Wikipedia contributors. (2023, December 27). Plasmon. In Wikipedia, The Free Encyclopedia. Retrieved March 11, 2024, from https://en.wikipedia.org/w/index.php?title=Plasmon&oldid=1192024201
In standard axiomatic set theory, by the principle of extensionality, two sets are equal if they have the same elements (that is, neither of them has an element not in the other). As a result, there can be only one set with no elements, hence the usage of “the empty set” rather than “an empty set”.
Some metals, such as copper and gold, have electronic interband transitions in the visible range, whereby specific light energies (colors) are absorbed, yielding their distinct color.