Refractive index Flashcards
refractive index of glass varies with
small changes in composition or manufacture
pros of RI
- can measure very precisely (+-0.0002)
- Doesn’t destroy the sample
- Superseded density measurements
- High degree of discrimination
cons of RI
intensive property
snells law
look at ppt for equation
v = velocity of light which is slowed down in the glass
what is RI
- snells law
- Refractive index is the dominant physical property of glass
- used for 60 years
density discrimination of samples with same RI
look on ppt for table
- nD, nC and nF are RI measured using light of different wavelengths
why is density discrimination used
for samples with similar/same refractive index
Becke Line method (1892)
- 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
Becke line example
look on ppt for diagram
- rays of light outside of glass for lower RI than the oil and vice versa
measurements of RI
emmons temperature variation method (1930)
emmons temperature variation method (1930)
- 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
calculating the mean match temp
look at ppt for graph
oil calibration
look at ppt for graph
GRIM 2
look at ppt for graphs
RI determination by GRIM 2
- 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
emmons double variation method
- 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.
characterisation of glass sources
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
Manufacturers use the same processes to produce each type of glass
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.
dispersion of electromagnetic waves
- 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
dispersion in lenses
- 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.
Dispersion is measured by the parameter Abbe’s number, v
look on ppt for equation and image
Dispersion is characterized by index of refraction (n) values at three standard wavelengths
- A low v value implies high dispersion
look on ppt
the variation of refractive index vs. vacuum wavelength for various glasses
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.
Flint glass were 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
crown glass is made form
alkali lime silicate
crown glass
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)
Becke line dispersion staining
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).
