If an individual breaks a window or has contact with broken glass, fragments can be transferred to their clothing or person.
Clothing, hair, tools or instruments can be searched for the presence of glass fragments. The forensic scientist will analyse the glass fragments to determine whether they are similar to the fragments found at the crime scene.
For instance, in the case of a house breaking, broken glass on the suspect's clothing would be compared with glass from the broken window at the crime scene, referred to as the control sample.
The glass can be examined in a number of different ways:
- Physical Matching
- Refractive Index
- Scanning Electron Microscope (SEM)
Physical matching remains the most definitive means of establishing a common origin between any samples. If the forensic scientist can fit the two samples of glass together to form one item then they can determine that the samples have only originated from that one item and nowhere else.
The forensic scientist will also look at the physical properties of the recovered glass such as its colour and thickness.
One of the characteristics of glass is the degree to which light is refracted, or how it bends, as it enters the glass. This is measured using an instrument called GRIM (Glass Refractive Index Measurement). Each type of glass will refract the light differently and will have its own refractive index. If two pieces of glass have the same refractive index, the forensic scientist can conclude that the glass originated from the same source.
Scanning Electron Microscope
The Scanning Electron Microscope is used to analyse the elemental composition of the glass.
The forensic scientist can also establish from which side of the glass a window was broken by looking at fracture marks on the broken edges.
Broken glass can also provide other crucial forms of evidence for the forensic scientist such as fingerprints, footwear marks, blood, hairs or fibres from clothing which have been caught on the sharp edge of the glass. Using their specialist skills the forensic scientist can analyse this evidence and help to uncover vital clues to solve the crime.