What is plasma frequency?

[DariusP]

Hello, I am having a very hard time understanding what is plasma frequency by reading definitions. I know, for example, that metals have a metallic bond and can be analysed like plasma because of an equal distribution of electrons in electron gas and positively charged atoms.

I know metals are very good at reflecting light but at certain frequency their reflectivity drops very fast and this frequency where the drop happens is, I think, what's called - plasma frequency.

The question: Why do metals reflect light and why at certain frequency they stop reflecting?

 

[tech99]

There is a description in the following video which is quite easy to follow:-