Light Refraction as a Forensic Tool

PWISTA 12/2/2006

Objectives: Matching Glass FragmentsMatching 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 FragmentsMatching Glass Fragments

Suspect and crime scene fragments must fit Suspect and crime scene fragments must fit together to be from same sourcetogether to be from same sourcePhysical properties of density and refractive index Physical properties of density and refractive index are used most successfully for characterizing are used most successfully for characterizing glass particles.glass particles.

1.1. Flotation test in density column!! Flotation test in density column!! 2.2. Immersion MethodImmersion Method3.3. GRIM 3: Glass RI measurement (GRIM 3: Glass RI measurement (automated)automated)

1. Flotation test in density column1. Flotation test in density column

Control glass added to liquidControl glass added to liquidDensity of liquid adjusted until Density of liquid adjusted until control glass suspendedcontrol glass suspendedUnknown is then added to see if it Unknown is then added to see if it floats or sinksfloats or sinks

1. Flotation test in density column1. 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 the1. 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 IndexRefractive Index

Ratio of speeds in a vacuum vs. a mediumRatio of speeds in a vacuum vs. a medium– At a specific temperatureAt a specific temperature– And Wavelength FrequencyAnd Wavelength Frequency

V of light in VacuumV of light in VacuumV of light in mediumV of light in medium

Refractive IndexRefractive Index

Water at 25C =1.333 (1.333 times Water at 25C =1.333 (1.333 times faster in a vacuum then in water at that faster in a vacuum then in water at that temp.temp.Dependent on temperature and Dependent on temperature and wavelength frequencywavelength frequency– Sodium D light: STANDARD wavelengthSodium D light: STANDARD wavelength– 589.3 nanometers589.3 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

Refractometer

Refractometer

Different Refractometers

Salinity Refractometer: Salinity vs Specific gravityBrix 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:Becke line: a bright halo that is observed near the a bright halo that is observed near the border of a particle immersed in a liquid of a border of a particle immersed in a liquid of a different RI different RI

Becke line:Becke line:nglass >nmedium nglass < nmedium

    nmedium  = 1.525

    nglass    = 1.60

     nmedium = 1.525

     nglass    = 1.34

Becke Lines: 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

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

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

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

Class Unknown Results????? 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

Unknowns????? Unknowns?????

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”

#1 picture frame glass: #2 Fish Tank#4 Beaker Pyex #4 Beaker non-Pyrex, Bottle

Soda-lime glassSoda-lime glass Soda (NaCo3)Soda (NaCo3)Lime (CaO)Lime (CaO)

Windows Windows BottlesBottles

Pyrex Pyrex BorosilicatesBorosilicates use Boron oxide, use Boron oxide, Can with stand Can with stand HIGH heatsHIGH heats

Test tubesTest tubesHeadlightsHeadlights

Tempered Glass:Tempered Glass: Rapid heating and Rapid heating and cooling does not cooling does not shattershatter

Shower doorsShower doorsSide + rear Side + rear windowswindows

Laminated GlassLaminated Glass Plastic or Glass and Plastic or Glass and glues and sandwichglues and sandwich

WindshieldsWindshields

Dats It

Remember Slides

Refractive IndexRefractive Index

Transparent solids immersed in a liquid Transparent solids immersed in a liquid having a similar RI, light will not be having a similar RI, light will not be refracted as it passes from liquidrefracted as it passes from liquidsolid.solid.Reason why the eye unable to distinguish Reason why the eye unable to distinguish between the solidbetween the solidliquid boundary.liquid boundary.

Solids are crystallineSolids are crystalline

Crystalline solids: have definite geometric Crystalline solids: have definite geometric forms because of the orderly arrangement of forms because of the orderly arrangement of particles (particles (Atoms).Atoms).

• Relative location/arrangement of atoms Relative location/arrangement of atoms repeatsrepeats

Atoms: smallest unit of an elementAtoms: smallest unit of an element

No!! Amorphous SolidsNo!! Amorphous Solids

Amorphous solids:Amorphous solids: atoms or molecules atoms or molecules are arranged RANDOMLYare arranged RANDOMLYNO regular order to the atomsNO regular order to the atoms– GlassGlass

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

Crystalline solidsCrystalline solids

Exhibits double refraction (double imagery Exhibits double refraction (double imagery produced)produced)Calcite, RI=1.486 and 1.658 Calcite, RI=1.486 and 1.658 The difference 0.172 is known as The difference 0.172 is known as birefringence. birefringence. Most CALCITE Most CALCITE

Dispersion: separation of light into its Dispersion: separation of light into its component wavelengthscomponent wavelengths