Review Sheet for MSE 257A at UA
Review Sheet for MSE 257A at UA
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Date Created: 02/06/15
Iridescence on Glass INTRODUCTION Iridescence is a complex optical effect and its study presents an opportunity to introduce or review some basic concepts regarding the properties and behavior of light Additionally the velvety appearance of the iridescent surface invites the discussion of re ectivity and how a surface with diffuse re ectivity was achieved by Tiffany s glass workers Thus the optical properties of the surface can be related to the nature of the material as well as its processing The techniques used to produce iridescent surfaces on Tiffany glass objects involved vapor deposition as well as a regimen of heating and cooling The latter operations resulted in the wrinkling of the glass surface which causes a velvety visual effect through diffuse re ection The outline below lists and organizes a wide array of concepts that may be discussed in relation to iridescence on glass CONCEPTS 1 Light and its wavelike behavior a Light can be described both in terms of particlelike as well as wavelike properties b Relations between wavelength 7 frequency v and energy v ck E h v h c A i E is energy J h is the Planck constant 6626quot 103934 Js v is the frequency ls s391 Hz c is the speed oflight in vacuum 2998 108 ms and 7 is the wavelength In ii The electromagnetic spectrum ranges from low energy power transmission and radio waves to high energy gamma and cosmic rays iii The visible light spectrum ranges from low energy red L700 nm to high energy violet L400 nm light c Interference between two light beams of the same wavelength can be examined in terms of the wavelike behavior of light The following cases involve two such beams traveling along the same path i If the points of maximum and minimum amplitude along the two wave forms coincide ie they are in phase then the two waves will add together to form a single wave with double the original amplitude The addition of the two electric fields is called constructive reinforcement ii If the points of maximum amplitude on one wave coincide with the points of minimum amplitude on the other wave ie they are out of phase then the two waves will add together and neutralize each other This destructive cancellation prevents either beam from being detected Prepared by Aniko Bezur Department of Materials Science and Engineering The University of Arizona 1999 Online notes on iridescence 2 iii In between the above two extremes the points of maximum amplitude on one wave do not exactly coincide with points of either the maximum or the minimum amplitude of another wave In this case the addition of the two electric elds will result in a wave of amplitudes that are less than double the original amplitudes but more than zero 2 The interaction of light with partly transparent matter Light impinging on a surface of an object with nite thickness can be re ected transmitted scattered and absorbed a Vocabulary used to describe the behavior of light C e i incident beam re ected beam angle of incidence angle of re ection ii angle of refraction refractive index Re ection and Scattering An incoming or incident beam is re ected in a specular manner if the surface is very smooth In this case the angle of incidence is the same as the angle of re ection The re ected beams of light are parallel and this causes shiny glossy surface Rougher surfaces will tend to scatter light from the surface causing dif ise re ectance Absorption Scattering and Transmission Not all light is re ected or scattered from the surface The remaining beam enters the material where it can be absorbed During absorption the constituents of the material molecules atoms ions etc capture the energy of the light beam and may convert it into heat or other type of radiation Alternatively constituents of the material may scatter the light sending it in many different directions Lastly part or all of the beam may pass through the material and exit on the other side as transmitted light incident beam interface norm al diffuse re ection scattering transm itte d re ected beam beam specular Based on illustration by K Nassau 1983 Figure 112 page 20 Refraction When light enters a transparent material at an angle other than perpendicular to the surface it is de ected In simple terms this phenomenon called refraction results from the slowing down of the light beam in denser materials If light moves from a less dense medium into a more dense medium for example from air into glass then the angle between the beam and the normal to the interface will be smaller than the angle of incidence For the case of light moving from a dense medium into a less dense medium the angle between the beam and the normal to the interface increases Online notes on iridescence 3 AIR GLASS incident beam angle of interface incidence 1 norm 31 r angle of refraction 1 ii Snell s law relates the velocity of light in vacuum c and the velocity of light in a transparent medium v to the angles of incidence and refraction The refractive index or a material n is expressed by the ratio cv c sini n v s1nr f Dispersion Different colors of light are bent to different extents when they enter the same medium In other words light waves with different frequencies have different refractive indices If a beam of white light light containing all colors of the visible spectrum impinges on a glass prism then as a consequence of the differing extent of refraction the beam is split into the its component colors This phenomenon is referred to as dispersion 3 Surface iridescence Iridescence is an optical effect which combines concepts related to the interaction of light waves with other light waves and the interaction of light waves with thin transparent materials thin lms Iridescence involves interference re ection and dispersion The phenomenon arises from the interference of light re ected from the two surfaces of a thin lm of material The interference causes parts of the spectrum to be reinforced while others are cancelled out This results in the viewer observing a rainbow effect the appearance of which depends on the angle of observation a Examples of iridescence While iridescence is a complex optical effect it can result from very simple situations that we may encounter under everyday circumstances Iridescence can be observed when we see i oil on the surface of a puddle of water ii soap bubbles on the surface of water iii old weathered glass surfaces for example glass pieces found in streams iv Tiffany glass products often possess an iridescent surface The surface of glass objects can be treated using chemicals and heat to produce an iridescent effect The essence of the treatments is to create a very thin layer of glass on the surface that is slightly different in composition than the glass making up the bulk of the object b Interaction of monochromatic light with a thin film To simplify matters it best to start to examine what happens when two parallel beams of the same wavelength and amplitude Online notes on iridescence 4 impinge on athin lm of glass As discussed before part of the beams may be re ected off the top surface of the partly transparent material while part of them may enter into the glass itself The latter parts may subsequently be re ected off the bottom surface of the thin film Beam B Destructive interference i Based on illustration by K Nassau 1983 Figure 126 page 254 ii Beam A As part of Beam A enters the glass it is bent toward the surface normal because it enters a medium of higher refractive index The beam is re ected off the bottom glassair interface without experiencing a phase change because the surface of re ection air has a lower refractive index than the medium in which the beam is traveling glass Upon reemerging from the glass the re ected beam is bent away from the surface normal because it enters a medium of lower refractive index iii Beam B Part of Beam B is re ected off the top airglass interface and experiences a phase change because the surface of re ection glass has a higher refractive index that the medium in which the beam is traveling air iv Interference of Beam A and Beam B When Beam A emerges from the thin glass film it encounters the re ected Beam B In the above example the two beams are out of phase ie they destructively interfere and cancel out each other Whether the two beams of the same wavelength are completely in phase or out of phase or somewhere in between is determined by the di erence in the path lengths that the beams travel This difference depends on o the angle of incidence o the thickness of the thin film and the refractive index of the thin film c Interaction of light with thin lms The above example considered beams of light that were parallel to each other and had the same wavelength and amplitude Under everyday circumstances however light is composed of many different colors ie beams of different wavelength frequency and energy It is important to recall at this point that light beams of different wavelength are bent to different extents as they move from one medium into another dispersion Thus the difkrence in path length between two beams of light will depend on their wavelength and their different refractive indices Online notes on iridescence d The rainbowlike effect afiridescence is thus a result of the interference between the beams of light re ecting from the top and bottom surfaces 0fa thin lm 4 Iridescent layers on glass a Louis Comfort Tiffany is said to have been inspired by ancient Roman glasses when he decided to develop a process for producing iridescent surfaces on glass It is important to point out that the iridescent surfaces on ancient Roman glasses were not intentionally created Rather they are a consequence of the degradation of the glass Iridescence on degraded glass i As glass is exposed to water in its burial environment some of its components can be dissolved by the water and carried away leached out This generates athin surface layer of glass that has a different composition than the undegraded bulk of glass Often there is a thin layer of air between the corroded surface and the bulk ii The corroded surface layer on old glass acts as a thin film and can give rise to iridescence Intentionally developed iridescent surfaces i It is explained above that to produce an iridescent surface it is necessary to superimpose a thin film over a bulk material For a glass object this can be accomplished by exposing the glass on the surface at high temperatures with compounds that will react with the glass and alter its composition and hence optical properties ii At the end of the manufacture of some Tiffany vases a thin surface film was produced by placing the piece in hot chamber with vapors that would react with the glass on the very surface This process now is referred to as chemical vapor deposition The vapor contained 0 Tin chloride SnClz andor 0 Iron chloride FeCl3 iii These chlorides form oxides on the surface SnO Fe203 Tin oxide on its own gives a more silvery color while iron oxide gives a golden tone to the iridescent effect it produces on the surface of the glass Mixing them gives something in between The size of the oxide crystals is very small and the thickness of the layer that they form on the glass surface is on the order of one micron 10396 m This is a film thin enough to give rise to iridescence 5 The velvety appearance of iridescent surfaces on Tiffany glass a If a surface is not perfectly smooth then parallel incident light beams will be re ected from the surface in many different directions As discussed under Reflection and Scattering quotthis type of re ection is called dif ise Surfaces with di use re ection have a matte and sometimes a velvety appearance Observing that a surface has a matte and velvety appearance allows us to conclude that it is most likely not a perfectly smooth surface This is the case with the iridescent surface of Tiffany glasses Examination using an electron microscope shows that in fact the iridescent surface is wavy and has many tiny ripples These ripples appear because the Tiffany glass objects were subjected to a regimen of heating and cooling This caused the surface layer and the base glass to expand as it was heated and shrink as it was cooled Online notes on iridescence 1 ii It is important to remember at this point that the surface layer is effectively a thin film containing very small tin oxide and iron oxide crystals The crystals expand and contract to a different extent than the base glass of the object In fact glass expands and shrinks almost twice as much upon heating than tin oxide or iron oxide Thus upon cooling the thin surface film that does not shrink as much as the bulk bottom glass to which it is attached In general this would cause the top layer to shrivel and fracture as the glass cooled However the extremely small particle size of the oxide surface layer gives it superplasticity Superplasticity is the ability of a material to deform uniformly by an exceptionally large amount 1 This property allows the metal oxide film to buckle on the glass surface instead of fracturing This leads to the appearance of small ripples and waves much like the surface of a lake when it is slightly windy Light is re ected in a diffuse manner from the rippled surface and this is what causes the velvety appearance of iridescent surfaces on Tiffany glass Donald R Askeland 1994 3rd edition The Science and Engineering anaIen39als PWS Publishing Company Boston Page 199
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