Refraction, reflection and absorption of light

Since the refractive index itself is a function of frequency, reflectance is also a function of the wavelength of radiation used. When one of the medium is vacuum or air ( vac is 1.000 at 15°C and 760 mm pressure for the Na-D line - /l=589.3nm - and which is 1.000275, under same conditions), reflectance can be written as ( -/) /( +/), where n is the refractive index of the medium. 

From Maxwell s relations, it can be shown that at very high frequencies = n. Therefore, refractive index, n itself can be written as a complex quantity. 

For convenience (and to avoid confusion) we write n as . k is known as the extinction coefficient and is responsible for attenuation of light in the same manner as is responsible for attenuation of electrical field causing dielectric loss. 

The general expression for attenuation or the intrinsic loss of light intensity in a glass is given by, 

There is the extreme situation where the refractive index for the given frequency is dominated by ic so that R tends to unity as it happens in metals like in molten Ge. It may be noted that both amorphous and crystalline Ge have reflectances of 38% at low frequencies, while it tends to 100% for molten Ge. This is attributed to the drastic changes in the local structure of Ge whereby the local coordination number changes from 4 in solid to 8 in the melt which is metallic. 

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