Total internal reflection in optical fibers
How does light travel all the way along a curving glass thread?
An optical fiber consists of a core with a higher refractive index surrounded by cladding with a lower one. Light entering the end face is reflected again and again at the core–cladding boundary: as long as the angle of incidence there is not smaller than the critical angle C, total internal reflection occurs and the light reaches the far end with almost no loss. If the entry angle is too large, the angle of incidence at the boundary falls below the critical angle, some light refracts into the cladding at every reflection, and the light cannot travel far.
Light kept per reflection vs. entry angle
For θ up to θmax there is total reflection, R = 100%; beyond that it drops sharply.
Think about it
If the cladding index is brought closer to the core index, how does the maximum acceptance angle change?
It gets smaller. sin θmax = √(n₁² − n₂²): the closer the two indices, the smaller the numerical aperture. The critical angle C grows, so total internal reflection at the boundary is harder to achieve, and only light entering closer to the axis gets through.
Model assumptionsOnly rays through the axis are drawn · Reflection loss at the end face ignored · Without total reflection, each reflected fraction follows the Fresnel equations · Refraction beyond the cladding not drawn
Data log
Set the parameters and click “Record data”. Record several rows with different parameters to plot their relationship and fit a straight line; pause and record at different times to study how a reading changes with time.