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.

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Objectives 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

Demo

Lab


Matching glass fragments
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
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



2 theory of refraction speed
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 angular
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



Refraction notation
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
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 index1
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
Theory of Refraction

  • Temperature dependence of refractive index for Sucrose.


Theory of refraction1
Theory of Refraction

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


Jell o refractive index lab demo
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


Wall demo
Wall Demo

Demo




Different refractometers
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

Immersion Method of Glass Identification


Becke line

nglass >nmedium

nglass < nmedium

    nmedium  = 1.525     nglass    = 1.60

     nmedium = 1.525      nglass    = 1.34

Becke line:



3 glass refractive index measurement grim 3
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



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????


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

Lab

Objective: To Identify the numeric refractive index of varying Liquid Standards.


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


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


Glass samples

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


Class results glass samples

Lab

Class Results “Glass Samples”

  • #1 picture frame glass:

  • #2 Fish Tank

  • #4 Beaker Pyex

  • #4 Beaker non-Pyrex, Bottle





Refractive index2
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
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


  • 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

    Lab

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


    Crystalline solids
    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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