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
Pure copper is orange-red and acquires a reddish tarnish when exposed to air. This is due to the low plasma frequency of the metal, which lies in the red part of the visible spectrum, causing it to absorb the higher-frequency green and blue colors.