About the Critical Angle Calculator
Critical angle for total internal reflection between two media — the basis of optical fibres.
Also known as: total internal reflection, tir calculator, fibre optic angle.
How to use this calculator
- Enter refractive index (denser medium), refractive index (lighter medium).
- The result updates instantly as you type — press Calculate (or Enter) at any time.
- Review the breakdown shown under the main result.
- Use Copy result or Share to save the answer or send a link with your inputs.
Formula
θ_c = arcsin(n₂ / n₁)
Worked example
Using the values pre-filled in the calculator above:
- Refractive index (denser medium) = 1.5
- Refractive index (lighter medium) = 1
the calculator returns:
- Critical angle: 41.8103 °
- Numerical aperture: 1.118
- Acceptance angle: 90 °