Physical-Trace Evidence

Parts of a microscope

Eyepiece, low-power objective, high-power objective, nose piece, body tube, coarse adjustment, fine adjustment, stage, stage clips, diaphragm, light source, base

Physical Evidence

tangible items that tend to prove some material fact; anything that can be touched.

who substantiates the identity of a substance?

the court of law/jury

individual vs class characteristics

class characteristics: the evidence is from a white man with brown hair, etc.

individual characteristics: the evidence is from this specific person

Locard's Exchange Principle

Every contact leaves a trace

Even if physical evidence can't positively ID a suspect, it is still valuable because what can it do?

it can classify the suspect

the 3 layers that make up hair

Keratin

a fibrous protein forming the main structural constituent of hair, feathers, hoofs, claws, horns, etc.

medullary ratio

diameter of medulla/diameter of hair shaft

human medullary ratio

less than 1/3

animal medullary ratio

greater than 1/2

patterns of medulla

How can the cuticle help identify a source of hair?

the scale pattern

Three phases of hair growth

anagen (hair is being produced), catagen (resting period), telogen (hair naturally falls out)

when are you most likely to get a follicular tag?

when the hair has been forcibly ripped out (usually during the anagen stage)

what is the forensic value of a follicular tag?

it has your dna

how can you tell if hair's been dyed or bleached?

a difference in color between the cuticle and cortex/sharp change in color close to the root

what can be determined by hair?

Human/Animal (species), body area

what can't be determined by hair?

race or gender, individual characteristics

How many head hairs should be obtained to get a good sample?

50

How many pubic hairs should be collected?

24

3 classifications of animal hair

Guard hairs - protection from elements

Fur/wool - insulation

Tactile hairs - sensory functions

Natural Vs. Man-made fibers

Natural fibers are derived from nature and include cotton, wool, silk, cashmere, mink, and other hairs/fur. Man-made fibers are made in the lab and include rayon, polyester, and spandex.

What is a polymer?

any synthetic or natural fiber made of a long chain of monomers

How should crime scene fibers be collected?

A paper bag and/or an envelope are used to collect fibers.

4 layers of paint

-electrocoat primer- provides corrosion resistance

-primer surface- completely smooth out and hide any seams or imperfections

-basecoat- resists weather, UV radiation, and acid rain

-clearcoat- outstanding etch resistance and appearance

How can paint formulations be distinguished?

Pyrolysis gas chromatography is used to distinguish most paint formulations by determining the dye used.

What is glass?

a transparent fusion product of inorganic substances that have cooled to a rigid state without crystallizing

What is the main ingredient of glass?

Sand

soda-lime glass

used in plate and window glass, glass containers, and electric light bulbs

Borosilicate glass

heat resistant, like Pyrex, used in scientific/kitchen equipment

laminated glass

two sheets of ordinary glass bonded together with a plastic film, used in car windshields

tempered glass

glass that is strengthened by introducing stress through rapid heating and cooling of the glass surfaces; used in car windows

How might glass samples vary from each other?

• surface striations and markings

• surface contaminants

• thickness

• hardness

• color

• density

• refractive index

what two features of glass are the most helpful in categorizing it?

density and refractive index

What is the Becke line?

a bright halo observed near the border of a particle immersed in a liquid of a different refractive index

How can you tell which way a bullet went through glass?

the exit hole is always larger than the entrance hole.

is a glass is shot more than once, how can you tell which was shot first?

the radial fracture lines

Should you use color when comparing glass samples? Why?

no; natural elements and manual tampering can easily alter the color of the glass.

How do calculate density?

mass divided by volume

How does density help in glass comparison?

each type of glass typically has a different density.

HONORS: How can you distinguish between the hairs from people of European, Asian, or African descent?

thickness, follicle shape, texture (straight, curly, wavy), and sometimes medulla.

HONORS: Asian Hair

• circular cross section

• thick cuticle

• prominent medulla

HONORS: European Hair

• round to oval cross sections

• straight to curly hair

• very thin to nonexistent medulla

HONORS: African Hair

• flattened cross section

• curly to coiled hair

• ringlets can form