The critical angle can be intuitively understood by Snell's law.If the incident medium has a bigger diffraction index than the refracted medium then according to Snell's law the refracted ray will be inside the incident medium which means it doesnt leave it in the first place as show in the picture:

However why is the ray reflected only at the angle of incidence?I dont get it.Shouldnt part of the ray be reflected at the angle of incidence and part of the ray be "reflected" according to Snell's law?Thanks


2 Answers 2


(1) Reflection is always at the same angle as the incident angle. Snell's law doesn't hold for reflection.

(2) Snell's law gives the critical angle above which total reflection, i.e., no refraction, occurs, because at the critical angle the refraction angle is 90 degrees and cannot get larger because there is no sine larger than 1.

(3) Thus Snell's law cannot hold for larger incident angles than the critical angle.


bigger diffraction index

You probably meant "bigger refractive index".

the refracted ray will be inside the incident medium which means it doesnt leave it

This statement seems debatable. Even in the case of total internal reflection the wave penetrates the medium with lesser index (although its amplitude decreases exponentially with the distance from the interface), so the refracted wave still exists, although one can dispute whether it's a "ray". For example, @freecharly says that "there is no sine larger than 1". This is true for real arguments but not for complex arguments. I believe the Snell law (maybe properly modified) works for total internal reflection if we extend the formulas to complex "angles".


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