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Glass

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Sangeetha Balakrishnan

Glass is an amorphous solid made through melting sand, lime, and sodium oxide at high temperatures. Its irregular atomic structure results in varied fracture patterns when broken. Glass properties like density and refractive index are used in forensic investigations. Refractive index is measured by placing glass on a slide with a liquid of known refractive index under a microscope, where Becke lines indicate if the glass or liquid has a higher index.

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Dr. Sangeetha Balakrishnan PG Department of Chemistry Women’s Christian College Chennai – 600 006 Glass 13 January, 2017 Forensic Science PCH/NM/01
What is Glass? • Glass is a hard, amorphous material made by melting sand (silicon dioxide), lime (Calcium oxide) and sodium oxide at very high temperatures. • Sodium oxide is added to reduce the melting point of sand. • Calcium oxide is added to prevent glass from being soluble in water. SiO2 + CaO + Na2O  (melt, high temp)  Glass
Glass is called an amorphous solid because its atoms are arranged in a random fashion. Because of its irregular atomic structure, when glass is broken, it produces a variety of fracture patterns.
Properties of glass that are used in forensic investigations • Density Density = mass/volume Unit: g/mL
• Refractive Index (µ): It is a measure of how light bends as it passes from one medium to another. µ = speed of light in vacuum / speed of light in medium (medium = glass)
Microscopic Determination of Refractive Index of Glass • Place the glass on a slide surrounded by a liquid of known refractive index. • Place the slide on the microscopic stage and focus the lens. i) If the glass seems to disappear, then the glass and the liquid have the same refractive index. ii) If the glass seems visible, then the glass and the surrounding medium have different refractive indices. • Look for Becke Lines.
Glass
• Becke Lines: When the refractive index of the glass is different from the refractive index of the medium, then a halo-like ring appears on the edge of the glass. This halo-like effect is called a Becke Line. The Becke Line is located in the medium that has a higher refractive index.
Info obtained from Becke Line • If the Becke Line is located inside the perimeter of the glass, then the refractive index of the glass is higher than the refractive index of the surrounding medium. • If the Becke Line is located on the outside the perimeter of the glass, then the refractive index of the surrounding medium is higher than the refractive index of glass.
Glass

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Glass

  • 1. Dr. Sangeetha Balakrishnan PG Department of Chemistry Women’s Christian College Chennai – 600 006 Glass 13 January, 2017 Forensic Science PCH/NM/01
  • 2. What is Glass? • Glass is a hard, amorphous material made by melting sand (silicon dioxide), lime (Calcium oxide) and sodium oxide at very high temperatures. • Sodium oxide is added to reduce the melting point of sand. • Calcium oxide is added to prevent glass from being soluble in water. SiO2 + CaO + Na2O  (melt, high temp)  Glass
  • 3. Glass is called an amorphous solid because its atoms are arranged in a random fashion. Because of its irregular atomic structure, when glass is broken, it produces a variety of fracture patterns.
  • 4. Properties of glass that are used in forensic investigations • Density Density = mass/volume Unit: g/mL
  • 5. • Refractive Index (µ): It is a measure of how light bends as it passes from one medium to another. µ = speed of light in vacuum / speed of light in medium (medium = glass)
  • 6. Microscopic Determination of Refractive Index of Glass • Place the glass on a slide surrounded by a liquid of known refractive index. • Place the slide on the microscopic stage and focus the lens. i) If the glass seems to disappear, then the glass and the liquid have the same refractive index. ii) If the glass seems visible, then the glass and the surrounding medium have different refractive indices. • Look for Becke Lines.
  • 8. • Becke Lines: When the refractive index of the glass is different from the refractive index of the medium, then a halo-like ring appears on the edge of the glass. This halo-like effect is called a Becke Line. The Becke Line is located in the medium that has a higher refractive index.
  • 9. Info obtained from Becke Line • If the Becke Line is located inside the perimeter of the glass, then the refractive index of the glass is higher than the refractive index of the surrounding medium. • If the Becke Line is located on the outside the perimeter of the glass, then the refractive index of the surrounding medium is higher than the refractive index of glass.