The velocity of propagation through the solid of complex refractive index N = n - ik is related to the speed of light in a vacuum, c, by V = C / N, then: Therefore, substituting 1/V into equation above produces: is a measure of the damping factor, or extinction coefficient (k). The more light a material absorbs, the higher its absorption coefficient will be. Absorption and extinction coefficient theory. In addition to this, boundary conditions are required to be defined to ensure waves in the two media match at the interface. https://encyclopedia2.thefreedictionary.com/Absorption+Coefficient. If a plane wave of frequency (f) propagates through a solid with velocity (v) in a direction defined by (x), the electric field (E) is described by the following progressive wave equation: Where, (E0)is the incident electric field vector and. where . The absorption of light per unit length in a medium is often quantified with an absorption coefficient α, also called attenuation coefficient. This information should not be considered complete, up to date, and is not intended to be used in place of a visit, consultation, or advice of a legal, medical, or any other professional. (13) As was the case for the impedance, the absorption coefficient may be a function of frequency, and measurements over the frequency range of interest may be required. The effects of [D.sub.t] and [D.sub.f] on the sound absorption coefficient [alpha] are plotted in Figure 19. From Maxwell's equations on electromagnetic theory, the speed of light in a vacuum c is related to the permittivity of free space ε0, (the degree to which a medium can resist the flow of charge, defined by the ratio of the electric displacement to the intensity of the electric field that produces it), and the permeability of free space µ0 (the ratio of the magnetic flux density in a solid to the external magnetic field strength inducing it, µ = B/H.) is the displacement at time t after a disturbance, created by the electric field at a point situated at x along the line of propagation. I a = sound intensity absorbed (W/m 2). As the power (P) or intensity of an incident wave through a solid is the conductivity (σ) of the solid multiplied by the square of the electric field vector (P=σE2), then using the damping factor term, the fraction of the incident power that has propagated from position (o) to a distance (x) through the material with conductivity (σ) is given by: from which the absorption coefficient (α) can be expressed in terms of the extinction coefficient (k) as: As the velocity of light in a vacuum, c = fλ, then α = 4πk/λ, and the power or intensity is P = Poexp-αx. The absorption coefficient describes how much light of a given color is absorbed by a material of a given thickness. The velocity of propagation of a electromagnetic wave through a solid is given by the frequency-dependent complex refractive index N = n - ik where the real part, n is related to the velocity, and k, the extinction coefficient is related to the decay, or damping of the oscillation amplitude of the incident electric field. a The fraction of the sound energy (incident at random angles on a surface) which is absorbed or otherwise not reflected by the surface. The sound absorption coefficient is the ratio of absorbed sound intensity in an actual material to the incident sound intensity and can be expressed as. At normal incidence, η = y = Y N, while at oblique incidence where the incident wave becomes polarised. ηp = y/cosθ = YN/cosθ and ηs = ycosθ = NY cosθ. If a flux through a material decreases with distance. It has units of 1/cm (1/length), because it describes the amount of light absorbed per thickness of material. Absorption coefficient - α - for some common materials can be found in the table below: Also known as absorption factor; absorption ratio; coefficient of absorption. In the case of an absorbing material, the behaviour of a beam of radiation incident in a medium of refractive index n1 on an absorbing medium of complex refractive index n2 = n2-ik2, with an angle of incidence θ1, using Snell's law in complex form is then defined by: Coherence of multiple internal reflections, Find out more about our products and services, Find out more about our research interests, Spectral Characterisation of Infrared Optical Materials and Filters, Absorption and extinction coefficient theory. The velocity of propagation of a electromagnetic wave through a solid is given by the frequency-dependent complex refractive index N = n - ik where the real part, n is related to the velocity, and k, the extinction coefficient is related to the decay, or damping of the oscillation amplitude of the incident electric field. The optical admittance of free space, Y, is given by Y = 1/Z0 = (ε0/µ0)½ = 2.654x10-3Ω-1. 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