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A first-surface mirror or front-surface mirror (also commonly abbreviated FS mirror or FSM) is a
mirror A mirror or looking glass is an object that Reflection (physics), reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the ...
with the reflective surface being above a backing, as opposed to the conventional, second-surface mirror with the reflective surface behind a transparent substrate such as
glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) o ...
or acrylic. Historically, the
bronze mirror Bronze mirrors preceded the glass mirrors of today. This type of mirror, sometimes termed a copper mirror, has been found by archaeologists among elite assemblages from various cultures, from Etruscan Italy to Japan. Typically they are round an ...
, an FSM type, was standard from ancient times until relatively recent centuries. These were simply highly polished pieces of
bronze Bronze is an alloy consisting primarily of copper, commonly with about 12–12.5% tin and often with the addition of other metals (including aluminium, manganese, nickel, or zinc) and sometimes non-metals, such as phosphorus, or metalloids such ...
or other metals, usually small and round, and designed for a person to see their face. First-surface mirrors are now made for applications requiring a strict reflection without a ''ghosting'' effect as seen with a second-surface mirror, where a faint secondary reflection could be observed, coming from the front surface of the glass. This includes most optics applications where light is being manipulated in a specific manner.
Reflecting telescope A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternat ...
s,
rear-projection television Rear-projection television (RPTV) is a type of large-screen television display technology. Until approximately 2006, most of the relatively affordable consumer large screen TVs up to used rear-projection technology. A variation is a video projec ...
s,
periscope A periscope is an instrument for observation over, around or through an object, obstacle or condition that prevents direct line-of-sight observation from an observer's current position. In its simplest form, it consists of an outer case with ...
s, non-reversing mirrors, high-quality
kaleidoscope A kaleidoscope () is an optical instrument with two or more reflecting surfaces (or mirrors) tilted to each other at an angle, so that one or more (parts of) objects on one end of these mirrors are shown as a regular symmetrical pattern when v ...
s, and the animation process. In cases where the mirror is subjected to extreme cold (as low as 33 K as in the
James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope which conducts infrared astronomy. As the largest optical telescope in space, its high resolution and sensitivity allow it to view objects too old, distant, or faint for the Hubble Spa ...
), a polished pure
beryllium Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to for ...
mirror is used without a first-surface coating in order to eliminate deformations caused by differing coefficients of
thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kinetic ...
.


Silvering

The "
silvering Silvering is the chemical process of coating a non-conductive substrate such as glass with a reflective substance, to produce a mirror. While the metal is often silver, the term is used for the application of any reflective metal. Process Mo ...
" on a front-surface mirror is usually
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ...
for
visible light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
and
gold Gold is a chemical element with the Symbol (chemistry), symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a Brightness, bright, slightly orange-yellow, dense, s ...
for
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from arou ...
radiation.


References

Mirrors {{optics-stub