Snell's Law · Critical angle · Numerical Aperture · V-number · Single vs multi-mode
Light confined inside a glass core by total internal reflection — no mirrors, no metal, just Snell's law at a glass-glass interface. This simulation lets you explore every key optical fiber parameter in real time.
When light hits the core-cladding boundary at an angle greater than the critical angle θc = arcsin(n2/n1), 100% of the energy reflects back. The numerical aperture NA = √(n1²−n2²) defines the cone of angles that can enter and be guided. The V-number V = πdNA/λ determines how many modes fit: single-mode fibers (V < 2.405) carry one electromagnetic mode, eliminating modal dispersion and allowing Tbit/s data rates over thousands of kilometres.
Use the preset buttons to switch between single-mode (thin core, small index difference) and multi-mode (large core, many bouncing rays). Drag the Core RI slider above the Cladding RI — when they are close the acceptance cone narrows and fewer rays are guided. Watch the V-number: when it drops below 2.405 only one ray path survives. The acceptance angle shows the maximum tilt from the axis at which you can shine a light source into the fiber.
The global internet runs on optical fiber — a single hair-thin glass strand carries over 100 Tbit/s. Fiber optics were commercially deployed in 1977; today there are over 1 billion km of installed fiber worldwide — enough to circle the Earth 25 000 times. Medical endoscopes use bundles of thousands of fibers so surgeons can see inside the body without open surgery. The 2009 Nobel Prize in Physics was awarded to Charles Kao for his pioneering work on light transmission in optical fibers.