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Light Refraction as a Forensic Tool

Light Refraction as a Forensic Tool. PWISTA 12/2/2006. Objectives: Matching 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. Demo. Lab.

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Light Refraction as a Forensic Tool

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  1. Light Refraction as a Forensic Tool PWISTA 12/2/2006

  2. Objectives: Matching 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 Demo Lab

  3. Matching Glass Fragments • Suspect and crime scene fragments must fit together to be from same source • Physical properties of density and refractive index are used most successfully for characterizing glass particles. • Flotation test in density column!! • Immersion Method • GRIM 3: Glass RI measurement (automated)

  4. 1. Flotation test in density column • Control glass added to liquid • Density of liquid adjusted until control glass suspended • Unknown is then added to see if it floats or sinks

  5. 1. Flotation test in density column

  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

  7. 2. Theory of Refraction, (Speed)

  8. 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

  9. 2. Theory of Refraction, (Angular)

  10. Refraction Notation • Since the index of refraction depends on both the • temperature of the sample • 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).

  11. Refractive Index • Ratio of speeds in a vacuum vs. a medium • At a specific temperature • And Wavelength Frequency • V of light in Vacuum • V of light in medium

  12. Refractive Index • Water at 25C =1.333 (1.333 times faster in a vacuum then in water at that temp. • Dependent on temperature and wavelength frequency • Sodium D light: STANDARD wavelength • 589.3 nanometers

  13. Theory of Refraction • Temperature dependence of refractive index for Sucrose.

  14. Theory of Refraction • Table 1. Temperature dependence of refractive index for selected substances.

  15. Jell-O (Refractive Index) Lab Demo Demo Objective: • Use gelatin as a smoked lens, to view total internal reflection and as a color filter. Jell-O (Refractive Index) Lab Demo

  16. Wall Demo Demo

  17. Refractometer

  18. Refractometer

  19. Different Refractometers • Salinity Refractometer: Salinity vs Specific gravity • Brix Refractometer: Sugar content

  20. 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: a bright halo that is observed near the border of a particle immersed in a liquid of a different RI Immersion Method of Glass Identification

  21. nglass >nmedium nglass < nmedium     nmedium  = 1.525     nglass    = 1.60      nmedium = 1.525      nglass    = 1.34 Becke line:

  22. Becke Lines:

  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 http://www.fosterfreeman.com/index.html

  24. 3. Glass Refractive Index Measurement (GRIM 3)

  25. Lab What is the refractive index of the Unknown Glass Sample?

  26. Lab What would You would need? • Standards????

  27. Lab How Can this be accomplished? • Procedure????

  28. Experiment #1 Using the Jell-O Activity: Mathematically Develop your standards. Experiment #2 Using the Immersion Method Identification: Develop your standards Lab Objective: To Identify the numeric refractive index of varying Liquid Standards.

  29. Lab Class Unknown Results????? 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

  30. Lab Unknowns????? 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

  31. Lab 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

  32. Lab Class Results “Glass Samples” • #1 picture frame glass: • #2 Fish Tank • #4 Beaker Pyex • #4 Beaker non-Pyrex, Bottle

  33. Lab

  34. Dats It

  35. Remember Slides

  36. Refractive Index • Transparent solids immersed in a liquid having a similar RI, light will not be refracted as it passes from liquidsolid. • Reason why the eye unable to distinguish between the solidliquid boundary.

  37. Solids are crystalline • Crystalline solids: have definite geometric forms because of the orderly arrangement of particles (Atoms). • Relative location/arrangement of atoms repeats • Atoms: smallest unit of an element

  38. No!! Amorphous Solids • Amorphous solids: atoms or molecules are arranged RANDOMLY • NO regular order to the atoms • Glass

  39. Lab What is the refractive index of Sodium Chloride (NaCl)?

  40. Crystalline solids • Exhibits double refraction (double imagery produced) • Calcite, RI=1.486 and 1.658 • The difference 0.172 is known as birefringence. Most CALCITE • Dispersion: separation of light into its component wavelengths

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