Download Refractive index

Catalyst 1/14/13
1. Discuss two of the fiber identification techniques
we mentioned last Friday. Be specific. What is the
investigator looking for in a sample?
Agenda 1/14/13
• Catalyst
• Announcements
– Last call for Hair Poster extra credit opportunity
– Reminder: Bring your textbook with you EVERYDAY
• Main Types of Fiber Identification Techniques
• Probability and Statistics
• Fiber Review Project
Class Points
Your class can earn class points if:
everyone in class:
 Comes to class quietly and on
time
 Stays focused and on task during
class
 Leaves classroom neat and
organized
 Students are teaching other
students
 Majority of class participates
 Follows all classroom
expectations and procedures
 And more…
P8: 10
(on time,
cleaned up
quickly)
Objectives 1/11/13
We will be able to:
• Describe the physical and chemical properties of
fibers
• Describe how density, refractive index, and
fluorescence are used to identify fibers
• Calculate the probability
Collection of Fiber Evidence
• Bag clothing items individually in paper
bags. Make sure that different items are
not placed on the same surface before
being bagged.
• Make tape lifts of exposed skin areas and
any inanimate objects.
• Removed fibers should be folded into a
small sheet of paper and stored in a paper
bag.
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Microscopy - Silk
Microscopy - Nylon
Microscopy - Cotton
C.used to analyze fiber evide
be
POLYSTER, or NYLON
· Explain how the following exa
used to distinguish between fi
techniques are not used often
Micro-Chemical Test)
· Identify
each of the following
C.
D.
following class characteristics
Identifying
Fibers
! POLYSTER, or NYLON
Scenario:!Summarize!the!case/cr
A.
collection and preservation method
!!
D.
Problem:!Using!the!scenario!deve
investigated.!
ario:!Summarize!the!case/crime!scene.!Explain
the
ction and preservation methods forA.
fiber samples.!
Researched>based!Hypothesis:!B
!!
B.
forensic!microscopy!and!physica
Testing for Identification
1. Microscopic observation✔
2. Burning—observation of
how a fiber burns, the odor,
color of flame, color of
smoke, and the appearance
of the residue ✔
3. Thermal decomposition—
gently heating to break down
the fiber to the basic
monomers
4. Chemical tests—solubility
and decomposition
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Testing for Identification
Density—the mass of an
object divided by the volume
of the object
Refractive index—
measurement of the bending
of light as it passes from air
into a solid or liquid
Fluorescence—absorption
and reemission of light; used
for comparing fibers as well
as spotting fibers for
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Density
• Density = mass/volume
• Submerge piece of fiber in a known liquid with
known density
Float or sink?
• Object density > fluid density, object will SINK
• Object density < fluid density, object will FLOAT
Density – Fiber Identification
• An unknown fiber sample is placed in a solution
of sodium chloride. You observe that it floats.
What are the possible identities of this fiber?
Density – Fiber Identification
• An unknown fiber sample is placed in a solution of
potassium carbonate. You observe that the sample
sinks. What are the possible identities of the fiber?
Refractive Index
• Refractive Index
– How light bends from one medium to another
• Represented by N
• n = c/v
 v= velocity of light in that
medium
 c= speed of light =
3 x 108 m/s
Refractive Index – Becke LIne
Liquid has
higher
refractive index
than the fiber
Nliquid > NFiber
Refractive index Liquid has
of fiber and liquid lower refractive
are the same (No index than fiber
Becke line)
Nliquid = NFiber
Nliquid < NFiber
Refractive Index – Which fiber could it
be?
• Open your book to page 148 and refer to
Table 6.1
• You take an unknown sample of fiber and drop
glycerin (N = 1.473) on the sample. Under a
compound microscope, you observe the
disappearance of the Becke line. What are the
possible identities of this fiber?
Fluorescence Test
• WARNING: Do NOT LOOK directly at UV light!
These will damage your eyes.
• Your goggles DO NOT protect you from UV light.
• Place the fiber sample under the UV light and
check for fluorescence. Jot down any other
observations you may have in your notebook.
• Read page 151 for more details.
Flourescence
Dyes
• Components that make up dyes can be
separated and matched to an unknown.
• There are more than 7,000 different dye
formulations.
• Chromatography is used to separate dyes for
comparative analysis.
• The way a fabric accepts a particular dye
may also be used to identify and compare
samples.
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Probability and Statistics
• Class evidence is useful if it can significantly
narrow down the suspects.
• Open your textbook to page 129.
• How many students are wearing blue?
• How many students are wearing green?
• What is the probability of finding a student
wearing blue and green?
• See if you can follow the steps on pages 43 – 45.
Fiber Review Chart
• Create a chart that compares and contrasts at
least 8 of the fibers we have explored. This chart
should include what the expected results of the
following tests should be:
– Burn Test
– View under a microscope
– Refractive Index
– Density
– Fluorescence? (If applicable)
More about Fibers
For additional information about fibers and other trace evidence, check out
truTV’s Crime Library at:
www.crimelibrary.com/criminal_mind/forensics/trace/1.html
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