Presentation on theme: "Light Refraction as a Forensic Tool"— Presentation transcript:
1 Light Refraction as a Forensic Tool PWISTA 12/2/2006
2 Objectives: Matching Glass Fragments Theory of RefractionSpeedAngularRefractometryDifferent RefractometersJell-O (Refractive Index) Lab DemoImmersion Method of Glass IdentificationBecke LinesUnknown labDemoLab
3 Matching Glass Fragments Suspect and crime scene fragments must fit together to be from same sourcePhysical properties of density and refractive index are used most successfully for characterizing glass particles.Flotation test in density column!!Immersion MethodGRIM 3: Glass RI measurement (automated)
4 1. Flotation test in density column Control glass added to liquidDensity of liquid adjusted until control glass suspendedUnknown is then added to see if it floats or sinks
6 2. Theory of Refraction, (Speed) The speed of light in a vacuum is always the same,but when light moves through any other medium it travels more slowly since it is constantly being absorbed and reemitted by the atoms in the material.The ratio of the speed of light in a vacuum to the speed of light in another substance is defined as the index of refraction (aka refractive index or n) for the substance.Refractometry Link
8 2. Theory of Refraction, (Angular) Light crossing from any transparent medium into another in which it has a different speed, is refractedi.e. bent from its original path (except when the direction of travel is perpendicular to the boundary).In the case shown, the speed of light in medium A is greater than the speed of light in medium B.refractometry
10 Refraction Notation Since the index of refraction depends on both the temperature of the samplethe wavelength of light used these are both indicated when reporting the refractive index:italicized n denotes refractive indexsuperscript indicates the temperature in degrees Celsiussubscript denotes the wavelength of light(in this case the D indicates the sodium D line at 589 nm).
11 Refractive Index Ratio of speeds in a vacuum vs. a medium At a specific temperatureAnd Wavelength FrequencyV of light in VacuumV of light in medium
12 Refractive IndexWater at 25C =1.333 (1.333 times faster in a vacuum then in water at that temp.Dependent on temperature and wavelength frequencySodium D light: STANDARD wavelength589.3 nanometers
13 Theory of RefractionTemperature dependence of refractive index for Sucrose.
14 Theory of RefractionTable 1. Temperature dependence of refractive index for selected substances.SubstanceIsopropanol1.38021.37721.3749Acetone1.36161.35881.3560Ethyl Acetate1.37471.37421.3700Water1.33341.33301.3325
15 Jell-O (Refractive Index) Lab Demo Objective:Use gelatin as a smoked lens, to view total internal reflection and as a color filter.Jell-O (Refractive Index)Lab Demo
19 Different Refractometers Salinity Refractometer: Salinity vs Specific gravityBrix Refractometer: Sugar content
20 Immersion Method of Glass Identification Glass put into liquidRI of liquid adjusted by temperature until a match point is reached.Point when Becke line disappears because both liquid and glass have same RI.Becke line: a bright halo that is observed near the border of a particle immersed in a liquid of a different RI
23 3. Glass Refractive Index Measurement (GRIM 3) GRIM3 can process glass fragments as small as 50 microns obtained from scenes-of-crime.Phase contrast optics and a Mettler hotstage, for temperature control.Varying temperature to alter the refractive index of a calibrated oil, the RI of an immersed fragment of glass can be determined at the point of null refraction, the point at which the refractive indices of glass and oil match
25 What is the refractive index of the Unknown Glass Sample? LabWhat is the refractive index of the Unknown Glass Sample?
26 What would You would need? LabWhat would You would need?Standards????
27 How Can this be accomplished? LabHow Can this be accomplished?Procedure????
28 LabObjective: To Identify the numeric refractive index of varying Liquid Standards.Experiment #1 Using the Jell-O Activity:Mathematically Develop your standards.Experiment #2 Using the Immersion Method Identification:Develop your standards
32 Class Results “Glass Samples” LabClass Results “Glass Samples”#1 picture frame glass:#2 Fish Tank#4 Beaker Pyex#4 Beaker non-Pyrex, Bottle
33 Soda-lime glass Lab Pyrex Borosilicates Tempered Glass: Soda (NaCo3)Lime (CaO)Windows BottlesPyrex Borosilicatesuse Boron oxide, Can with stand HIGH heatsTest tubesHeadlightsTempered Glass:Rapid heating and cooling does not shatterShower doorsSide + rear windowsLaminated GlassPlastic or Glass and glues and sandwichWindshieldsLab
36 Refractive IndexTransparent solids immersed in a liquid having a similar RI, light will not be refracted as it passes from liquidsolid.Reason why the eye unable to distinguish between the solidliquid boundary.
37 Solids are crystalline Crystalline solids: have definite geometric forms because of the orderly arrangement of particles (Atoms).Relative location/arrangement of atoms repeatsAtoms: smallest unit of an element
38 Amorphous solids: atoms or molecules are arranged RANDOMLY No!! Amorphous SolidsAmorphous solids: atoms or molecules are arranged RANDOMLYNO regular order to the atomsGlass
39 What is the refractive index of Sodium Chloride (NaCl)? LabWhat is the refractive index of Sodium Chloride (NaCl)?
40 Crystalline solidsExhibits double refraction (double imagery produced)Calcite, RI=1.486 and 1.658The difference is known as birefringence. Most CALCITEDispersion: separation of light into its component wavelengths