Light Refraction as a Forensic Tool

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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

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)

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

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
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).

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 589.3 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 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

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

Wall Demo Demo

Refractometer

Refractometer

Different Refractometers
Salinity Refractometer: Salinity vs Specific gravity Brix Refractometer: Sugar content

Immersion Method of Glass Identification
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: nglass >nmedium nglass < nmedium

Becke Lines:

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

What would You would need?
Lab What would You would need? Standards????

How Can this be accomplished?
Lab How Can this be accomplished? Procedure????

Lab 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 Unknown Results????? Lab Refractive index at 20°C
Baby oil: UNKNOWN #2 PYREX Canola oil: Olive oil: Soybean oil: Grape Seed Oil: , UNKOWN #4 frame Castor Oil: UNKNOWN #1 AQUARIUM Corn Oil: Xylene: 1.505 Clove Oil: 1.543

Unknowns????? Lab Refractive index at 20°C
Baby oil: UNKNOWN #2 PYREX Canola oil: Olive oil: Soybean oil: Grape Seed Oil: , UNKOWN #4 frame Castor Oil: UNKNOWN #1 AQUARIUM Corn Oil: Xylene: 1.505 Clove Oil: 1.543

Glass Samples Lab #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

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

Soda-lime glass Lab Pyrex Borosilicates Tempered 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 Lab

Dats It

Remember Slides

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 Atoms: smallest unit of an element

Amorphous solids: atoms or molecules are arranged RANDOMLY
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)?
Lab What is the refractive index of Sodium Chloride (NaCl)?

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

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