This youtube video is something that I'll have to remember for a future demo. It shows that cellophane tape makes (1-side) frosted glass appear to be transparent. Quite striking! The reason this works is pretty straightforward from the physics perspective. Frosted glass looks whitish because its surface has been covered (by sandblasting or something analogous) with little irregularities that have a typical size scale comparable to the wavelengths of visible light. Because of the different in index of refraction between glass and air, these little irregularities diffusely scatter light, and they do a pretty equitable job across the visible spectrum. (This is why clouds are white, too, by the way.) By coating the glass intimately with a polymer layer (with an index of refraction closer to the glass than that of the air), one is effectively smoothing out the irregularities to a large degree. As far as I know, this is essentially the same physics behind why wet fabrics often appear darker than dry fabrics. Some of the apparent lightness of the dry material is due to diffuse scattering by ~ wavelength-sized stray threads and fibers. A wetting liquid acts as an index-matching medium, effectively smoothing out those inhomogeneities and reducing that diffuse scattering.
3 comments:
Light scattering depends on wavelength vs. scatter dimensions. Excluding short wavelengths excludes scattered noise, hence yellow to orange shooting spectacles to remove atmospheric haze.
CCD webcams are sensitive to below 1000 nm. An IR cutoff filter directly above the CCD excludes the IR signal. Remove the IR cutoff filter, reassemble, shoot through an orange filter. White cotton clothing becomes largely transparent.
http://hackaday.com/2005/03/14/make-an-infrared-webcam/
http://www.youtube.com/watch?v=KyZbkM0dZHw
be wary if Uncle Al approaches you on the beach and he is carrying a digital camera with orange gel over the lens...
Smoothly varying the refractive index in this manner is also used (sometimes) for making anti-reflective coatings for photovoltaic applications. Over a limited wavelength range, Si wafers reflect ~60 percent of the incident light. If one uses a coating with a refractive index between the RI of Si and the RI of air, this can be reduced significantly.
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