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.
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

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

16. Wall Demo
Demo

17. Refractometer

18. Refractometer

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

20. 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

21. Becke line: nglass >nmedium nglass < nmedium

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

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

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

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

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

28. 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

29. 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

30. 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

31. 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

32. Class Results “Glass Samples”
Lab
Class 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 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

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. 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

39. What is the refractive index of Sodium Chloride (NaCl)?
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 is known as birefringence. Most CALCITE
Dispersion: separation of light into its component wavelengths