Editing Mirror (section)

===Reflectivity===
[[File:Four mirrors - dielectric aluminum silver and chrome.jpg|thumb|Four different mirrors, showing the difference in reflectivity. Clockwise from upper left: dielectric (80%), aluminium (85%), chrome (25%), and enhanced silver (99.9%). All are first-surface mirrors except the chrome mirror. The dielectric mirror reflects yellow light from the first-surface, but acts like an [[antireflection coating]] to purple light, thus produced a ghost reflection of the lightbulb from the second-surface.]]
[[File:Image-Metal-reflectance.png|thumb|Spectral [[reflectance]] curves for [[aluminium]] (Al), [[silver]] (Ag), and [[gold]] (Au) metal mirrors at normal incidence.]]
The reflectivity of a mirror is determined by the percentage of reflected light per the total of the incident light. The reflectivity may vary with wavelength. All or a portion of the light not reflected is [[Absorption (electromagnetic radiation)|absorbed]] by the mirror, while in some cases a portion may also transmit through. Although some small portion of the light will be absorbed by the coating, the reflectivity is usually higher for first-surface mirrors, eliminating both reflection and absorption losses from the substrate.

The reflectivity is often determined by the type and thickness of the coating. When the thickness of the coating is sufficient to prevent transmission, all of the losses occur due to absorption. Aluminium is harder and more resistant to tarnishing than silver, and will reflect 85 to 90% of the light in the visible to near-ultraviolet range, but experiences a drop in its reflectance between 800 and 900&nbsp;nm. Gold is very soft and easily scratched, but does not tarnish. Gold is greater than 96% reflective to near and far-infrared light between 800 and 12000&nbsp;nm, but poorly reflects visible light with wavelengths shorter than 600&nbsp;nm (yellow). Silver is expensive, soft, and quickly tarnishes, but has the highest reflectivity in the visual to near-infrared of any metal. Silver can reflect up to 98 or 99% of light to wavelengths as long as 2000&nbsp;nm, but loses nearly all reflectivity at wavelengths shorter than 350&nbsp;nm.

Dielectric mirrors can reflect greater than 99.99% of light, but only for a narrow range of wavelengths, ranging from a bandwidth of only 10&nbsp;nm to as wide as 100&nbsp;nm for [[tunable laser]]s. However, dielectric coatings can also enhance the reflectivity of metallic coatings and protect them from scratching or tarnishing. Dielectric materials are typically very hard and relatively cheap, however the number of coats needed generally makes it an expensive process. In mirrors with low tolerances, the coating thickness may be reduced to save cost, and simply covered with paint to absorb transmission.<ref name=walk1998/>