The thin silver behind a reflection

A coating does the optical work

Silver-coated mirrors feature in astronomical engineering, including the Gemini Observatory's protected-silver programme. A thin layer of metal does the reflecting, with other layers contributing adhesion or protection. Gemini's coating research compared performance across visible and infrared wavelengths and examined how protective layers affected it. That is why there is no single reflectivity figure that describes every silver mirror under every condition: the complete coating system matters.

The reflecting surface has a position

In a first-surface mirror, the reflective coating faces the incoming light. In a second-surface mirror, light passes through a transparent substrate before reaching the coating. That arrangement helps shelter the metal, but reflections from the front surface can produce a secondary, or ghost, image. Precision optical systems often use first-surface mirrors because this extra reflection can interfere with a clean optical path.

Protection matters

Silver exposed to sulphur-containing compounds can form dark silver sulphide on its surface. This tarnish changes the bright finish that makes the metal useful. Optical coatings also need protection against handling damage. A transparent dielectric overcoat can improve durability while retaining useful reflection. Protected silver therefore describes an engineered combination of layers, rather than a promise that the reflective surface can never deteriorate.

Read the whole specification

A LIGO specification offers a concrete example: its protected-silver coating for certain telescope mirrors calls for reflectivity above 98% at a wavelength of 1064 nanometres, with incidence below eight degrees. Those conditions are essential to the claim. The same document also specifies the coated face, usable aperture and durability requirements. Optical engineering assesses the complete surface in its intended setting, rather than choosing a metal by appearance alone.

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Sources & further reading

Sources checked 14 November 2026. Reference organisations do not endorse this website. Price examples are hypothetical and educational.