This article explains how total internal reflection (TIR) can trap light inside a material with a high index of refraction.

swimming pool under water
From underneath, the surface of the water in a swimming pool looks like a mirror
Total internal reflection (TIR) starts to occur when refraction is so strong that the light can not escape the water.

You have probably noticed that, if you are underwater in a swimming pool, you can only see out of the water if you look straight up. When you look further down the length of the pool, the surface of the water appears to be a perfect mirror, reflecting only the images of objects beneath the waves. That should strike you as odd—water is transparent and air is transparent, so how is the surface acting as a mirror?

The previous blog established a rule of thumb for refraction:

“Light refracts toward the material with the higher refractive index.”

If you think about that, you might anticipate a problem. If light is trying to move from a material with a higher refractive index into a material with a lower refractive index, but the light always bends back toward the higher index material, then there is going to be an angle at which the light cannot escape. Because the light cannot break through the surface of the water it reflects from it.

The angle at which light starts reflecting instead of refracting is called the “critical angle”. Because none of the light is leaving, it is called “total internal reflection” (TIR).

Total Internal Reflection (TIR) occurs when light cannot transmit from a high refractive index material into a lower refractive index material and is therefore reflected back into the high refractive index material.

The Critical Angle is the angle at which total internal reflection occurs, and light at grazing angles beyond the critical angle becomes trapped within the material with the higher refractive index.

So why does TIR matter?

TIR traps light within an optical fiber because the inner portion of the fiber (the “core”) has a higher refractive index than the outer portion of the fiber (the “cladding”).

TIR really is *total* reflection. For all practical purposes, TIR reflects 100% of the light—better than any kind of metal mirror. The most ubiquitous application of TIR is optical fiber. Fiber optic cables trap light and transmit it for kilometers. Fiber optics will be the subject of the next article.

Swimming pool photo copyright:  cookelma / 123RF Stock Photo

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