Total internal reflection

Total internal reflection is an optical phenomenon. When light crosses materials with different refractive indices , the light beam will be bent at the boundary surface (→ Light Refraction ). At a certain angle of incidence (the critical angle), the light will stop crossing the boundary but instead reflect back internally at the boundary surface. This occurs only at a high- Refractive Index /low- Refractive Index boundary, not the other way around. For example it will occur when passing from glass to air, but will not occur when passing from air to glass. (Source: Wikipedia)

In Wide Field Microscopes and Confocal Microscopes this loss of high-angle rays reduces the effective Numerical Aperture of the objective, and therefore the resolution. (See Image Formation ). This will happen in a Refractive Index Mismatch situation, which should be avoided during the image acquisition.

There is a type of microscope called TIRF that make use of this phenomenon to image thin layers right at the boundary by using high-angle incidence illumination: only in this case the total internal reflection is a benefit. See Total internal reflection fluorescence microscope. To deconvolve images from these microscopes, see Total Internal Reflection System Imp Faq .



Image from Harvard University Natural Science Lecture Demonstrations

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