This simulator sends a ray from a dense medium toward a less-dense one and sweeps the incidence angle past the critical angle, where refraction becomes mathematically impossible and all the light reflects back into the dense medium instead.
• A ray diagram showing the incident ray from the dense medium, the refracted ray while below the critical angle, and the internally reflected ray once past it. • Three live controls: incidence angle (0°-89°), n₁ for the dense medium (1.10-2.60) and n₂ for the less-dense medium (1.00-1.60). • A live critical-angle readout, computed as sinθc = n₂/n₁, marked directly on the diagram. • A mode readout that switches between "Refracting" and "Total internal reflection" as you cross the critical angle. • Four preset material pairs: glass-to-air, water-to-air, diamond-to-air, and glass-to-water.
The critical angle sinθc = n₂/n₁ only has a valid solution when n₁ > n₂ — going from a denser to a less-dense medium. Beyond that angle, Snell's law would require sinθ₂ to exceed 1, which is impossible for a real angle, so no light can refract through at all; 100% of it reflects internally. Going the other direction (less-dense into denser) never produces total internal reflection, no matter how steep the angle.
Optical fibers rely entirely on total internal reflection to guide light down a glass or plastic core with essentially no loss at each internal bounce, as long as the light stays within the fiber's numerical aperture (its critical-angle window). Diamonds appear especially brilliant because their very high refractive index (about 2.42) gives them an unusually small critical angle, trapping light inside through many internal reflections before it escapes.
When light traveling in a denser medium hits a boundary with a less-dense medium at an angle steeper than the critical angle, no refraction is possible — all the light reflects back into the denser medium instead of transmitting through.
sin θc = n₂ / n₁, where n₁ is the denser medium's refractive index and n₂ is the less-dense medium's index. This only has a solution when n₁ is greater than n₂.
Light injected into a glass fiber core at a shallow enough angle relative to the fiber wall repeatedly undergoes total internal reflection, letting it travel long distances down the fiber with very low loss at each bounce.
No. The critical-angle condition requires n1 to exceed n2; when light moves from a lower-index medium into a higher-index one, it always refracts through at some angle, never totally reflects.