"Critical" in reflected and refracted rays

[Sabalaba]

Hello,

I didnt understand what the "critical" in critical rays are. I didnt understand the meaning.
Can someone help, please.

Thanks.

 

[nrqed]

Hello,

I didnt understand what the "critical" in critical rays are. I didnt understand the meaning.
Can someone help, please.

Thanks.

Hard to tell without more context. I assume you are talking about critical angles, for total internal reflection (ray coming from side 1 and $n_1>n_2$)? This is when the angle of refraction reaches 90 degrees and pretty much all the energy is reflected inside the material (I assume n1 > n2). Then we have $theta_2=90$ and you can find the critical angle inside the medium.

 

[Sabalaba]

Thank you for you help