Re: Thin Films

[Doug Craigen <dcc@ESCAPE.CA>]

David Abineri wrote:
> A high school student of mine asked when and why the thin film
> interference effect stops working.  If one looks into a transparent
> container of oil no effect is seen but a thin film on a puddle shows a
> definite interference pattern.  How thick does the oil need to be to
> lose the effect and what is the mechanism involved?  Is it perhaps just
> that too much absorption is taking place?
>
> Thanks for any help on this.      David Abineri

This effect (also true for soap bubbles, coatings on glass, the thin
space between microscope slide, etc) is noticeable for films whose
optical thickness is of the same order as visible wavelengths.  It
occurs from the interference of light reflected off the front and back
faces of the film.  If the optical path length of the ray which passes
through the film  is an integral number of wavelengths - you have
constructive interference for those colors.  If the optical path length
is 1/2, 3/2, 5/2 etc of a given wavelength - it results in destructive
interference.
There is one qualifier - an additional phase change of pi (equivalent to
another half wavelength of path) for reflection when the incident medium
has a higher index than the reflecting medium.

To demonstrate the effect I like to use two clean microscope slides.
You can easily press them together close enough that the air gap takes
on beautiful coloration on reflection. The colors are easily seen to be
related to the thickness by the rings they form around the place where
you are pressing.  You notice that the wider the gap the more rapid the
change from ring to ring and the less distinct the colors.

Suppose the optical thickness effect is 1000 nm, where is there
constructive interference?
#*wavelength=1000

# = 1  - 1000 nm (infrared)
# = 2  - 500 nm  (visible)
# = 3  - 333 nm (UV)

For this thickness, there is one maximum interference color in the
visible region.  The color will be quite distinct.

Now suppose it is 4000 nm:
# = 1 - 4000 nm (infrared)
# = 2 - 2000 nm (infrared)
# = 3 - 1333 nm (infrared)
# = 4 - 1000 nm (infrared)
# = 5 - 800 nm (red - visible)
# = 6 - 667 nm (visible)
# = 7 - 572 nm (visible)
# = 8 - 500 nm (visible)
# = 9 - 444 nm  (blue visible)
# = 10 - 400 nm (edge of UV)
... in UV

So there are 6 interference maximum colors within the visible and of
course 5 minimum in between. So the overall color will be indistinct,
and the thicker the film is the less distinct it will become.  So by the
time you consider the microscope slide, it is so thick that it will
probably have hundreds of max and min in the red region alone.  The same
argument holds for comparing an oil slick with a bottle of oil.



\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\_/^\

Doug Craigen
          http://www.dctech.com/physics/about_dc.html