WHAT IS GLASS?FORCES THAT FRACTURE GLASS

GLASS

HISTORICALLY

• Obsidian (volcanic glass) use as cutting tool

• 2500 B.C. glass beads in Egypt• 1st Century B.C. glass blowing• 1291 Murano glass, Venice• Industrial Revolution – mass production of

glass

SOLID OR LIQUID? NEITHER?

Glass is considered a solid because it is rigid

Crystalline solid Fluid Glass

Amorphous

WHAT IS GLASS?

• One of the oldest of all manufactured materials

• Hard, amorphous solid• Without shape, particles are arranged

randomly instead of in a definite pattern• A simple fusion of sand, soda, and lime

produces a transparent solid when cooled

WHY STUDY GLASS?

• Glass has stable, physical properties which can be measured

• May link a suspect to a crime scene• Can determine a sequence of events

WHAT PROPERTIES CAN BE USED TO DISTINGUISH

BETWEEN OR MATCH GLASS SAMPLES?

• Appearance – shape, color, thickness• Density• Refractive Index (and Becke lines) • Fracture patterns• Chemical analysis

• For example, a chip of glass from a broken window may fall into a perp’s trouser cuff or shoes.

• A forensic scientist can identify the chips as part of the broken window.

• Similarly, parts of a broken headlight found at the scene of a hit and run can be used to identify the suspected vehicle.

SAFETY GLASS

• Broken glass can be sharp and dangerous

• car manufacturers use tempered and safety glass in vehicles.

• Tempered glass is made strong by a rapid heating and cooling process that introduces stress to the glass surface

• When tempered glass breaks, it fragments into small squares that do not have sharp edges

LAMINATED GLASS

• Windshields are made of laminated or safety glass.

• This type of glass is strong and break resistant because it is made by sandwiching a layer of plastic between two ordinary pieces of window glass.

DIFFERENT DENSITIES FOR DIFFERENT GLASS

• Forensic scientists use physical properties of glass to associate one type of glass fragment with another.

• One of these properties is density• Density refers to a material’s mass per unit volume

• D=mass/volume

DENSITY

• Density of a substance remains constant, no matter what the size of the substance

• 3 steps to determine density:• 1. weigh the sample to find its mass• 2. determine the volume of the sample• 3. Divide the mass of the sample by its

volume

HOW TO FIND VOLUME USING WATER DISPLACEMENT METHOD

• Initial volume of water in beaker = 300 mL

• Add a rock• New volume= 500 mL• Volume of rock 500-300 = 200 mL

• Now it is your turn to calculate the densities of various types of glass using the water displacement method

• You will need• Glass density handout• Glass samples – Be careful!!• Graduated cylinder • Water• Scale• Calculator

• Clean up your area when you are done.• Turn in your completed handout at the end of the

period

PART II – REFRACTION AND PATTERNS

REFRACTION: BENDING OF LIGHT AS IT PASSES FROM ONE MEDIUM TO

ANOTHER

No refraction –

beads disappear

1

2

34

WHAT IS REFRACTIVE INDEX?

• Comparison of speed of light in vacuum

speed of light in material

• R.I. in vacuum = 1• R.I. in air = 1.0003 or ~1.00

REFRACTION Ex. Air to water

Less dense to more dense

Light is bent toward the normal

Ex. Water to air

More dense to less dense

Light is bent away from the normal

Water

Air

Normal

1

2

Angle 1 = angle of incidence

Angle 2 = angle of refraction

WHICH MEDIUM IS MORE DENSE?

Medium 1

Normal

2

1

Medium 2

Medium 1

Interface

SNELL’S LAW (n1)(sin angle 1) = (n2)(sin angle 2)

(n1) = refractive index of first medium

(n2) = refractive index of second medium

Angle 1 & 2 measured to the normal

Normal

1

2

Medium 1

Medium 2

R.I. USING SUBMERSION METHOD

Glass will seem to disappear when submerged in a liquid with the same refractive index

Notice - Glass disappears in test tube 4. The refractive

index of the glass and the liquid are the same.

1 2 3 4 5

Glass

disappears in

vegetable oil

Glass & oil

have same

R.I.CSI NY video clip

TABLE OF REFRACTIVE INDICES

Refractive index video clip

ACTIVITY: HOW DO WE CALCULATE THE

REFRACTIVE INDEX OF A LIQUID?

LET’S TRY IT!HERE’S THE PLAN.

• Draw two perpendicular lines• Draw a third line 30° from the normal• Position your liquid-filled plastic dish• Using the laser pointer, determine the angle of

refraction• Calculate the refractive index of the liquid

30°

Calculating the refractive index of a liquid

Piece of paper

Normal

Oil

MANY CHOICES FOR PAPER

• Plain, unlined paper• Paper with lines• Polar graph paper (with or without degrees)

either unmarked or with lines

Laser path

Place a dot

along image line.

Draw in line.

Measure angle 2 and apply Snell’s Law

Solve for refractive index of liquid

(n1)(sin first angle) = (n2)(sin second angle)

• (n1) = refractive index of first medium

• (n2) = refractive index of second medium

• Angle 1 & 2 measured to normal

1

2

WHAT IT LOOKS LIKE

Normal

30°

47°

TO REVIEW - SNELL’S LAW

Medium 1(liquid) Medium 2 (air)(n1)(sin first angle) = (n2)(sin second angle)

(n1) (sin 30° ) = (1) (sin 47°)

(n1) (.5000) = (1) (.7313)

(n1) = .7313/.5000 = 1.46

TIME TO WORK (AGAIN)!

• Obtain liquid, hemispherical dish, laser pointer, two pins, paper and a partner

• Set up dish as directed• Determine the angle of incidence and

refraction• Calculate the refractive index of the liquid• What steps should be taken to ensure

reasonable results?

GLASS FRACTURES

GLASS CAN PROVIDE VALUABLE EVIDENCE ABOUT A CRIME

•Comparisons possible with broken or fractured glass include: •physical match•probability of common origin•direction of impact•Sequence of impact

• The penetration of glass by a high speed projectile, such as a bullet, can leave evidence as to the direction of impact.

• If there is more than one hole in glass from flying projectiles, the sequence of their impact can be determined

FRACTURING GLASS

STRAIGHT AND CIRCULAR LINES

• When glass is penetrated by a projectile, it fractures in two ways

• Radially• Extends from the point of impact• Outward lines found opposite side of impact

• Concentrically• Circular line of broken glass around the point of

impact, on the same side

When a high speed projectile hits glass, it bends the glass as far as possible, then breaks it

GLASS FRACTURE COMPARISON

Glass• Is considered a slightly elastic solid• Flexes, then breaks to relieve stress at its elastic limit

BULLET HOLES• When a high-speed projectile, such as a bullet,

penetrates glass, it leaves an exit hole that is larger than its entrance hole.

• This helps to determine the direction of impact.

• The hole produced is often crater-shaped, and surrounded by concentric and radial fractures.

• A piece of glass may be penetrated by more than one projectile

PATH OF PROJECTILE

HIGH OR LOW VELOCITY PROJECTILES

•An impression or hole size in glass can determine whether it was a high or low velocity object

•Check area for evidence – stone v. bullet

VELOCITY OF IMPACT

High velocity (bullet) impact Lower velocity (hammer) impact

Not to scale

• It is possible to determine the order in which the penetrations occurred by examining the fracture lines.

• A new fracture line will always stop when it reaches an existing fracture line

• Therefore, fracture lines from the first penetration will not end at any other fracture lines.

The left fracture cameFirst, because the rightLines terminate at theLines of the left one

DETERMINE THE SEQUENCE

A

B