= Frustrated total internal reflection
<Frustrated total internal reflection> occurs when a nearby second high-index <dielectric> couples to the <evanescent field in total internal reflection>, allowing <power> to cross a finite low-index gap. The extra boundary changes the <dielectric-interface boundary conditions>; the gap field need not be a single decaying <evanescent wave>. Consequently, the zero normal <Poynting vector> calculation for two infinite <half-spaces> does not forbid transmission in this three-medium geometry.
Harvard’s actual demonstration uses two right-angle glass prisms, a <helium-neon laser>, a small clamp and two perpendicular screens as detectors. The contact faces are cleaned and placed opposite each other, with the clamp protecting the corners. Increasing pressure narrows the intervening air film; the observed beam routing changes gradually from <total internal reflection> toward onward transmission. The broad-gap reflecting configuration supplies the comparison for the narrow-gap transmitting configuration. This is a qualitative readout: the page explicitly says the gap is not measured.<Frustrated Total Internal Reflection by Harvard Natural Sciences Lecture Demonstrations>[How it works and Setting it up: two prisms, HeNe laser, perpendicular screens, pressure adjustment and unmeasured gap]{ref}
The inference is that the lower-index region can mediate <electromagnetic energy> transfer when another boundary is nearby. The demonstration does not determine a numerical <evanescent penetration depth>, resolve gap uniformity, or establish a three-layer transmission coefficient. For the constructed $50^\circ$ example, $\delta\approx178\,\mathrm{nm}$ identifies the scale over which the isolated-interface field remains appreciable; substituting $e^{-2g/\delta}$ for a gap transmission fraction would omit the changed boundary fields and their <interference>. A quantitative gap calculation must solve the full <Maxwell equations> with both boundaries.
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