# Reflectivity and absorption

by clanx
Tags: absorption, reflectivity
 P: 5 i was reading papers and also doing my own simulations for a reflectivity experiment. i noticed though that reflectivity peak say at 3eV will correspond to a absorption peak (in the complex dielectric) at about 3eV too. I'm just confused here because we all know that R + T +A = 1. But yet when R is high, it corresponds to a absorption peak too? what is wrong with my concept?
 Sci Advisor P: 1,477 What you observe is probably a manifestation of the Kramers-Kronig relations, that dictate that an absorption peak (i.e. a peak in the imaginary part of the refractive index) corresponds to an inflection point in the real refractive index (reflectivity). Claude.
 P: 5 yes I understand that. but what about the physical picture? say if we happen to have this material with 100% reflection, shouldn't the absorption be 0?
 P: 5 the 100% reflection is just a hypothetical case. for the general case I'm referring to can be found on many scholarly articles. here is one I found on Google http://www.scielo.br/img/fbpe/bjp/v29n4/a06f1.gif you can see clearly that a reflection peak corresponds to a complex dielectric peak at the same energy.
 Sci Advisor P: 3,628 What makes you think that a large imaginary part of the dielectric constant corresponds necessarily to large absorption?
 Sci Advisor P: 3,628 This effect is quite easy to observe btw. Make a line with a foil marker on a black sheet of paper or look at some crystals of a dye. The colour you see is the complementary colour of the one seen in transmission. Reflection is always high when there is a large difference between the refractive indices (whether real or imaginary isn't important). If the imaginary part is high, the light won't enter deeply into the material. However, absorption is nevertheless low, as most light is reflected. In former times, this was exploited to generate monochromatic light in the far IR. It is called Reststrahlen method.
 P: 5 hmm. because the imag dielectric can also be written as 2nk where k is the extinction coefficient, and the absorption coefficient is directly proportional to the extinction coefficient? is there something wrong with concept?
 Sci Advisor Thanks PF Gold P: 1,911 You will find a nice introduction here: http://www.tf.uni-kiel.de/matwis/ama...ne/r3_7_2.html This goes on for several pages; follow the links at the bottom.