William Angerer - Department of Physics and Astronomy - University ...

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thickness of the film is greater than the wavelength of light, the phase difference. (/),
between the wave reflected from the surface and the wave reflected off of back surface
of the film is
( .. 1. 1)
where 0 :::; cPo < 27r and m is an integer. Since the interference of these waves
depends only on rPo, a fit to the reflection data yields a series of thicknesses if the
effective thickness is greater than the wavelength of light (m > 0). vVe removed this
uncertainty in the reflection measurement by performing film thickness measurements
with a HeNe laser (). = 632.8 nm) and the 488 nm line of an Ar laser. Fits to both sets
of reflection data produced two sets of possible film thicknesses. The film thickness
that appeared in both sets of data was the correct thickness .
.-\ second experimental difficulty is the transparent sapphire substrate. Reflection
from the back surface of the substrate can contribute to the measured signal. \\"e
suppressed this contribution by putting a thin layer of index matching fluid between
the back surface of the sapphire substrate and a glass slide. The other surface of the
glass slide was sand blasted to suppress specular reflections. Although the indices
of the index fluid and glass slide (nrv1.5) do not match that of sapphire (n"-'1.76).
this technique effectively suppressed nearly all of the reflected light from the sapphire
back surface. This was experimentally verified by accurately fitting the reflected light
from a sapphire slab/index fluid/glass slide with a semi-infinite slab theory.
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