Interaction of Light With Matter PDF

INTERACTION OF LIGHT WITH MATTER The optical properties of engineering materials are useful in different applications. Ex.: domestic, medicine, astronomy, manufacturing Light is an electromagnetic wave or photon. In the classical sense, electromagnetic radiation is considered to be wave-like, consi...

INTERACTION OF LIGHT WITH MATTER The optical properties of engineering materials are useful in different applications. Ex.: domestic, medicine, astronomy, manufacturing Light is an electromagnetic wave or photon. In the classical sense, electromagnetic radiation is considered to be wave-like, consisting of electric and magnetic field components that are perpendicular to each other and also to the direction of 2𝜋 propagation a wave k= 𝜆 ` E B k (1) ) Optical property of a material Optical property of a material is defined as its interaction with electro- magnetic radiation in the visible. Electromagnetic spectrum of radiation spans the wide range from γ-rays with wavelength as 10-12 m, through x-rays, ultraviolet, visible, infrared, and finally radio waves with wavelengths as along as 105 m. Visible light is one form of electromagnetic radiation with wavelengths ranging from 390 to 770 nm (1) Light Interactions With Solids: The intensity IO of the beam incident to the surface of the solid medium must equal the sum of the intensities of the transmitted, absorbed, and reflected beams, denoted as IT, IA, and IR respectively, or IO = IT + IA + IR T: Transmissivity A = IA /I0 Absorptivity R = IR /I0 Reflectivity T~1 : Transparent T~0 : Opaque T+A+R=1 Question 1 Classify the interaction of visible light with material ? answer 1-Materials that are capable of transmitting light with relatively little absorption and reflection are called transparent materials i.e. we can see through them. 2- Translucent materials are those through which light is transmitted diffusely i.e. objects are not clearly distinguishable when viewed through.  3- Those materials that are impervious to the transmission of visible light are termed as opaque materials. These materials absorb all the energy from the light photons Example (1): Distinguish between materials that are opaque, translucent, and transparent in terms of their appearance and light transmittance. answer Opaque materials are impervious to light transmission; it is not possible to see through them. Light is transmitted diffusely through translucent materials (there is some internal light scattering). Objects are not clearly distinguishable when viewed through a translucent material. Virtually all of the incident light is transmitted through transparent materials, and one can see clearly through them. 1- Light Absorption – Lambert-Beer Law* The last expression may be generalized for any cuvette with light path- length l, where k is a coefficient which depends on concentration, c, and molar absorptivity, ε Absorption : Nonmetallic materials may be opaque or transparent to visible light; and, if transparent they often appear colored. In principle, light radiation is absorbed in this group of materials by two basic mechanisms, which also influence the transmission characteristics of these nonmetals. One of these is electronic polarization. Absorption by electronic polarization is important only at light frequencies in the vicinity of the relaxation frequency of the constituent atoms. The other mechanism involves valence band-conduction band electron transitions, which depend on the electron energy band structure of the material.  Absorption of a photon of light may occur by the promotion or excitation of an electron from the nearly filled valence band, across the band gap, and into an empty state within the conduction band Optical Properties of Metals: Metals are opaque because the incident radiation having frequencies within the visible range excites electrons into unoccupied energy states above the Fermi energy, as demonstrated in Figure (a) below. Total absorption is within a very thin outer layer, usually less than 0.1 µm thus only metallic films thinner than 0.1 µm are capable of transmitting visible light. In fact, metals are opaque to all electromagnetic radiation on the low end of the frequency spectrum, from radio waves, through infrared, the visible, and into about the middle of the ultraviolet radiation. Metals are transparent to high-frequency (x- and γ-ray) radiation. All frequencies of visible light are absorbed by metals because of the continuously available empty electron states, which permit electron transitions. Most of the absorbed radiation is reemitted from the surface in the form of visible light of the same wavelength, which appears as reflected light. Aluminum and silver are two metals that exhibit this reflective behavior. Copper and gold appear red-orange and yellow, respectively, because some of the energy associated with light photons having short wavelengths is not reemitted as visible light.  Metals are, however, transparent to high end frequencies i.e. x-rays and γ-rays.  Absorption of takes place in very thin outer layer. Thus, metallic films thinner than 0.1 μm can transmit the light.  The absorbed radiation is emitted from the metallic surface in the form of visible light of the same wavelength as reflected light. The reflectivity of metals is about 0.95 Atomic And Electronic Interactions: The optical phenomena that occur within solid materials involve interactions between the electromagnetic radiation and atoms, ions, and/or electrons. Two of the most important of these interactions are electronic polarization and electron energy transitions. Refraction : Light that is transmitted into the interior of transparent materials experiences decrease in velocity and as result is bent at the interface; this phenomenon is termed refraction. The index of refraction   Snell’s law of light refraction Example 2: Compute the velocity of light in diamond, which has a dielectric constant 𝜖𝑟 of 5.5 (at frequencies within the visible range) and a magnetic susceptibility of -2.17 x 10-5. Solution: 2-Reflection: When light radiation passes from one medium into another having a different index of refraction, some of the light is scattered at the interface between the two media even if both are transparent. The reflectivity R represents the fraction of the incident light that is reflected at the interface, where I0 and IR are the incident and reflected beams intensities respectively. If the light is normal (or perpendicular) to the interface, then where and are the indices of refraction of the two media. If the incident light is not normal to the interface, R will depend on the angle of incidence Materials with a high index of refraction have a higher reflectivity than materials with a low index. Because the index of refraction varies with the wavelength of the photons, so does the reflectivity. The high reflectivity of metals is one reason that they are opaque. High reflectivity is desired in many applications including mirrors, coatings on glasses, etc Question 2: Describe briefly the Absorption mechanisms Answer: Rayleigh scattering: where photon interacts with the electrons, it is deflected without any change in its energy. This is significant for high atomic number atoms and low photon energies. Ex.: Blue color in the sunlight gets scattered more than other colors in the visible spectrum and thus making sky look blue. Tyndall effect is where scattering occurs from particles much larger than the wavelength of light. Ex.: Clouds look white.  Compton scattering – interacting photon knocks out an electron loosing some of its energy during the process. This is also significant for high atomic number atoms and low photon energies.  Photoelectric effect occurs when photon energy is consumed to release an electron from atom nucleus. This effect arises from the fact that the potential energy barrier for electrons is finite at the surface of the metal. Ex.: Solar cells. Example (3): The index of refraction of quartz is anisotropic. Suppose that visible light is passing from one grain to another of different crystallographic orientation and at normal incidence to the grain boundary. Calculate the reflectivity at the boundary if the indices of refraction for the two grains are 1.544 and 1.553 in the direction of light propagation Example (4): The fraction of non-reflected radiation that is transmitted through a 5-mm thickness of a transparent material is 0.95. If the thickness is increased to 12 mm, what fraction of light will be transmitted? I𝑻 ′ = 𝑰𝟎 𝒆 −𝜷𝒙 , ln (IT' / I0') = − βx β =(−1 / x) ln IT ' / I0' = − (1/5mm) ln(0.95) = 1.026 x 10-2 mm-1 And computation of IT'/I0' when x = 12 mm , (IT / I0)= exp (− βx) =exp[−(1.026x10−2 mm−1 )(12mm)]= 0.88415 Transmission : The phenomena of absorption, reflection, and transmission may be applied to the passage of light through a transparent solid Examples 5: The transmissivity T of a transparent material 15 mm thick to normally incident light is 0.80. If the index of refraction of this material is 1.5, compute the thickness of material that will yield a transmissivity of 0.70. All reflection losses should be considered. Solution: Optical applications: Light interacts with a material in many ways. Depending on the material, its crystal-/micro-structure, and also on the characteristics of incident light, there are many phenomena occurs, which are known as optical phenomena. These include: 1. luminescence 2. lasers 00 3. thermal emission 4. photo-conductivity 5. optical fibers 1- Luminescence It is the process where a material absorbs energy and then immediately emits visible or near-visible radiation. It consists of electron excitation and then dropping down to lower energy states. If the emission of radiation occurs within 10 -8 sec. after excitation, the luminescence is called fluorescence, and if it takes longer than 10-8 s, it is known as phosphorescence.  Ordinarily pure materials do not display this phenomenon. Special materials called phosphors have the capability of absorbing high-energy radiation and spontaneously emitting lower-energy radiation. Ex.: some sulfides, oxides, tungstates, and few organic materials.  The intensity of luminescence is given as where I 0 – initial intensity of luminescence,  I – fraction of luminescence after time, t,  τ - relaxation time, constant for a material. Luminescence process is classified based on the energy source for electron excitation as photo-luminescence,  Photo-luminescence, cathode-luminescence, and electro- luminescence. Photo-luminescence Photo-luminescence occurs in fluorescent lamps. Arc between electrodes excites mercury in lamp to higher energy level. Electron falls back emitting UV light. Fluorescent lamps consist of a glass housing, coated on the inside with specially prepared tungstates or silicates. Ultraviolet light is generated within the tube from a mercury glow discharge, which causes the coating to fluoresce and emit white light. Antimony, Sb 3+ , ions provide a blue emission while manganese, Mn 2+ , ions provide an orange-red emission band. Cathode-luminescence: Cathode-luminescence is produced by an energized cathode which generates a beam of high-energy bombarding electrons. Applications of this include electron microscope; cathode-ray oscilloscope; color television Electro-luminescence: Electro-luminescence occurs in devices with p-n rectifying junctions which are stimulated by an externally applied voltage. When a forward biased voltage is applied across the device, electrons and holes recombine at the junction and emit photons in the visible range (mono- chromatic light i.e. singe color). These diodes are called light emitting diodes (LEDs). LEDs emit light of many colors, from red to violet, depending on the composition of the semiconductor material used. Ex.: GaAs, GaP, GaAlAs, and GaAsP are typical materials for LEDs. 2- Lasers : Laser is an acronym for light amplification by stimulated emission of radiation., which produce incoherent light, the light produced by laser emission is coherent.  The The emitted light has the same energy and phase as the incident light (= coherent) 3- Thermal emission ▪ When a material is heated, electrons are excited to higher energy levels, particularly in the outer energy levels where the electrons are less strongly bound to the nucleus. ▪ These excited electrons, upon dropping back to the ground state, release photons in process what is called thermal emission. ▪ During thermal emission a continuous spectrum of radiation is emitted with a minimum wavelength and the intensity distribution is dependent on the temperature. ▪ Higher the temperature, wider will be the range of wavelengths emitted. By measuring the intensity of a narrow band of the emitted wavelengths with a pyrometer, material’s temperature can be estimated 4- Photo-conductivity Bombardment of semiconductors by photons, with energy equal to greater than the band gap, may result in creation of electron-hole pairs that can be used to generate current. This process is called photoconductivity. It is different from photo-electric effect in the sense that an electron-hole pair is generated whose energy is related  to the band gap energy instead of free electron alone whose energy is related to the Fermi level. The current produced in photo-conductivity is directly related to the incident light intensity.  This phenomenon is utilized in photographic light meters. Cadmium sulfide (CdS) is commonly used for the detection of visible light, as in light meters.  Photo-conductivity is also the underlying principle of the photo-voltaic cell, known to common man as solar cell, used for conversion of solar energy into electricity 5- Optical fibers  Optical fibers have revolutionized the communication industry.  It primarily consists of core, cladding and coating. The core transmits the signals, while the cladding constrains the light beam to the core; outer coating protects the core and cladding from the external environment.  Typically both the core and cladding are made of special types of glass with carefully controlled indices of refraction.  The indices of refraction are selected such that  Once the light enters the core from the source, it is reflected internally and propagates along the length of the fiber.  Internal reflection is accomplished by varying the index of refraction of the core and cladding glass materials. Usually two designs are employed Types of optical fibers  In step-index optical fiber, there is a sharp change in refractive index between the core and cladding. In this design output pulse will be broader than the input one. It is because light rays traveling in different trajectories have a variety of path lengths.  It is possible to avoid pulse broadening by using graded- index fiber. This results in a helical path for the light rays, as opposed to zig-zag path in a step-index fiber.  Here impurities such as boron oxide (B2O3 ) or germanium dioxide /GeO2 ) are added to the silica glass such that the index of refraction varied gradually in parabolic manner across the cross section. This enables light to travel faster while close to the periphery than at the center. This avoids pulse broadening.  Both step- and graded- index fibers are termed as multi-mode fibers