Absorption
Absorption refers to the process by which energy, such as light or sound, is taken in and retained by a substance rather than being reflected or transmitted. In the context of physics, absorption is a fundamental concept in the study of wave behavior and the interaction of electromagnetic radiation with matter. It plays a crucial role in various phenomena, including the heating of materials and the creation of color.
5 Key excerpts on "Absorption"
eBook - ePub- George Wypych, George Wypych(Authors)
- 2015(Publication Date)
- ChemTec Publishing(Publisher)
...Scattering effects diminish the amount of energy which is transferred to the material. Compared specimens must have a similar surface structure. The wavelength of incident radiation must also be the same for any set of samples compared within the range of the experiment. 2.2 Absorption OF RADIATION BY MATERIALS Materials exposed to sunlight receive a very wide spectrum of energy levels. These include the high energy of UV radiation, the lower energy of visible light, and the even lower energy level of infrared radiation. The energy of incoming radiation is quantified such that Absorption occurs in one and only one step where all the energy of a single photon is either absorbed or rejected (a quantum of photon energy cannot be divided). This restriction determines which specific wavelength of radiation is absorbed. When UV radiation energy is absorbed by a molecule, the molecule attains an excited state but only when the difference energy between the states before and after Absorption equals hν. The quantity of energy absorbed determines whether a bond can be broken (see Table 2.2). The amount of energy carried by a particular photon (determined by its wavelength) must exactly match the level of energy required by the electronic structure of the molecule for it to absorb this photon and elevate it to its excited state. The difference between the normal and the excited states must be equal hν. Although this explains the selectivity of this process, it does not fully reflect the complexity of all the processes which occur on exposure. The energy of radiation may cause different molecular transitions. We have described the so-called electronic transitions in which the position of an electron in the molecule may change from molecular bonding to non-bonding or anti-bonding. These are very drastic changes in molecular structure and they usually require the highest energy levels available in the UV range...
eBook - ePub- Z. Maekawa, Jens Rindel, P. Lord(Authors)
- 2010(Publication Date)
- CRC Press(Publisher)
...4 Sound Absorption Materials and construction The Absorption coefficient is a useful concept when using geometrical acoustic theory to evaluate the growth and decay of sound energy in a room. Any material absorbs sound to some extent. However, when sound is considered as a wave motion it is necessary to use the concept of acoustic impedance. In this chapter the fundamental characteristics of the terms which describe sound Absorption, an outline of the performance of various absorbing materials, details of construction and their practical application in architecture are discussed. 4.1 Types of sound Absorption mechanisms Absorptive materials and construction can be divided into three fundamental types as shown in Figure 4.1. A. Porous Absorption When sound waves impinge on a porous material containing capillaries or continuous airways, such as are found in glass wool, rock wool and porous foam, they propagate into the interstices where some of the sound energy is dissipated by frictional and viscous losses within the pores and by vibration of small fibres of the material. The Absorption is large at high frequencies and small at low frequencies. B. Panel and membrane type Absorption An impervious material, such as a thin plywood panel, painted canvas, etc., causes sound-induced vibration. A portion of the energy incident upon the material is dissipated by the internal loss of the vibrating system. The sound Absorption characteristics produce a peak in the low frequency range; however, the peak is not very high unless a porous material is inserted in the back cavity. Figure 4.1 Absorption mechanisms and characteristics outline. C. Resonator Absorption Sound incident on a resonator, consisting of a cavity with an opening, excites large-amplitude air vibrations at the opening in the resonant-frequency range, dissipating the sound energy by means of viscous losses...
eBook - ePub- Simonpietro Agnello, Simonpietro Agnello(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
...This is defined by (wavelength) −1, 1/ λ, and, using Eq. (1.18), it is shown that Figure 1.2 Bottom: Typical Absorption spectrum registered as a function of wavelength. Top: Representative experimental Absorption (continuous line) and emission (dashed line) spectra registered as a function of wavelength. (1.20) The wavenumber is usually reported in units of cm −1. Combining Eqs. (1.19) and (1.20), it is found that (1.21) Concluding this paragraph, it is worth mentioning that the Absorption phenomenon is one of the basic processes of the radiation–matter interaction and it is extended in a wide range of energy of the electromagnetic spectrum. The underling physical process is related to the specific atomic or molecular species absorbing the energy from the electromagnetic wave [ 8, 9 ]. The frequency range of interest for this chapter includes the visible (Vis) radiation and goes from the near infrared (NIR) to the ultraviolet (UV). In particular, the visible range in vacuum extends in frequency from about 3.8⋅10 14 to 7.5⋅10 14 Hz, in wavelength from 800 to 400 nm, and in energy from 1.6 to 3.1 eV [1]. 1.1.2 Emission: Fluorescence and Phosphorescence The Absorption of light at a given wavelength λ by a sample is physically associated to an energy transfer from the radiation electromagnetic field to the electrons of the matter constituting the sample. In this phenomenon, the electrons are typically promoted from one energy level (ground state) to another level of higher energy (excited state). In the process, the electron system is put out of thermal equilibrium; so, each electron spontaneously tends to return to its initial energy level, releasing the acquired energy. In an ideal experiment, a stationary state can be observed in which by continuously illuminating the sample with a radiation at λ, in a given spatial direction, another radiation is emitted by the sample isotropically in space with wavelength λ′ > λ...
eBook - ePub- Irving P. Herman(Author)
- 1996(Publication Date)
- Academic Press(Publisher)
...CHAPTER 8 Transmission (Absorption) Transmission through, and reflection from, a medium both depend on the refractive (n) and absorptive (k) properties of the medium. Consequently, it is not always straightforward to classify interactions into the separate categories of transmission, reflection, and Absorption spectroscopies. Transmission clearly monitors Absorption in gases (except for the reflection from the chamber windows). In probing solids, however, transmission depends on the reflections at the ambient/material interfaces as well as on Absorption in the material. This chapter covers spectroscopies that probe Absorption in a medium by transmission, while Chapter 9 discusses most reflection spectroscopies, whether they depend primarily on refractive or absorptive effects. For example, infrared reflection-Absorption spectroscopy (IRRAS) and attenuated total internal reflection (ATR) spectroscopy, which probe the region near the surface, are treated in Section 9.7. However, probes that involve a roundtrip transmission through a wafer, which is aided by a single reflection at a wafer surface, are closely tied to “single-pass” transmission probes and thus are included in this chapter (Section 8.4). This chapter covers only those “Absorption” spectroscopies in which resonant light enters the medium and is partially absorbed, after which the transmitted beam is detected. Spectroscopies in which the Absorption of light is monitored by other means are treated in other chapters. For example, in laser-induced fluorescence (LIF, Chapter 7) and photoluminescence (PL, Chapter 14), Absorption induces emission of light at a different wavelength, which is then detected. In some cases the Absorption of light leads to the production of electrons, as in photoionization of atoms and molecules, including resonant multiphoton ionization and optogalvanic spectroscopy of plasmas, and visible/near-ultraviolet photoelectron spectroscopy of solids (photoemission)...
eBook - ePub- Vadim Backman, Adam Wax, Hao F. Zhang(Authors)
- 2018(Publication Date)
- CRC Press(Publisher)
...We also see it in everyday life; for example, it explains why the color of water changes with depth. Contrary to elastic scattering, Absorption is a function of chemical composition of tissue and not so much of its structure (although we will see in Chapter 6 that the spatial organization of absorbers does play a part in how much light gets absorbed). Although most tissue molecules have light-absorbing properties, most of the molecules have a negligible Absorption cross-section; thus, their light-absorbing properties do play a significant part in light–tissue interactions. A handful of molecules, however, absorb light efficiently—typically within a specific bandwidth. The single most prominent absorber in the visible part of the spectrum is hemoglobin contained in red blood cells or erythrocytes. Hemoglobin absorbs prominently in the blue and green parts of the spectrum, thus giving perfused tissue its pinkish color. (What is your skin color when you blush?) This property of hemoglobin makes it easy to detect by means of visible light spectroscopy. Other absorbers in the visible spectrum include bilirubin and beta-keratin. In the near-infrared part of the spectrum, however, hemoglobin no longer has the dominant position in terms of light Absorption, and other molecules may have comparable Absorption, such as lipids. This is the principle behind near-infrared (NIR) tissue spectroscopy and NIR optical tomography that can measure or image concentrations of substances such as hemoglobin and lipids in tissue. Other interactions include fluorescence, a variety of inelastic scattering phenomena (i.e., a scattering event in which the wavelength of light changes as a result of the scattering) such as Raman scattering, and dynamic inelastic scattering (e.g., Brillouin scattering)...
