How this calculator works
Light changes direction when its phase speed changes at an interface. Snell law relates the sine of the angle on each side to the refractive indices. When light tries to enter a lower-index material above the critical angle, no real transmitted angle exists and the result reports total internal reflection. Material presets are approximate visible-light values; enter measured indices for wavelength-dependent work.
Formula
n1 sinθ1 = n2 sinθ2; θcritical = asin(n2/n1) when n1 > n2.
Worked example
With n1 = 1, n2 = 2 and incidence at 90 degrees, the limiting refracted angle is 30 degrees. Going from index 1.5 to 1 gives a critical angle of about 41.81 degrees.
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Frequently asked questions
Are angles measured from the surface?
They are measured from the normal, so a ray along the surface has an angle of 90 degrees.
Why is there no refracted angle above the critical angle?
The required sine would exceed one. The transmitted field is evanescent rather than a propagating ray.
Are the material presets exact?
No. Index depends on wavelength, temperature, composition and sometimes polarization.