Editing Rain sensor (section)

==Physics of rain sensor==
[[File:Rain sensor en.svg|right|thumb|A diagram showing the operation of an optical rain sensor]]
The most common modern rain sensors are based on the principle of [[total internal reflection]]. At all times, an [[infrared]] light is beamed at a 45-degree angle into the windshield from the interior. If the glass is dry, the critical angle for total internal refraction is around 42°. This value is obtained with the total internal refraction formula.

<math>\sin (\theta_\text{c}) = \frac{n_1}{n_2}</math>

where <math>n_1 = 1</math> is the approximate value of air's refraction index for infrared <ref>{{Cite web|url=https://refractiveindex.info/?shelf=other&book=air&page=Ciddor|title = Refractive index of Air - Ciddor}}</ref> and <math>n_2 = 1.5</math> is the approximate value of the glass refraction index, also for infrared.<ref>{{Cite journal | doi=10.1088/1742-6596/249/1/012034| title=Refractive index of glass and its dispersion for visible light| journal=Journal of Physics: Conference Series| volume=249| pages=012034| year=2010| last1=Smith| first1=D. Y.| last2=Karstens| first2=W.| issue=1| bibcode=2010JPhCS.249a2034S| doi-access=free}}</ref> Since the incident angle of light is 45°, all the light is reflected, and the detector receives maximum intensity.

If the glass is wet, the critical angle changes to around 60° because the refraction index of water is higher than air (<math>n_1 = 1.3</math>).<ref>{{Cite web|url=http://www.philiplaven.com/p20.html|title = Refractive index}}</ref> In that case, because the incident angle is 45°, total internal reflection is not obtained. Part of the light beam is transmitted through the glass, and the intensity measured for reflection is lower: the system detects water, and the wipers turn on.