6. Snell's Law
• Describe the relationship between the
angles of incidence and refraction,
• Snell's law is used to determine the direction
of light rays through refractive media with
varying indices of refraction. The indices of
refraction of the media, labelled n1, n2 and
so on, are used to represent the factor by
which a light ray's speed decreases when
travelling through a refractive medium, such
as glass or water, as opposed to its velocity
in a vacuum.
• As light passes the border between media,
depending upon the relative refractive indices
of the two media, the light will either be
refracted to a lesser angle, or a greater one.
These angles are measured with respect to
the normal line, represented perpendicular to
the boundary. In the case of light travelling
from air into water, light would be refracted
towards the normal line, because the light is
slowed down in water; light travelling from
water to air would refract away from the
normal line.
7. • Theory
• Specific refraction: Denoted By ‘R’
• Independent on temp and pressure
• Lorentz and Lorenz give equation for specific refraction
• R= n2 -1/d(n2 +2)
• Molar refraction: Denoted By “Rm”
• Specific refraction when multiplied by molecular weight we get
molar refraction
• Rm= M(n2 -1)/d(n2 +2)
• Both are constant at given temp
• It is characteristic of numerous type of atoms and arrangements found in
a molecule
• Useful in finding the nature of bonding in molecules and dipole movement
Molar fraction of solid is total sum of mole fraction of the solvent and the solute
along with molar retroactivities R=N1R1+N2R2
8. PRINCIPLE & THEORY
• The main principle involved in refractometry is the refraction based
on the speed of the light that passes in the different mediums.
• Light enters into the light denser medium to high denser medium at an
angle, that is, with bent. The bent in the light ray is known as the
refraction. The relation between the refraction of the light between the air
and the medium is given by Snell's law:
Refractive Index (n) = sin i/sin r
• where n = refractive index; sin i = angle of incidence; sin r = angle of
refraction.
• Samples with different refractive index will produce different angles of
refraction. This helps in the assessment of the compounds’ composition and
the purity of the compounds.
9. FACTORS AFFECTING THE REFRACTIVE INDEX
Temperature
Wavelength
Viscosity
• Temperature: It is inversely proportional to
the refractive index.
• Wavelength: The sodium D-line at 589 nm
(Yellow light) is the appropriate wavelength for
the determination of the refractive index.
• Viscosity: It is inversely proportional to the
refractive index.
10. Measurement of RI
• Two types of instruments
1. REFRACTOMETER
Based on measurement of critical ray (angle) or
by displacement of image
2. INTERFEROMETER
Use interference phenomenon to find out RI
11.
12.
13. INSTRUMENTATION
Types of Refractometers
OTHER REFRACTOMETERS
• Traditional Hand held Refractometer
• Digital Hand held Refractometer
• Inline Process Refractometer (for continuous
liquids through pipes)
24. +ve or –ve Relief
A mineral may exhibit positive or negative
relief:
• +ve relief - index of refraction for the
material is greater than the index of the
oil.
e.g. Garnet 1.76
• -ve relief nmin < noil
e.g. Fluorite 1.433
26. METHODS TO DETERMINE RI
1.Becke line method
2.Immersion Contrast Method
3. Direct measurement Method
4.Minimum Deviation Method
5. Central illumination Method
27. 1.Becke Lines
• To determine whether a mineral has positive or negative
relief, the Becke Line method is used.proposed by
“Mittchel”
• Becke Lines are a band or rim of light visible along the grain
boundary in plane light when the grain mount is slightly out of
focus.
• The Becke line may lie inside or outside the mineral grain
depending on how the microscope is focused
• Becke lines are interpreted to be produced as a result of:
– the lens effect and/or
– internal reflection effect.
28. This method involves the determination of
RELIEF & Appearance of grindle & the facet
edges when immersed in known liquid.
2.Immersion Contrast Method
3. Direct measurement Method
Involves the measurement of RI by using a
microscope with a vernier scale.
By using Table spectrophotometr we can
determine the RI .
Most Accurate & ideal Method
Requires Skill & Ideal Conditions
4.Minimum Deviation Method
29. Central Illumination Method
A – When the refractive index of the mineral fragments
is greater that that of the immersion medium, a white
line (Becke line) forms around the fragments and
moves into the fragments as the focus is raised.
B – When the refractive index of the fragments is less
than that of the immersion medium, the white line
moves outward away from the fragments into the
immersion medium, as the focus is raised.
5.Central Illumination Method
31. Image displacement refractometer
• Any prism spectrometer is refractometer.
• Prism shaped vessel containing sample
Is used instead of prism of
spectroscope, the image of slit will be
displaced proportional to RI of sample
prism.
• Refractive index is read from graduated
arc provided to a movable telescope
arm.
• Accuracy is about ± 10-6 in unit of n.
• Large range of wavelength from UV or
IR can be used as source of radiation.
• Used in gel permeation chromatoghraphy.
32. Principle on which instrumentworks
• Fresnel refractometer
• Deflection refractometer
35. Advantages and disadvantages
Disadvantages
Temperature Corrections (0.00045 per degree
Celsius is used)
Refractive index not determined in Temperature 20 º
or if 20º cooling water is not used to cirucalate in the
instrument.
Optical Exaltation( Conjugated Double bond)
Difference between Observed & Calculated Values of
atomic & Structural Constants.
Advantages
Simple Operation
