FORS-2107EL Study Guide - Quiz Guide: Conchoidal Fracture, Refractive Index, Trace Evidence
27 Jun 2018
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Glass Analysis Quiz
-Glass is trace evidence
-Can easily be transferred to hair, clothing, shoes or other
objects
-When collecting glass as evidence it is important to note which
surface is the outside of the glass and which is the inside,
when it is intact with the rest of the glass
-When not intact, it can still be analyzed, so they will look for dir
on one side, to determine the orientation of glass before it was
broken
-Fracture Patterns
-Tell us whether the break was due to a high velocity or or
velocity object, direction of impact, and sequence in which
the breaks occurred
-When forced is applied to a glass surface, it will bend until
the elastic limit of the glass is met, here a break or fracture
will occur
-The hole can be examined to see if it was high or low
velocity, low velvocity -> more shattering around the hole and hole can be irregular shape,
fast velocity -> exit side in hole is wider then the entry side (in bullets)
-Conchoidal fractures:
-Concentric fractures: circular around the centre hole
-Radial fractures: radiate out of centre of the hole
-We can see the type of conchoidal patterns in the glass by looking at the side of the glass,
while known if it is radial or concentric
-Radial conchoidal lines: curve originates on the side of the force, and curves away from
this source
-Concentric fractures: curves originate on the opposite side of the force, and curve towards
the source
-Sequence of events can be established by looking at the radial fracture patterns, a fracture
pattern will stop when it intersects with an existing fracture line, so from this we can see
where lines stop and see the series of events
-Refractive index
-When light enters one medium through another with a different density, it will not travel in a
straight line, but bend at the boundary between the two mediums
-Light slows down when it goes from a lower density compound to a higher density
compound, so it will bend towards the normal line
-Light speeds up when it goes from a higher
density compound to a lower density compound,
so it will bend away from the normal line
-RI (refractive index) (n) = velocity of light in
vacuum / velocity of light in material
-We can use Snells law to find the refractive
index, by finding the angle of incidence and
angle of refraction with a light pointer, through a
beaker with the sample
-Becke lines: lines that show the outline of the glass sample in a liquid due to differences in
refractive indices
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