Optical absorption of small copper particles and the optical properties of copper.

Small copper particles in a silicate glass have an optical absorption band at 0.565 microm. Transmission electron micrographs of the particles show they are ~4.5 nm in diameter. The measured optical absorption agrees closely with that calculated from measurements by Otter and Roberts of the optical properties of copper, in which they are corrected for the small size of the particles. Optical absorption by small metallic particles can help one to choose the most reliable measurements of optical properties of the bulk metal.