Chapter 3 & 4: Glass Evidence. What Is Glass?. Glass is considered a very slow moving liquid Glass is a hard, brittle, amorphous substance that is composed of silicon oxides mixed with various metal oxides. Amorphous solids have their atoms arranged randomly, unlike crystals.
Amorphous Structure: Glass
Crystalline Structure: Salt (NaCl)
A telephone booth with smashed tempered glass in Holloway, London.
Perpendicular marks (~90 degree angles) are found on the edge opposite the way that the force was pushing
This photo depicts two bullet holes in safety glass. Which hole was created first? How can you tell?
The hole on the right was created first. Cracks radiating out from the hole will stop when they encounter another crack. Stress placed on the glass (causing it to crack) will be transferred along the existing crack rather than across it.
speed of light in medium
Becke line on outside
Becke line on inside
RI of glass (1.525 > RI of medium (1.34)
RI of glass (1.525) < RI of medium (1.6)
-automated system for matching glass RI
-uses a step-heating stage and a RI liquid that varies
GRIM II identifies the RI match by monitoring a
video image of the glass fragment in the liquid
-as it is heated/cooled the contrast is measured until
a minimum is reached-the match point
(n1)(sin first angle) = (n2)(sin second angle)
Lower RI to Higher RI = Angle bends toward normal line
Higher to Lower RI = Angle bends away from normal line
Pick a liquid with a refractive index that matches the crime scene glass, and the glass will disappear when submerged!