Matter, light, and glass

Forensics Final SP26

physical properties

weight, volume, color, boiling point, and melting point

chemical property

the behavior of a substance when it reacts or combines with another substance

weight

the force with which gravity attracts a body

mass

the amount of matter an object conatins; independent of gravity

refraction

the bending of light waves because of change in velocity

refractive index

the ratio of the velocity of light in a vacuum to that in the medium under examination

birefringence

the numerical difference between these two refractive indices

crystalline solids

have definite geometric forms because of the orderly arrangement of their atoms

double refraction

crystalline solids refract a beam of light in two different light-ray components

Theory of Light

light is a continuous wave, a stream of discrete energy particles

wavelength

the distance between two successive crests (or one trough to the next trough)

frequency

the number of crests (or troughs) passing any one given point per unit of time

frequency and wavelength

inversely proportional to one another

electormagnetic spectrum

the entire range of radiation energy from the most energetic cosmic rays to the least energetic radio waves

density and refractive index

physical properties to evaluate glass fragments

density

mass per unit volume; intensive property of matter

refractive index

ratio of the velocity of light in a vacuum to that in the medium under examination

procedure for determining density of a solid

measure its mass, then measuer its volume by noting the volume of water it displaces

glass composition

silicon oxides (sand) mixed with various metal oxides; sodium carbonate (soda) is normalled added to the sand to lower melting point; calcium carbonate (lime) aslo added to the sand mix to prevent the glass from dissolving in water

“soda lime”

used to manufacture most window and bottle glass

float glass

molten glass is cooled on a bath of molten tin

tempered glass

stronger than normal glass due to rapid heating and cooling of the glass surfaces

laminated glass

found in car windsheilds; has a layer of plastic between two pieces of ordinary window glass

annealed glass shatter

breaks easily; produces long, sharp, splinters

tempered glass shatter

shatters completely under high levels of impact energy, few pieces remain in the frame

laminated glass

may crack under presuure, but tends to remain integral, adhering to plastic vinyl interlayer

flotation method

unknown glass fragment is immersed in a liquid of known density; density is adjusted until glass chip florats; meaning liquid and glass density is the same

immersion method

used to determine a glass fragment’s refractive index; same as floatation method but with refractive index; Becke line disappears

Becke line

a bright halo near the border of a particles that is immersed in a liquid of a different refractive index

radial fractures

something produces cracks in glass that radiate outward

concentric fractures

cracks in glass that encircle the hole of a bullet or stone

production of radial and concentric fractures

radial cracks are formed first on the opposite side of the destructive force, concentric cracks occur afterward on the same side as the force

3R Rule for direction of impact

Radial cracks form a Right angle on the Reverse side of the force

suquence of impacts

determined because a fracture always terminates at an existing line of fracture

collection of glass

even if the remotest possibility exists that glass fragments may be pieced together, every effort must be made to collect all the glass found; evdidence collectoru must submit all glass evidence found with suspect along with sample of glass at the rime scene

glass fragments

packaged in solic containers to avoid further breakage; shoes/clothing should be indvidually wrapped in paper and trasmitted to the lab