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About this tool
Interactive Snell's Law light refraction simulator with material refractive index controls, critical angle detection, and Total Internal Reflection.
The Light Refraction Simulator draws a laser hitting a boundary between two media and solves Snell's law, n₁ sin θ₁ = n₂ sin θ₂, for the refracted angle in real time as you drag the incident angle from 0° to 89°. Pick the incident and refracting medium from air (n = 1.0003), water (1.333), glass (1.52) or diamond (2.417) and the canvas redraws the incident, reflected and refracted rays against the normal line. When the geometry has no solution the tool flags Total Internal Reflection and reports the critical angle from sin θc = n₂ / n₁.
Open Light Refraction Simulator on AltFTool — it loads instantly in your browser.
Pick "Incident Medium 1 (n₁)" and "Refracting Medium 2 (n₂)" from Air (1.0003), Water (1.333), Glass (1.52) or Diamond (2.417).
Drag the "Incident Angle (θ₁)" slider anywhere from 0° to 89°; the canvas redraws the incident, reflected and refracted rays against the dashed Normal Line.
Read the Optical Telemetry panel for Refraction Angle (θ₂), Critical Angle (θc) and Relative Ratio (n₁/n₂) — θ₂ reads TIR once the ray is totally internally reflected.
θ₂, the critical angle and the n₁/n₂ ratio update on every slider step instead of after a submit.
When sin θ₂ exceeds 1 the refracted ray disappears and the reflected ray brightens, so the cutoff is something you see, not just read.
Either medium can be the incident one, so you can compare bending toward the normal and away from it with the same setup.
Snell's law states that n₁ sin θ₁ = n₂ sin θ₂, where n is a medium's refractive index and θ is the angle measured from the normal — the line perpendicular to the boundary, not the surface itself. The simulator solves it for θ₂ = arcsin((n₁/n₂) sin θ₁) each time you move the angle slider.
About 48.6°, from sin θc = n₂/n₁ = 1.0003/1.333. Beyond that angle light hitting the water surface from below is entirely reflected back into the water, which is why the surface looks mirrored when you are underwater looking up at a shallow angle.
Because glass has the higher refractive index (1.52 against 1.0003 for air), so sin θ₂ must be smaller than sin θ₁ to keep both sides of Snell's law equal. Going the other way, from the denser medium into the rarer one, the ray bends away from the normal and can reflect entirely once past the critical angle.
Only when light travels from a higher index into a lower one and the incident angle exceeds the critical angle — 41.15° for glass to air and 24.45° for diamond to air. If n₂ is greater than n₁ there is no critical angle at all, and the tool shows N/A.