Reflection and filtering at optical component interfaces

In Section 10.2 we briefly mentioned that optical elements are not perfect insofar that the physical laws governing reflection and transmission tend to be limiting factors in ideal experiments . 

Let us briefly recapitulate what happens when hght is incident on the boundary between two media, and speciflcally at polished optical surfaces. It was shown that some hght is reflected and some is transmitted (undergoing refraction) into the second medium, and that this phenomenon followed physical laws, which encompass direction (angle), phase, polarization and relative amplimde of the reflected and transmitted light. 

When a beam of light is incident on a plane surface at normal incidence, the relative intensity of the reflected light /r as a fraction of the incident light Iq is given by 

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