Refractive index

Question 1

refractive index of glass varies with

Answer 1

small changes in composition or manufacture

Question 2

pros of RI

Answer 2

• can measure very precisely (+-0.0002)
• Doesn’t destroy the sample
• Superseded density measurements
• High degree of discrimination

Question 3

cons of RI

Answer 3

intensive property

Question 4

snells law

Answer 4

look at ppt for equation

v = velocity of light which is slowed down in the glass

Question 5

what is RI

Answer 5

• snells law
• Refractive index is the dominant physical property of glass
• used for 60 years

Question 6

density discrimination of samples with same RI

Answer 6

look on ppt for table

• nD, nC and nF are RI measured using light of different wavelengths

Question 7

why is density discrimination used

Answer 7

for samples with similar/same refractive index

Question 8

Becke Line method (1892)

Answer 8

• started analysis of rocks
• 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 9

Becke line example

Answer 9

look on ppt for diagram

• rays of light outside of glass for lower RI than the oil and vice versa

Question 10

measurements of RI

Answer 10

emmons temperature variation method (1930)

Question 11

emmons temperature variation method (1930)

Answer 11

• As increase temperature then decrease R.I. for oils, but little change in RI for glass.
• a circulating water bath heats the oil (silicone oils are well characterised) with an immersed glass in it
• When Becké line disappears measure the oil R.I. with a refractometer at that temperature
• 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 computor converts to RI from references in data base
• Reproducibility of repeated measurements - standard RI deviation under 0.00002

Question 12

calculating the mean match temp

Answer 12

look at ppt for graph

Question 13

oil calibration

Answer 13

look at ppt for graph

Question 14

GRIM 2

Answer 14

look at ppt for graphs

Question 15

RI determination by GRIM 2

Answer 15

• 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

Question 16

emmons double variation method

Answer 16

• 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 17

characterisation of glass sources

Answer 17

 It is useful to determine

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

Question 18

Data can be obtained for:

Answer 18

1) Within source studies (literature)
(containers, float sheet, vehicle windows, headlamps)
2) Reference Databases

Question 19

Manufacturers use the same processes to produce each type of glass

Answer 19

that fragments from different sources may have similar RI or density.
 Glass with RI of 1.5278 was found in only 1 out of 2,337 specimens in FBI data base.
 Glass of RI 1.5184 was found in 100 out of 2,337 specimens.

Question 20

dispersion of electromagnetic waves

Answer 20

• Velocity of a wave (e.g. light, acoustic, gravity waves) depends on its frequency
• In optics dispersion is 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

look on ppt for image

Question 21

dispersion in lenses

Answer 21

• in lenses, dispersion causes chromatic aberration, degrading images in microscopes, telescopes, and photographic objectives
• RI decreases as wavelength increases, blue light traveling more slowly in the transparent material than red light.

Question 22

Dispersion is measured by the parameter Abbe’s number, v

Answer 22

look on ppt for equation and image

Question 23

Dispersion is characterized by index of refraction (n) values at three standard wavelengths

Answer 23

• A low v value implies high dispersion

look on ppt

Question 24

the variation of refractive index vs. vacuum wavelength for various glasses

Answer 24

look at ppt.

The wavelengths of visible light are shaded in red.
Note that n decreases with increasing λ, which means that the velocity of light in the medium increases with increasing λ.

Question 25

Flint glass were lead oxide glasses with high RI.

Answer 25

 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 26

crown glass is made form

Answer 26

alkali lime silicate

Question 27

crown glass

Answer 27

 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 28

Becke line dispersion staining

Answer 28

 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).