Chapter 4

Section 2, Part A

Glass Analysis

Pg. 96 - 100

Refraction

the bending of a light wave as it passes from one medium to another which causes a change in velocity

Refractive index

ratio of the velocity of light in a vacuum to that in any medium; expressed as the equation:

Refractive index = velocity of light in a vacuum velocity of light in medium

ex: refractive index of water is 1.333 meaning light travels 1.333 times faster in a vacuum

than it does in water at 25 degrees Celsius

Refractive index

Varies with temperature and the frequency of the wavelengths

Testing of a substance must be performed under carefully controlled temperature and lighting conditions

Refractive Index cont.

All refractive indices are determined at a standard wavelength of 589.3 nanometers the predominant wavelength of sodium light Commonly known as sodium D light

A solid or liquid exhibit only one refractive index value for each frequency of light except crystalline solids which have two

refractive indices = called double refraction

Vocabulary

Crystalline solids have definite geometric forms due to the orderly arrangement of the atoms that make up that solid (ex. Sodium chloride)

Amorphous solids have their atomic arrangement randomly throughout (ex. Glass)

Vocabulary

Birefringence – the difference between two refractive indices exhibited by crystalline solids

Dispersion – the separation of light into its component wavelengths

Glass hard, brittle, amorphous

substance composed of silicon oxides and various metal oxides

Types of Glassa. soda-lime-silicate glass (Na, Ca, Mg, Al, O)

used for flat glass, bottles, light bulbs

b. borosilicate glass car headlights, pyrex, thermometers, lab glass

c. Aluminosilicate thermal resistant and used in lab glassware, stove

top cookware

d. lead-alkali-silicate glass commonly known as lead crystal and used in

decorative glassware and neon signs

Glass cont.

Tempered glass – produced by heating and cooling of the glass surfaces so that it fragments rather than shatters used in side and rear car windows and in

foreign made car windshields Laminated glass – the layering of plastic

between two pieces of glass used in US car windshields

Glass cont.

Glass evidence has its greatest value when it can be individualized such as piecing together fragments from a crime scene matching irregular pieces and striations on the

surface Physical properties of density and refractive

index are most useful for characterizing glass particles considered to be a class characteristic

In-Class Assignment/Homework