What happens at the water/glass interface if the incoming beam hits it above the
critical angle?
The term "total internal reflection" (TIR) is a little misleading in this case. However,
the not so frequently used expression "attenuated total reflection" (ATR) describes the
phenomenon a bit better. Looking at the interface and especially into the medium with
the lower index of refraction (water), a standing (evanescence) wave is established
whose intensity decreases exponentially with distance. In other words, part of the
wave (intensity) penetrates into the surrounding medium. The penetration depth depends
on the angle, the wavelength, and the index of refraction of the two media involved.
Coming back to the example given above the penetration depth slightly above, the critical
angle (60°) equals roughly 1.5 times the wavelength used.
