Refractive index, Refraction of light, Measurement of Refractive index, Refractometer, Abbe's refractometer

1. Refractive index
Mr. V. A. Sansare
Asst. Professor of Pharmaceutics.
Indira Institute of Pharmacy, Sadavali, Maharashtra.

2. Introduction: Refraction
• When monochromatic light passes through a
less dense medium such as air or vacuum and
enters a denser medium, the advancing waves
at interface are modified and brought closer
together.
• This leads to decrease in speed and shortening
of wavelength.

4. • When light passes the denser medium, a part
of wave slows down more quickly as it passes
through interface and makes it bend towards
the interface. This phenomenon is called as
refraction.
• If light passes from denser medium to less
denser medium then it is refracted away from
the interface.

7. Refractive index
• The refractive index is a constant for a given pair
of materials under specified conditions. It can be
defined as ratio of speed of light in material 1 to
the speed of light in material 2.
• This is usually written 1n2 and is the refractive
index of material 2 relative to material 1.
• The incident light is in material 1 and the
refracted light is in material 2.
• When the incident light is in a vacuum this value
is called the absolute refractive index of material.

8. • In other words refractive index of substance is
the ratio of velocity of light in vacuum or air to
that in the substance.
n = Velocity of light in air/vacuum
Velocity of light in material

9. Snell’s Law
• In 1621, a Dutch physicist named Willebrord Snell derived the
relationship between the different angles of light as it passes from
one transparent medium to another.
• When light passes from one transparent medium to another, it
bends according to Snell’s law which states:
Sin i
n = Sin r

11. • Refractive indices of most substance are more
than air because the velocity of light in air is
greater than in the substance.
Air 1.00029 Crystal 2.00
Water 1.330 Diamond 2.417
Glass, soda-lime 1.510 Ethyl Alcohol 1.36
Vacuum 1.000000 (exactly) Glass 1.5
Air (STP) 1.00029 Ice 1.309

12. Measurement of Refractive Index:
• Refractive index is determined by using
instrument called refractometer.
• Abbes refractometer
• Immersion refractometer
• Pulfrich refractometer

13. Abbes refractometer
Advantages:
• Most convenient, reliable and simple
instrument.
• Small sample size requirement.
• Ordinary light source can be used.
• Easy maintenance and economy.
• Easy determinations of RI

14. Components:
1. Light source (D-line of sodium)
2. Light reflection mirror
3. Prism box (2 prisms)
4. Telescope (Amici prisms)

17. Principle
• In the Abbe refractometer the liquid sample is
sandwiched into a thin layer between an
illuminating prism and a refracting prism.
• The refracting prism is made of a glass with a
high refractive index (e.g. 1.75) and the
refractometer is designed to be used with
samples having a refractive index smaller than
that.

18. • A light source is projected through the
illuminating prism, the bottom surface of
which is ground (i.e., roughened like a ground-
glass joint),
• So each point on this surface can be thought
of as generating light rays travelling in all
directions.

19. • Light travelling from point A to point B will have
the largest angle of incidence (θi) and hence the
largest possible angle of refraction (θr) for that
sample.
• All other rays of light entering the refracting
prism will have smaller θr and hence lie to the left
of point C.
• Thus, a detector placed on the back side of the
refracting prism would show a light region to the
left and a dark region to the right.

21. • Samples with different refractive indices will
produce different angles of refraction and this
will be reflected in a change in the position of
the borderline between the light and the dark
regions.
• By appropriately calibrating the scale, the
position of the borderline can be used to
determine the refractive index of any sample.

22. • Abbes refractometer may be calibrated with
anyone of the following liquid,
1. Water: 1.3325
2. Carbon tetrachloride: 1.4969
3. Toluene:1.4969
4. a-methylnaphthalene: 1.6176

23. Specific Refraction
• In 1880, scientist Lorentz showed the
property specific refraction which was found
to be more useful in characterization of
substance independent of temperature.
• The specific refraction is mathematically
expressed as

24. 11
S
Where,
Rs is specific refraction of substance
n is refractive index of substance
p is density of substance

25. Molar refraction
• It is defined as the product of specific
refraction and molecular mass of substance.

26. Where,
R is molar refraction of substance
n is refractive index of substance
M is molecular mass of substance
p is density of substance

27. Applications
• Refractive index values are useful in
determination of molecular weights and
structures of organic compounds from their
molar refraction values.
• Most commonly it is used to measure the
concentration of a solute in an aqueous
solution. For a solution of sugar, the refractive
index can be used to determine the sugar
content.

28. • Alcohol content in bioproduction is also
determined from the refractometry.
• Refractive index of a material is the most
important property of any optical system that
uses refraction for example, lenses and
prisms.

29. References
• Textbook of Physical Pharmaceutics I, Hajare
A. Nirali prakashan, 1st Ed; 2.62-2.66.