I'm aware that in Optics, the complex index-of-refraction $\eta = n+ik$ is used, which famously leads to the reflection property at an incident angle, i.e. Fresnel's law:


However, $k$ is the absorption (edit: extinction or attenuation) coefficient, indicating a loss of energy when the wave propagates through the medium. Fresnel's law refers to the reflection, so it is unclear to me why the absorption should have any effect on the reflected ray.

  1. So, why is it that $k$ plays a role in the reflection? (An argument using Maxwell's Equations would be perfect.)
  2. Has Fresnel's law (in particular, w.r.t. measuring the reflected ray), ever been validated by an experiment?
  • $\begingroup$ If you follow and believe the derivation from Maxwell's equations for a medium with purely real index, then you are almost done. The derivation is perfectly valid for complex index. Replace $n$ with $n+ik$ and calculate the modulus. Note that your equation for $R$ is only good when the incident angle is zero. $\endgroup$ – garyp Jun 20 '19 at 23:58
  • $\begingroup$ $k$ plays a role because it changes the phase of the polarization of the medium. The fields just inside the surface are not the same as they are for a pure real $n$, so by the boundary conditions, the fields just outside the surface must also be different. Fresnel's laws are tested billions of times every day in all sorts of optical equipment. If you have an anti-reflection coating on your eyeglasses, you are testing them yourself. $\endgroup$ – garyp Jun 21 '19 at 0:00

$k$ is the extinction coefficient. For metals it is what drive reflection. The cause of the reflection is that the wave cannot propagate in the metal - and therefor is extinguished. Your expression for R is derived from Maxwell's equation by Fresnel in the 19th century. His solution works incredibly well.

  • $\begingroup$ I guess different authors use different terms. When you say extinguished, what physical phenomenon is causing it? What is causing my concern is the following Wikipedia explanation: "Here, the real part n is the refractive index and indicates the phase velocity, while the imaginary part κ is called the extinction coefficient — although κ can also refer to the mass attenuation coefficient—[29]:3 and indicates the amount of attenuation when the electromagnetic wave propagates through the material.[1]:128" $\endgroup$ – Pål-Kristian Engstad Jun 20 '19 at 22:56
  • $\begingroup$ I agree with the Wikipedia quote. It is extinction, to be distinguished from absorption. $\endgroup$ – my2cts Jun 20 '19 at 23:02
  • $\begingroup$ Which one do you agree with, it should always be called extinction, and not attenuation, or vice versa? $\endgroup$ – Pål-Kristian Engstad Jun 20 '19 at 23:07
  • $\begingroup$ Attenuation is correct as well. $\endgroup$ – my2cts Jun 20 '19 at 23:08
  • $\begingroup$ So, if I understand you correctly, the extinction coefficient has nothing to do with any kind of absorption along the path, so it has nothing to do with Beer's law, it purely happens at the interface? $\endgroup$ – Pål-Kristian Engstad Jun 20 '19 at 23:10

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