Thin Film Interference
Physics
Thin Film Interference
http://graphics.stanford.edu/courses/cs348
b-competition/cs348b-08/finals/soap.png
Thin Film Interference
Thin Film Interference
http://laser.physics.sunysb.edu/~hilary/grap
hics/interference.gif
Thin Film
Interference
Thin Film
Interference
Ray 1 travels into the
film and back it
travels 2t (twice the
thickness of the film).
The path difference
between the rays
when they come out
of the film is 2t.
Condition for Constructive
Interference:
The reflected rays coming out of the film (5)
and the rays that reflected on the film
interface with the air(2), must have a path
difference of an integer multiple of /2.
This is because (2) flipped going from a less
dense to a more dense medium.
Note:
is the wavelength in the film.
Condition for Constructive
Interference (Color)
Path Difference= 2t =m /2
so thickness, t= m /4
where m=1,3, 5…
t is the thickness of the film.
Minimum thickness occurs when m=1.
Why do bubbles have different
colors?
The thickness of the bubble continuously
changes causing different colors to be “inphase” (constructive interference).
If the reflected and refracted waves are “out
of phase” (path difference not
then no
color will be seen.
.
Problem
Calculate the minimum thickness of a
soap bubble that results in
constructive interference when a light
of wavelength 500nm shines on it.
Answer: 125nm
Summary – Thin Film
Interference
• Reflection at a boundary
• Refraction – different n values
• Varies w/ thickness (colors)
• Superposition of light waves