https://macosxpowertools.com/what-are-fiber-optic-feedthroughs/

Total internal reflection may be your happening that retains the ray of light in the tube, instead of allowing it to only pass it and then leak out. It happens when the angle that the light strikes the glass tubing wall is more shallow, much less compared to 42 degrees.

Fiber optic feedthroughs would be those devices at each conclusion of a fiber optic communication and will be multichannel or only channel. Multichannel feedthroughs are successful, since they combine a number of fiber optic wires to economically move considerable quantities of data while still consuming minimal space.

Fiber optic feedthroughs make it possible for information to be transmitted further and more quickly compared to wire choices, such as aluminum. All these feedthroughs do not conduct power nor are they influenced by electromagnetism, therefore they are protected and transport data using minimal lack of quality. 4q8ifa6zeg.