Presentation on theme: "Light Refraction as a Forensic Tool PWISTA 12/2/2006."— Presentation transcript:
Light Refraction as a Forensic Tool PWISTA 12/2/2006
Matching Glass Fragments 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
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. 1.Flotation test in density column!! 2.Immersion Method 3.GRIM 3: Glass RI measurement ( automated)
1. Flotation test in density column 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
1. Flotation test in density column 1. Flotation test in density column
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
2. Theory of Refraction, ( Speed)
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
2. Theory of Refraction, (Angular)
Refraction Notation Since the index of refraction depends on both the 1.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).
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
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 – nanometers
Theory of Refraction Temperature dependence of refractive index for Sucrose.
Theory of Refraction Table 1. Temperature dependence of refractive index for selected substances. Substance Isopropanol Acetone Ethyl Acetate Water
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
Different Refractometers Salinity Refractometer: Salinity vs Specific gravity Brix Refractometer: Sugar content
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: a bright halo that is observed near the border of a particle immersed in a liquid of a different RI
Becke line: n glass >n medium n glass < n medium n medium = n glass = 1.60 n medium = n glass = 1.34
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
3. Glass Refractive Index Measurement (GRIM 3)
What is the refractive index of the Unknown Glass Sample?
What would You would need? Standards????
How Can this be accomplished? Procedure????
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
Class Results Glass Samples #1 picture frame glass: #2 Fish Tank #4 Beaker Pyex #4 Beaker non-Pyrex, Bottle
Soda-lime glass Soda (NaCo3) Lime (CaO) Windows Bottles Pyrex Borosilicates use Boron oxide, Can with stand HIGH heats Test tubes Headlights Tempered Glass: Rapid heating and cooling does not shatter Shower doors Side + rear windows Laminated Glass Plastic or Glass and glues and sandwich Windshields
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.
Solids are crystalline Crystalline solids: have definite geometric forms because of the orderly arrangement of particles ( Atoms). Relative location/arrangement of atoms repeats Relative location/arrangement of atoms repeats Atoms: smallest unit of an element
No!! Amorphous Solids No!! Amorphous Solids Amorphous solids: atoms or molecules are arranged RANDOMLY NO regular order to the atoms –Glass
What is the refractive index of Sodium Chloride (NaCl)?
Crystalline solids Exhibits double refraction (double imagery produced) Calcite, RI=1.486 and The difference is known as birefringence. Most CALCITE Dispersion: separation of light into its component wavelengths