固体材料的宏观光学性质.pptVIP

第四章 固体材料的宏观光学性质 Overview The study of the optical properties of materials is a huge field and we will only be able to touch on some of the most basic parts So we will consider the essential properties such as absorption/reflection/transmission and refraction Then we will look at other phenomena like luminescence and fluorescence Finally we will mention applications, in particular optical fibres and lasers Nature of light Light is an electromagnetic wave: with a velocity given by c?=?1/?(?0?0) = 3 x 108 m/s In view of this, it is not surprising that the electric field component of the wave should interact with electrons electrostatically Many of the electronic properties of materials, information on the bonding, material composition etc. was discovered using spectroscopy, the study of absorbed or emitted radiation evidence for energy levels in atoms evidence for energy bands and band-gaps photoelectric effect General description of absorption Because of conservation of energy, we can say that I0 = IT + IA + IR Io is the intensity (W/m2) of incident light and subscripts refer to transmitted, absorbed or reflected Alternatively T + A + R = 1 where T, A, and R are fractions of the amount of incident light T = IT/I0, etc. So materials are broadly classed as transparent:relatively little absorption and reflection translucent:light scattered within the material (see right) opaque:relatively little transmission If the material is not perfectly transparent, the intensity decreases exponentially with distance Consider a small thickness of material, ?x The fall of intensity in ?x is ?I so ?I = -a.?x.I where ? is the absorption coefficient (dimensions are m-1) In the limit of ?x ? 0, we get The solution of which is I?=?I0?exp(–?x) Taking “ln” of both sides, we have: which is known as Lambert’s Law (he also has a unit of light intensity named for him) Thus, if we can plot -ln(I) against x, we should find ? from the gradient Depending on the material and the wavele