Glass Analysis

Question 1

what is tempered glass?

Answer 1

• type of flat glass
• surface of glass heated and cooled rapidly (cool on top)
• much more resistant to breakage than regular glass
• must be cut and shaped prior to tempering
• in back and side windows of cars (tiny, non-sharp pieces)

Question 2

what is laminated glass?

Answer 2

• type of flat glass
• layer of plastic heat-sealed between two layers of non-tempered, heat-strengthened glass
• retains its integrity (makes hole rather than shattering)
• in windshields of cars

Question 3

what is continuous filament used for?

Answer 3

• type of glass fiber
• fiber optics and fiber-reinforced plastic composites

Question 4

what is non-continuous filament used for?

Answer 4

• type of glass fiber
• insulation
• three types: fiberglass wool (glass wool), mineral wool (slag and rock wools), and glass ceramic fibers

Question 5

where does glass fracture from first?

Answer 5

tension side at surface defect called the “stress concentrator”

Question 6

what is the 4R rule?

Answer 6

Radial Ridges form Right angles from the Rear (conchoidal ridges form right angles on the side opposite of where the force was applied)

Question 7

what are the ways you can examine glass?

Answer 7

• physical comparison of fractured glass
• non-optical physical properties (density and thickness)
• optical physical properties (absorption/color, fluorescence, RI/dispersion)
• chemical properties (elemental analysis through ICP)

Question 8

what type of glass examination is the most probative?

Answer 8

physical comparison of large fragments (fit together)

Question 9

what are the major factors affecting glass transfer and persistence?

Answer 9

• position of person relative to window
• nature of clothing worn by person

Question 10

what did glass transfer studies reveal?

Answer 10

• transfer process is highly variable
• 3-4 m max
• thick/wired glass produce more particles
• force influences number of particles backspattered

Question 11

where are glass particles most likely to be found in relation to clothing?

Answer 11

footwear/socks rather than pants

Question 12

what did glass persistence studies reveal?

Answer 12

• as activity vigor increases, the size of retained particles decreases
• very small proportion of fragments on one item are likely to be transferred to a second item by contact between them
• two stage process: initial loss within 30-60 min is very rapid and second process is slower

Question 13

what can you say with an expert opinion about glass?

Answer 13

• inference of source
• individualization or conclusion
• often consistent but not I or C…match/consistent with/etc.

Question 14

how is RI measured in the lab?

Answer 14

• use of Becke line (immersion method)
• halo effect around particle that moves into medium of higher RI when stage is lowered

Question 15

what is an isotropic material?

Answer 15

• a single RI value (n1 = n2 = n3)
• light is NOT able to pass through with crossed polars (no rotation of polarity)

Question 16

what is an anisotropic material?

Answer 16

• more than one RI value (birefringent)
• uniaxial (n1 = n2 x n3)
• biaxial (n1 x n2 x n3)
• light is able to pass through with crossed polars (rotation of polarity)
• no light when aligned at axes (extinction)

Question 17

why does birefringence produce color?

Answer 17

• two paths from two RIs = one faster = change color
• ordinary ray (continues straight path) vs extraordinary ray (bends)
• slow ray = when light separates into two rays, the ray that experiences higher RI

Question 18

what are extinction angles?

Answer 18

• when one of the axes of the indicatrix is aligned with either analyzer or polarizer, the light passing through sample only experiences one RI (extinct)
• when sample is rotated midway between extinction angles, light passing through experiences two RIs (maximum birefringence, bright)

Question 19

what is parallel extinction?

Answer 19

• when length of sample is aligned with one of the polarizing filters
• optic axis is aligned either parallel or perpendicular to sample (dark on crosshairs)

Question 20

what is symmetric extinction?

Answer 20

sample appears extinct midway between either polarizing filter

Question 21

what is oblique or inclined extinction?

Answer 21

sample appears extinct when sample is slightly off any axis (not parallel or symmetrical)

Question 22

what is undulose extinction?

Answer 22

• sample is twisted, bent, or warped
• only a portion of the sample which is aligned with extinction will be dark
• rotating the sample causes the extinguished area to change

Question 23

what is sign of elongation?

Answer 23

• relationship between physical structure of anisotropic crystal and the optical indicatrix
• only for crystals that exhibit parallel or oblique extinction

Question 24

what is positive and negative sign of elongation?

Answer 24

positive = length slow, higher RI (aligned with accessory plate/length of fiber)
negative = length slow aligned with width of fiber

Question 25

in a biaxial crystal, how is optic axis determined?

Answer 25

- two planes have same RI in all directions - direction perpendicular to each planes defines optic axis - angle between two axes = 2V - if 2V is along z-axis = positive - if 2V is along x-axis = negative

Question 26

what is the advantage of conoscopic illumination over orthoscopic?

Answer 26

- swing-out lens causes most of the light rays to pass through sample at an angle (except those that pass through center of lens) - most of the light rays experience two different RIs as they pass through sample - Bertrand lens refocuses these off-axis rays allowing image to be focused normally (can observe interference figures)

Question 27

for optic sign, what does gray or white observed in spaces between interference figure mean?

Answer 27

crystal is first-order

Question 28

for optic sign, what does blue observed in the upper right and lower left after accessory plate is inserted mean?

Answer 28

crystal is positive

Question 29

for optic sign, what does blue observed in the upper left and lower right after accessory plate is inserted mean?

Answer 29

crystal is negative

Question 30

how can interference figures help you determine if a crystal is uniaxial or biaxial?

Answer 30

if you rotate the stage 45 degrees, an uniaxial crystal will keep a cross shape while a biaxial crystal will have a baseball stitch shape (isogyres separate)

Question 31

for optic sign, what does blue observed in middle of the baseball stitch shape (or convex side) mean?

Answer 31

crystal is positive

Question 32

for optic sign, what does blue observed on outsides of the baseball stitch shape (or concave side) mean?

Answer 32

crystal is negative