How to use (3 steps)
- Select the mode: refraction (Snell's law) or critical angle / total internal reflection.
- Enter the refractive indices and angles. Pick which quantity is unknown—the field turns off automatically.
- Tap Compute to see the solved value, the critical-angle check, and the step-by-step log. Copy URL shares the exact setup.
A common air → glass example is preloaded and computed on page load so you can see the results immediately.
Inputs
Angles are measured from the normal (0–90°). Values stay on this device.
Results
Ray diagram
Angles are measured from the normal; illustration not to scale.
How it is calculated
FAQ
What is Snell's law?
It states n₁ sin θ₁ = n₂ sin θ₂, linking indices and angles measured from the normal. Enter any three of n₁, n₂, θ₁, θ₂ and the tool solves the last one.
When does total internal reflection happen?
When light goes from a higher to a lower refractive index and the incidence angle exceeds the critical angle. The calculator shows the critical angle and whether your chosen incidence angle triggers total internal reflection.
How accurate are the presets?
Air, water, and glass presets use typical values and ignore small dispersion or temperature effects. They are intended for quick learning examples rather than optical design.
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