# Light reflection from a charged surface

Will we see differences (in intensity or spectrum) of visible light reflection from surface of positively versus negatively charged metal plate (or any non-conducting materials)? Are there data about such experiments?

It is unlikely that you would be able to see a change in reflectivity. Note that to change the electron density appreciably would require an unphysical amount of charge.

• Agree, some other experimental setup is needed to test the principle.
– VYT
Commented Nov 6, 2020 at 10:57

In case only charges are opposite, while masses remain the same, I would guess that you would only see the phase difference of $$\pi$$ in the reflected wave in respect to the reflected wave with the original charges.

If masses are different, you can think of how mass is influencing the charge movement upon electric field, and I think you will find the answer.

• I would like to see the experimental data. (It would be strange if no such experiments were done.)
– VYT
Commented Nov 27, 2018 at 11:10
• I doubt it is feasible to conduct such experiment. Think of its requirements. On the other side, you can find answer through thought experiments. As for the non-conducting materials, that's very well established field. Check things such as plasma frequency, fluorescence, phosphorescence and most important $\textbf{Ewald's extinction theorem}$. Here is the approach, I like the most, about the Ewald's theorem: aapt.scitation.org/doi/10.1119/1.18315 Commented Nov 27, 2018 at 11:57
• What is the problem with feasibility of the experiment? Charge the aluminum plate (for instance using capacitor), take laser pointer (red light) and light intensity detector (for red color), so the reflecting light luminosity from plate can be measured as the luminosity of the reflected spot.
– VYT
Commented Nov 27, 2018 at 13:02
• In case of metal not charged, the reflection is governed by the mobility of the electron gas, which is loosely bonded via electric field to the positively charged atom cores. This bonding tells you that the electrons will oscillate upon some disturbance. The oscillation frequency of the electrons is called plasma frequency. As long as the light frequency is lower than the plasma frequency (typically in UV), the metal is reflective. Now, if you add some more electrons to the metal, I have impression that the plasma frequency would shift deeper into UV. Commented Nov 27, 2018 at 13:12
• On the other hand, if you take away some electrons, but leave majority of them, there will still be enough of them to oscillate, but would also be stronger bonded to the cores, I get impression the plasma frequency would go towards infrared. In an extreme case, if you take all of the free electrons away, you would be having a positively net charged plate, but you would have no free charges that could oscillate and reflect the incident light. Commented Nov 27, 2018 at 13:17