The sensor structure shown below allows total reflection (TIR) to measure the amount of condensation of raindrops on the windshield surface of a car (Left: no rain; Right: Rain). This means that the sensor can automatically change the speed of the wiper according to the number of drops. When the refractive index of air is 1, that of glass and prism is 1.5, and that of water is 1.33: ?1?Finding critical Angle at glass/air interface (left figure) ?2?Finding the critical Angle of glass/water interface (right figure) ?3?Describe the sensing principle using TIR