This simulator shows a magnified step-index optical fiber in cutaway. Adjust the core index, index contrast, core radius, segment length, launch angle and wavelength, and watch light markers show which rays are guided by total internal reflection, how many boundary encounters occur, and how much power escapes once the launch leaves the acceptance cone.
• A magnified 3D step-index fiber with a launch lens and angular mount, a higher-index glass core, a lower-index cladding, a protective coating and sleeve, a meridional light path, an escaping branch and an end-face power monitor. • Nine controls: core index, core-minus-cladding index, core radius, illustrated segment length, external launch angle, vacuum wavelength, a ±3° launch sweep and two marker toggles. • Twelve readouts including numerical aperture, air acceptance half-angle, incidence at the core boundary, critical angle, boundary encounters, end power, normalized frequency and transit time. • Four guided experiments, a verification bench, Curves & measurements and a Learn & assess tab.
The cladding index is nclad = ncore − Δn. The numerical aperture is NA = √(ncore² − nclad²), and light entering from air is accepted up to θaccept = asin(NA). Inside, the ray angle from the axis is α = asin(sin θlaunch / ncore), so the incidence angle at the core boundary is 90° − α. Total internal reflection holds when that angle exceeds the critical angle θc = asin(nclad/ncore).
Within the acceptance cone the ray alternates between reflections without loss at the wall; beyond it, each wall encounter becomes leaky and the end-face power falls. With zero index contrast the NA is zero and nothing is guided.
This is geometric ray tracing for a straight, circular, step-index fiber with a centered meridional launch from air and separate s and p power accounting. It is not a modal field solution: no bending, scattering, absorption, dispersion, skew rays or wave interference are included, and the longitudinal and transverse drawing scales differ. A low normalized frequency V is flagged because a many-mode ray picture stops being reliable there.
The core is the higher-index glass that carries the light, and the cladding is a lower-index layer around it. The index difference makes rays striking the boundary at shallow angles reflect totally back into the core.
NA = √(ncore² − nclad²) measures how steeply light can enter and still be guided. Its arcsine is the acceptance half-angle in air, about 13.93° in the simulator's centered-launch experiment.
The ray hits the wall at an angle below the critical angle, so each encounter partly transmits into the cladding. You can see escaped power and decreasing ray markers.
No. Acceptance depends only on the index values. A wider core reduces the number of boundary encounters and increases the normalized frequency V, but the cone stays the same.