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.

  • $\begingroup$ Beyond the critical angle, refraction out of the medium is impossible. $\endgroup$ – probably_someone Sep 14 '18 at 13:15
  • 1
    $\begingroup$ Because the equations require it. $\endgroup$ – Jon Custer Sep 14 '18 at 13:15
  • 4
    $\begingroup$ 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 $k$. $\endgroup$ – Emilio Pisanty Sep 14 '18 at 13:19

you can see the Fresnel coefficients for reflectivity and phase-change behavior below for dense to light-medium propagation that cause TIR (total internal reflection): TE polarization: enter image description here TM polarization: enter image description here

As a mathematical answer, near critical angle phase matching forces boundary to have to mirror-like behavior so reflection has to jumps to 1.

  • $\begingroup$ Thank you for answering. What is theta B in the phase changing graph of TM wave? $\endgroup$ – Koreyuki Sep 15 '18 at 5:18
  • $\begingroup$ it's Brewster angle. in this angle, TM mode gets removed from reflection. $\endgroup$ – Persian_Gulf Sep 15 '18 at 8:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.