Velocity of light in medium

Velocity of light in

vacuum

R.I. =

----------------------------------

-

FACTS:

Refractive IndexRefractive Index

Liquid RI Glass RIWater 1.333 Vitreous silica 1.458

Olive oil 1.467 Headlight 1.47-1.49

Glycerin 1.473 Window 1.51-1.52

Castor oil 1.82 Bottle 1.51-1.52

Clove oil 1.543 Optical 1.52-1.53

Bromobenzene 1.560 Quartz 1.544-1.553

Bromoform 1.597 Lead 1.56-1.61

Cinnamon oil 1.619 Diamond 2.419

For example, at 25°°C the refractive index of water is

1.333.

• This means that light travels 1.333 times faster in a

vacuum than it does in water.

•

Application of Refractive Application of Refractive Index to Forensics Index to Forensics

Place the glass Place the glass fragment into fragment into different liquids different liquids of known of known refractive indexesrefractive indexes

The glass fragment The glass fragment will seem to will seem to disappear when disappear when placed in a liquid placed in a liquid of the same of the same refractive indexrefractive index

Amorphous vs. Crystalline Amorphous vs. Crystalline StructureStructure

Amorphous Structure: Glass

Crystalline Structure: Salt (NaCl)

Types:

• float glass (Annealed) - typical window glass

• Pyrex – heat resistant

• Tempered glass – side and rear window of cars, when it breaks - it shatters into small squares with little splintering.

• Laminated glass - windshields – strong b/c a sheet of plastic is sandwiched btw two glass windows.

Overview of Tempered Glass:Tempered glass is made using special heated furnaces, which heat the annealed glass to a uniform temperature of approximately 700ºC. When the glass exits the furnace, it is rapidly cooled. This rapid cooling puts the glass surface into a state of compression while the center core remains in tension. Mechanical Strength of Tempered Glass = 4 x Mechanical strength of Annealed GlassWhen tempered glass is broken, all the stored energy is released at once and the glass breaks into small pieces.

Laminated Glass: Car Front Laminated Glass: Car Front WindshieldsWindshields

Overview of Laminated GlassLaminated glass, regard as “Safety glass”, consist of two or more panes of glass with one or more layers of polyvinyl butyral (PVB) sandwiched between them and treated. The glass panes can be basic float glass or tempered or heat strengthened panel. If the glass is broken fragments tend to adhere to the PVB interlayer thereby reducing the risk of injury from falling glass and helping to resist further impact or weather damage.

Comparing glass fragments-

• Measuring properties that will associate one fragment with another – Density , refractive index, striations, fit, type

Determining Density –

Floatation – a known standard glass fragment is immersed into a magic liquid, the composition of the liquid is changed by adding different magic potions to it. This is done until the glass fragment becomes magically suspended in the liquid.

Refractive index – The glass fragments are immersed in another magic liquid (one whose refractive index is known), the liquid is then adjusted until the index reaches that of the glass. When this happens the “Becke Line” has completely disappeared.

Becke line – Indication of Becke line – Indication of Refractive IndexRefractive Index

Notice the halo of Notice the halo of light on the inside light on the inside perimeter of the perimeter of the glass sample.glass sample.

When the Becke line When the Becke line is inside the is inside the perimeter of the perimeter of the glass fragment, the glass fragment, the refractive index of refractive index of the glass is higher the glass is higher than the refractive than the refractive index of the index of the surrounding medium.surrounding medium.

Becke line Indication of Becke line Indication of Refractive Index Refractive Index

Notice the halo of light Notice the halo of light (Becke line) is outside (Becke line) is outside the perimeter of the glass the perimeter of the glass fragment.fragment.

When the Becke line is When the Becke line is outside the perimeter of outside the perimeter of the glass sample, the the glass sample, the refractive index of the refractive index of the medium is higher than the medium is higher than the refractive index of the refractive index of the glass.glass.

Glass has higher refractive index-note white line inside

Glass has lower refractive index-note white line outside

Becke Lines

Becke Lines from GlassBecke Lines from Glass

RI of glass (1.525) > RI of medium (1.6) RI of glass (1.525 < RI of medium (1.34)

Becke line on insideBecke line on outside

For example, at 25°°C the refractive index of water is

1.333.

• This means that light travels 1.333 times faster in a

vacuum than it does in water.

•

SnellSnell’’s Laws Law

N=1.52

The higher the n, the more the light bends

N=1.33

frequency of occurrence of refractive index values of approx 2000 flat glass specimens obtained by the FBI

Radial fractures – fracture pattern in which cracks radiate outward

Concentric fracture - fracture pattern in which cracks encircle the hole

- Typical of a rock, bullet, hammer, etc

How does Glass break?Forensic Fracture Analysis

Determining direction Determining direction of forceof force

Radial edge of glassRadial edge of glass– 3 R3 R’’ss

Radial cracks have Right anglesRadial cracks have Right angles

on the Reverse side of applied forceon the Reverse side of applied force

Concentric edge of glassConcentric edge of glass

Direction of force

Direction of force

• Stress lines on the glass edge of concentric cracks form a right angle on the same side as the force.

Exceptions to 3R rule….Exceptions to 3R rule…. Tempered glassTempered glass

– ““dicesdices”” without forming ridges without forming ridges Very small windows held tightly in frameVery small windows held tightly in frame

– cancan’’t bend or bulge appreciablyt bend or bulge appreciably Windows broken by heat or explosionWindows broken by heat or explosion

– no no ““point of impactpoint of impact””– curved, smooth edges at break pointscurved, smooth edges at break points

• 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