# Why does reflectivity suddenly change around critical angle?

Why does reflectivity suddenly change around critical angle? For example, consider the situation when incident light travel in dielectrics A and go in to dielectric B. Assume the dielectric constant $\epsilon_A=2.25$ and $\epsilon_B=1.25$. Total internal reflection occur when incident angle is around 46,7 degree. I'm wondering why reflectivity arise so sharply and suddenly? The reflectiviy is almost 0 when incident angle < 30 degree. And suddenly arise around critical angle. This is weird.

P.S Sorry. I only focus on TM wave unconsciously.

• Beyond the critical angle, refraction out of the medium is impossible. – probably_someone Sep 14 '18 at 13:15
• Because the equations require it. – Jon Custer Sep 14 '18 at 13:15
• It doesn't. To first order in the difference between the incidence angle $\theta_i$ and the critical angle $\theta_c$, the reflectivity increases linearly, $R \approx k |\theta_i-\theta_c|$ for some constant $k$. – Emilio Pisanty Sep 14 '18 at 13:19