Properties of Light Forensic Light

PROPERTIES OF LIGHT

FORENSIC LIGHT

Test: Friday Oct. 7*properties of matter

*density• *properties of light• *fracture patterns

PROPERTIES OF LIGHT Light waves:

Differ in frequency and wavelengthspeed of light—299,792,458 m/s

ELECTROMAGNETIC SPECTRUM

PROPERTIES OF LIGHT Dispersion:

Process of separating light into its component colors (ex: prism)

As visible light passes through the glass of a prism each color composition of light is slowed to a speed slightly different from the others.

Causes each component to bend at different angles as it emerges from the prism

PROPERTIES OF LIGHT Refraction:

The bending of light waves because of a change in velocity

PROPERTIES OF LIGHT Refractive Index:

How much an object slows the speed of light

Ratio of velocity of light in a vacuum to its velocity in any medium

RI = velocity of light in vacuum velocity of light in medium Varies with temperature of substance and

wavelength of light passing through it

FORENSIC LIGHT AND GLASS Forensic scientists use two methods

when analyzing glass1. Determining the density of the shard of

glass in question2. Determining the refractive index of the

shard in question

FORENSICS LIGHT AND GLASS Refractive Index of Glass:

A piece of glass in question is immersed in a liquid medium whose refractive index is adjusted until it equals that of the glass particles

At this point (match point), the observer notes the disappearance of the Becke line

Becke line—halo near the border of glass particle that is immersed in a liquid of different refractive indices

The Becke line disappears when the medium and fragment have similar indices

BECKE LINES

Each of these three identical jars contains a glass stir rod. One jar is empty, and the entire

rod is visible. The second jar contains immersion oil, which has the same refractive index as glass, making the rod invisible. The third jar contains water, and the rod appears bent.

FORENSIC LIGHT SOURCES Forensic light source is made up of a

powerful lamp containing UV, visible, and infrared components of light

3 techniques:Fluorescence (evidence glows)Absorption (evidence darkens)Oblique lighting (small particles of evidence revealed—hair, fibers)

FORENSIC LIGHT USES Detecting latent fingerprints Detecting bodily fluids Hair and fibers Bruises, bite marks, pattern wounds Questioned documents—forgery Gun shot & explosive residue Bone fragments, tire tracks, shoe prints,

drugs

LATENT FINGERPRINTS

BODILY FLUIDS Semen, saliva,

and vaginal fluidsall glow

Blood does not glow—it

appears black

Saliva

Semen

DOCUMENTS--FORGERYIn works of art, modern paint will fluoresce or glow under a black light, older paints will not. Thus, pictures that have been "touched up" with modern paint will glow.

Bank notes also incorporate fluorescent dyes, which glow under exposure to UV illumination. The color and location of the security thread in US dollar bills are unique to the value of the bill.

GUN SHOT RESIDUE

Under infrared light

BONESUV fluorescence of burnt bone

Bone fragmen

ts