We demonstrate analytically that discontinuity at critical angle can be used to reach extremely high sensitivities against any optical properties that modify this angular value. To test in practice the approach we fabricated cheap and basic sensors by which we could demonstrate extremely high index of refraction discrimination ability in the range of one part over a million. Moreover, we constructed a similar apparatus to detect in a very simple and low-cost manner nanometer scale mechanical vibrations. Also in this case we obtained very high sensitivity, tiny vibrations of 2.4nm were resolved, this correspond to an angular shift of about 50 * 10-6 deg, as far as we know the best angular sensitivity recorded so far. The critical angle detection method discussed in this paper is applicable generally to any optical system and may pave the way for next generation optical sensing devices. © 2010 Optical Society of America.