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Mirrors are popular optical elements due to their light weight and because they do not introduce chromatic aberration. This is particularly true in the world of large optical telescopes. The majority of large-aperture telescopes, greater than about 1 m in diameter, use reflectors rather than refractors as their primary optic. This is primarily due to the weight reduction provided by the reduced amount of glass in mirrors compared to lenses. The mirror is formed by coating the glass to improve its reflectivity.
Optical thin film coatings can be composed of dielectric or metallic layers. The index of refraction is often represented as a real number but is more correctly described as a complex number. The real component of the index of refraction describes the relationship between the speed of light in vacuum and the speed of light in the material. This real component determines how much the light is bent when entering a material at an angle.
Large telescope mirrors rely on metal coatings that are on the order of 100 nm thick. In certain lighting, these metal layers are translucent and pass light. As the coating increases in thickness, the amount of transmitted light is reduced and the reflected light increases. The amount of light passing through a material will always be less than the amount that entered. This attenuation in the amount of light is represented by the complex representation of the index of refraction. When the real and imaginary components are combined, the index of refraction takes on new meaning, and thin metal films become much more interesting than just acting as reflectors.
In this chapter we explore how to use the complex index of refraction to work with thin metallic films on a glass substrate.
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