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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 wavelength speed of light—299,792,458 m/s. Electromagnetic spectrum. Properties of Light.

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properties of light forensic light

Properties of LightForensic Light

Test: Friday Oct. 7

*properties of matter

*density

*properties of light

*fracture patterns

properties of light
Properties of light
  • Light waves:
    • Differ in frequency and wavelength
    • speed of light—299,792,458 m/s
properties of light1
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 light2
Properties of Light
  • Refraction:
    • The bending of light waves because of a change in velocity
properties of light3
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 light and glass
  • Forensic scientists use two methods when analyzing glass
    • Determining the density of the shard of glass in question
    • Determining the refractive index of the shard in question
forensics light and glass
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
slide11

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 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
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
bodily fluids
Bodily Fluids
  • Semen, saliva, and vaginal fluids
    • all glow
  • Blood
    • does not glow—it appears black

Semen

Saliva

documents forgery
Documents--Forgery

In 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
Gun shot residue

Under infrared light

bones
Bones

UV fluorescence of burnt bone

Bone fragments