light refraction as a forensic tool pwista 12/2/2006
TRANSCRIPT
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Light Refraction as a Forensic Tool
PWISTA 12/2/2006
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Objectives: Matching Glass FragmentsMatching Glass Fragments
Theory of Refraction– Speed – Angular
Refractometry – Different Refractometers
Jell-O (Refractive Index) Lab Demo Immersion Method of Glass Identification– Becke Lines
Unknown lab
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Matching Glass FragmentsMatching Glass Fragments
Suspect and crime scene fragments must fit Suspect and crime scene fragments must fit together to be from same sourcetogether to be from same sourcePhysical properties of density and refractive index Physical properties of density and refractive index are used most successfully for characterizing are used most successfully for characterizing glass particles.glass particles.
1.1. Flotation test in density column!! Flotation test in density column!! 2.2. Immersion MethodImmersion Method3.3. GRIM 3: Glass RI measurement (GRIM 3: Glass RI measurement (automated)automated)
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1. Flotation test in density column1. Flotation test in density column
Control glass added to liquidControl glass added to liquidDensity of liquid adjusted until Density of liquid adjusted until control glass suspendedcontrol glass suspendedUnknown is then added to see if it Unknown is then added to see if it floats or sinksfloats or sinks
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1. Flotation test in density column1. Flotation test in density column
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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
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2. Theory of Refraction, (Speed)
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2. Theory of Refraction, (Angular)
Light crossing from any transparent medium into another in which it has a different speed, is refracted– i.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
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2. Theory of Refraction, (Angular)
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Refraction Notation
Since the index of refraction depends on both the1. temperature of the sample 2. the wavelength of light used these are both indicated
when reporting the refractive index:• italicized n denotes refractive index • superscript indicates the temperature in degrees Celsius • subscript denotes the wavelength of light
– (in this case the D indicates the sodium D line at 589 nm).
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Refractive IndexRefractive Index
Ratio of speeds in a vacuum vs. a mediumRatio of speeds in a vacuum vs. a medium– At a specific temperatureAt a specific temperature– And Wavelength FrequencyAnd Wavelength Frequency
V of light in VacuumV of light in VacuumV of light in mediumV of light in medium
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Refractive IndexRefractive Index
Water at 25C =1.333 (1.333 times Water at 25C =1.333 (1.333 times faster in a vacuum then in water at that faster in a vacuum then in water at that temp.temp.Dependent on temperature and Dependent on temperature and wavelength frequencywavelength frequency– Sodium D light: STANDARD wavelengthSodium D light: STANDARD wavelength– 589.3 nanometers589.3 nanometers
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Theory of Refraction
Temperature dependence of refractive index for Sucrose.
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Theory of Refraction
Table 1. Temperature dependence of refractive index for selected substances.
Substance
Isopropanol 1.3802 1.3772 1.3749
Acetone 1.3616 1.3588 1.3560
Ethyl Acetate
1.3747 1.3742 1.3700
Water 1.3334 1.3330 1.3325
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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
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Wall Demo
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Refractometer
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Refractometer
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Different Refractometers
Salinity Refractometer: Salinity vs Specific gravityBrix Refractometer: Sugar content
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Immersion Method of Glass Identification
Immersion method:– Glass put into liquid– RI 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:Becke line: a bright halo that is observed near the a bright halo that is observed near the border of a particle immersed in a liquid of a border of a particle immersed in a liquid of a different RI different RI
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Becke line:Becke line:nglass >nmedium nglass < nmedium
nmedium = 1.525
nglass = 1.60
nmedium = 1.525
nglass = 1.34
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Becke Lines: Becke Lines:
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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
http://www.fosterfreeman.com/index.html
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3. Glass Refractive Index Measurement (GRIM 3)
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What is the refractive index of the Unknown Glass Sample?
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What would You would need?
Standards????
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How Can this be accomplished?
Procedure????
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Objective: 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
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Refractive index at 20°C
Baby oil: 1.45 UNKNOWN #2 PYREX
Canola oil: 1.465-1.467
Olive oil: 1.467-1.4705
Soybean oil: 1.470-1.472
Grape Seed Oil: 1.471-1.478, UNKOWN #4 frame
Castor Oil: 1.4750 - 1.4850 UNKNOWN #1 AQUARIUM
Corn Oil: 1.4735 - 1.4785
Xylene: 1.505
Clove Oil: 1.543
Class Unknown Results????? Class Unknown Results?????
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Refractive index at 20°C
Baby oil: 1.45 UNKNOWN #2 PYREX
Canola oil: 1.465-1.467
Olive oil: 1.467-1.4705
Soybean oil: 1.470-1.472
Grape Seed Oil: 1.471-1.478, UNKOWN #4 frame
Castor Oil: 1.4750 - 1.4850 UNKNOWN #1 AQUARIUM
Corn Oil: 1.4735 - 1.4785
Xylene: 1.505
Clove Oil: 1.543
Unknowns????? Unknowns?????
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Glass Samples
#1 picture frame glass: 1.48, 1.47, 1.48, 1.36
#2 Fish Tank: 1.50, 1.466, 1.45, 1.48, 1.45
#4 Beaker Pyrex : 1.471, 1.48, 1.47
#1 picture frame glass: 1.47, – Grape Seed, Soybean
#2 Fish Tank: 1.50, 1.466, 1.45, 1.48, 1.45
#4 Beaker Pyrex : 1.471, 1.48, 1.47
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Class Results “Glass Samples”
#1 picture frame glass: #2 Fish Tank#4 Beaker Pyex #4 Beaker non-Pyrex, Bottle
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Soda-lime glassSoda-lime glass Soda (NaCo3)Soda (NaCo3)Lime (CaO)Lime (CaO)
Windows Windows BottlesBottles
Pyrex Pyrex BorosilicatesBorosilicates use Boron oxide, use Boron oxide, Can with stand Can with stand HIGH heatsHIGH heats
Test tubesTest tubesHeadlightsHeadlights
Tempered Glass:Tempered Glass: Rapid heating and Rapid heating and cooling does not cooling does not shattershatter
Shower doorsShower doorsSide + rear Side + rear windowswindows
Laminated GlassLaminated Glass Plastic or Glass and Plastic or Glass and glues and sandwichglues and sandwich
WindshieldsWindshields
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Dats It
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Remember Slides
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Refractive IndexRefractive Index
Transparent solids immersed in a liquid Transparent solids immersed in a liquid having a similar RI, light will not be having a similar RI, light will not be refracted as it passes from liquidrefracted as it passes from liquidsolid.solid.Reason why the eye unable to distinguish Reason why the eye unable to distinguish between the solidbetween the solidliquid boundary.liquid boundary.
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Solids are crystallineSolids are crystalline
Crystalline solids: have definite geometric Crystalline solids: have definite geometric forms because of the orderly arrangement of forms because of the orderly arrangement of particles (particles (Atoms).Atoms).
• Relative location/arrangement of atoms Relative location/arrangement of atoms repeatsrepeats
Atoms: smallest unit of an elementAtoms: smallest unit of an element
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No!! Amorphous SolidsNo!! Amorphous Solids
Amorphous solids:Amorphous solids: atoms or molecules atoms or molecules are arranged RANDOMLYare arranged RANDOMLYNO regular order to the atomsNO regular order to the atoms– GlassGlass
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What is the refractive index of Sodium Chloride (NaCl)?
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Crystalline solidsCrystalline solids
Exhibits double refraction (double imagery Exhibits double refraction (double imagery produced)produced)Calcite, RI=1.486 and 1.658 Calcite, RI=1.486 and 1.658 The difference 0.172 is known as The difference 0.172 is known as birefringence. birefringence. Most CALCITE Most CALCITE
Dispersion: separation of light into its Dispersion: separation of light into its component wavelengthscomponent wavelengths