Comprehensive Glass and Light Properties: Density, Refractive Index, and Fracture Analysis

Physical properties

Examples include weight, volume, density, color, boiling point, melting point, refractive index, etc.

Chemical properties

Examples include burning, formation of gas, rusting decomposition, color change.

Wavelength and frequency relationship

They have an inverse relationship meaning that as one increases, the other decreases and vice versa.

Spectrophotometer

An instrument that measures the quantity of radiation (light) that a particular material absorbs as a function of wavelength and frequency.

Intensive property

A property that does not depend on the amount of material (ex: density, refractive index).

Formula for density

Density = mass/volume (D=M/V).

Density of glass fragment

Density = mass/volume = 0.195 g/mL.

Refraction

The bending of light caused by the change of velocity from one medium to another.

Refractive index

The measure of how much the light is bent, defined as the ratio of the speed of light in a vacuum (air) to the speed of light in the medium under examination.

Factors affecting density & refractive index

Temperature.

GRIM 3

A computer/video system used with a hot stage microscope to automatically adjust the temperature of the immersion oil & calculate RI of glass at match point.

Crystalline solid refraction

It splits the beam into two, causing double refraction.

Birefringence

The numerical difference between the two refractive indices of the double refraction.

Type of solid glass is considered

Amorphous, noncrystalline solid.

Birefringence in glass

No, glass does not exhibit birefringence.

Soda-lime glass composition

Sand (SiO2), Soda (NaO2) and lime (CaO).

Examples of soda-lime glass

Windows and bottles.

Borosilicate glass composition

Sand (SiO2), Boron oxide.

Purpose of boron oxide in glass

To increase thermal (heat) resistance.

Examples of borosilicate glass

Test tubes, laboratory glassware, headlights, kitchenware.

Tempered glass production

By rapidly heating and cooling it.

Laminated glass

Made by sandwiching a plastic layer between two pieces of glass.

Flotation Method

Used to compare the density of glass fragments by suspending them in a liquid.

Density

A physical property used to identify and compare glass fragments.

Immersion Method

Used to find the refractive index of glass by immersing it in a liquid with a known refractive index.

Becke Line

A line that appears around a glass sample when immersed in a liquid; its position indicates the relationship of refractive indices.

Radial cracks

Cracks that form on the opposite side of the force's origin.

Concentric cracks

Cracks that form on the same side as the force's origin.

3R Rule

To determine the direction of impact; Radial cracks form a Right angle on the Reverse side of the force.

Gunshot penetrations

The first shot's crack will be uninterrupted by the following shots.

Linking glass fragments

The only way to link two glass fragments to a common source is by physically fitting them together.

Collection of glass fragments

If there is a possibility that glass fragments can be pieced together for an individual fit, collect all the glass found.

Evidence collection when individual fit is not possible

Submit all glass evidence found in the possession of the suspect along with a representative sample of broken glass remaining at the crime scene.

Best containers for glass collection

Rigid, sealable containers like pill bottles.

Packaging for clothing and shoes

Individually wrapped in paper.

Example of laminated glass

Windshields.

Example of glass that shatters into small squares

Shower doors, side car windows, rear car windows.

Density comparison in flotation

If the evidence sample floats on top of the liquid medium, the glass evidence is less dense than the liquid.

Refractive index comparison

When the evidence sample is immersed in the liquid and the Becke Line around the sample disappears, the refractive indices are the same.

Higher refractive index indication

If the Becke Line is inside the border of the sample, the glass has a higher RI than the liquid.

Lower refractive index indication

If the Becke Line is outside the border of the sample, the glass has a lower RI than the liquid.