Refractive Index

Question 1

how does refractive index of glass vary

Answer 1

small changes in composition or by how it is manufactured

Question 2

see powerpoint for

Answer 2

snell’s law

Question 3

refractive index

Answer 3

Dominant physical property of glass

High degree of discrimination

Used for 60 years

Superseded density measurements

Question 4

Becké Line Method 1892

Answer 4

Started with analysis of rock samples

When the objective of the microscope is raised (focus up), a bright line moves into the direction of the material of higher R.I

Once the line disappears or doesn’t move, the R.I. of the oil can be measured by a refractometer.

The Becké line is best observed with contrast microscopy.

As Becke line near match point is influenced by glass shape, size and degree of microscope defocussing

Question 5

measurement of RI

Answer 5

Emmons temperature variation method (1930)

As temp increases then RI decreases for oils, but little change in RI for glass

A circulating water bath heats the oil with an immersed glass in it

When Becké line disappears measure the oil RI with a refractometer at that temp

RI of all window glass and most bottles can be measured using silicone oil by varying temp between 35 °C and 100° C

Mettler Hot-Stage can be used for better temperature control

Glass fragment and immersion oil are placed on a microscope slide

Warmed slowly using a heater with temperature control of ±0.1°C

RI of oil decreases by about 0.004 per 1°C

Automated systems Glass Refractive Index measurement (GRIM) videos glass on heating

At match point the temperature of the oil is recorded and computer converts to RI from references in data base

Reproducibility of repeated measurements - standard RI deviation under 0.00002

Question 6

see powerpoint for

Answer 6

calculating the mean match temp

Question 7

RI determination by GRIM2

Answer 7

Good Precision: SD’s of 0.00002 over 5 hr. period (using optical reference glass) and 0.00003 over 5 days.

Fast analysis routine (~ 5-10 min. / reading)

Semi-automated, reduced operator bias

Improved data handling, reduces transcription errors, facilitates data manipulation

Published by ASTM E-30 as a standard method of analysis.

Question 8

Emmons double variation method

Answer 8

variation of both the wavelength of the light coming through the sample and the temperature.

3 separate wavelengths are used:

• nD, nC and nF
• nD is the sodium D line at 589nm
• nC is the hydrogen lamp C line at 656nm
• nF is the hydrogen lamp F line at 486nm

Gives more precise RI measurements

Glass sample is crushed and placed in silicone oil on hot stage. Temp increases and measurements are taken.

Question 9

characterisation of glass sources

Answer 9

It is useful to determine

1) variation within a single source and
2) variation within all sources

Data can be obtained for:

1) Within source studies (literature) (containers, float sheet, vehicle windows, headlamps)
2) Reference Databases (FBI - RI and ICP-AES from casework, or Florida International University (FIU)- RI and ICP-MS from surveys)

Question 10

velocity of a wave depends on what

Answer 10

its frequency

Question 11

what is dispersion

Answer 11

seen as change in refractive index of different components of light by a prism

• Different colours refract at different angles, so that glass prism splits white light into a spectrum
• Dispersion by rain droplets causes rainbows

Question 12

dispersion causes what in lenses

Answer 12

chromatic aberration, degrading images in microscopes, telescopes, and photographic objectives

Question 13

RI decreases as what increases

Answer 13

as wavelength increases, blue light travelling more slowly in the transparent materials than red light

Question 14

see powerpoint for

Answer 14

Abbe’s number

Question 15

what is dispersion measured by

Answer 15

the parameter Abbe’s number, v

Question 16

what is dispersion characterised by

Answer 16

index of refraction (n) values at three standard wavelengths

see pp

Question 17

flint glass

Answer 17

lead oxide glasses with high RI.

Now lead replaced by titanium and zirconum oxides

Used for simulated diamonds ( rhinestones), electric bulbs and spectacle lens

Originally developed by George Ravenscroftin 1675

Question 18

crown glass

Answer 18

made from alkali-lime silicate

Relatively low RI

Contains 10% potasium oxide and other oxides e.g. zinc, phosphorus, barium and lanthanum oxides and fluorite.

Used in lenses etc.

Crown and flint glass are cemented together to

make lenses corrected for chromatic

aberration (colour defects)

Question 19

becke line dispersion staining

Answer 19

particle is mounted in a liquid with a matching refractive index in the visible range of wavelengths

When the oil has refractive indices near to those of the mineral, Becke lines for shorter wavelengths (blues) will move into the oil while Becke lines for longer wavelengths (reds) will move into the mineral, as the stage is lowered.

This is because liquids tend to have a steeper dispersion curve than colorless solids.

If white light is used, then one tries to find the match for yellow light (nD where D = 589 nm).