phase change
THIN-FILM INTERFERENCE
There is NO phase
change at air – oil
interface.
There is 180o phase
change at oil – water
interface.
C 2001 Wiley, Physics Cutnell & Johnson 5th Ed.
180o
l oil = l vac / n
0o
D path = 2t
If there are no phase changes upon reflection
constructive (max intensity) interference
will occur if the top film (oil) is: 2 t = m ln
2t = loil, 2 loil, 3 loil, …
If there is a 180o phase change upon reflection
constructive (max intensity) interference
will occur if top film (oil) is 2 t = (m + ½) ln
2t + loil/2 = loil, 2 loil, 3 loil, …
IMPORTANT: l oil = l vac / n
For destructive (min intensity) interference,
interchange the two equations!
If there are no phase changes upon reflection
destructive (min intensity) interference will
occur if top film (oil) is: 2 t = (m + ½) ln
2t = ½ loil, 1.5 loil, 2.5 loil, 3.5 loil, …
If there is a 180o phase change upon reflection
destructive (min intensity) interference will
occur if top film (oil) is: 2 t = m ln
2t + loil/2 = loil/2, 3 loil /2, 5 loil /2, …
IMPORTANT: l oil = l vac / n
Fig. 37-12 Upper figures: Light waves striking
an interface between optical materials
at (approximately) normal incidence.
na > n b
na = n b
na < n b
Mechanical wave pulses on ropes:
NO PHASE
CHANGE
NO REFLECTION
180o PHASE
CHANGE
Fig. 37-15 A nonreflective coating
has an index of refraction
that is intermediate
between those of glass and air.
Destructive interference*
occurs upon reflection
when film thickness is
¼l
in the film.
*No
reflected
light here!
n=
1.00
1.38
1.50
180o
180o
Fig. 37-16 A
schematic
diagram of a
Michelson
interferometer.
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