An optical element formed of a plurality of materials includes a middle layer between a base material and a reflecting member so as to suppress stripping, cracking and the like of the optical surface due to the difference in coefficients of thermal expansion among the component materials, in the case where a temperature difference in the service environment or a temperature difference between a manufacturing environment and the service environment is large.

Soluble copper oxide powder capable of preventing a decrease in quality of a copper film formed by plating is disclosed. The copper oxide powder contains copper and impurities including sodium. A concentration of the sodium is not more than 20 ppm. The copper oxide powder is regularly supplied into a plating solution. A voltage is applied between an insoluble anode and a substrate immersed in the plating solution, thereby plating the substrate.

Soluble copper oxide powder capable of preventing a decrease in quality of a copper film formed by plating is disclosed. The copper oxide powder contains copper and impurities including sodium. A concentration of the sodium is not more than 20 ppm. The copper oxide powder is regularly supplied into a plating solution. A voltage is applied between an insoluble anode and a substrate immersed in the plating solution, thereby plating the substrate.

Soluble copper oxide powder capable of preventing a decrease in quality of a copper film formed by plating is disclosed. The copper oxide powder contains copper and impurities including sodium. A concentration of the sodium is not more than 20 ppm. The copper oxide powder is regularly supplied into a plating solution. A voltage is applied between an insoluble anode and a substrate immersed in the plating solution, thereby plating the substrate.

Soluble copper oxide powder capable of preventing a decrease in quality of a copper film formed by plating is disclosed. The copper oxide powder contains copper and impurities including sodium. A concentration of the sodium is not more than 20 ppm. The copper oxide powder is regularly supplied into a plating solution. A voltage is applied between an insoluble anode and a substrate immersed in the plating solution, thereby plating the substrate.