This document discusses refractive indices and how they are used to identify minerals. It defines refractive index as the ratio of the speed of light in a vacuum to its speed in a particular material, which is constant for each substance. Refractive indices are determined through several methods, including immersion and using a refractometer. They can indicate whether a mineral is isotropic or anisotropic. Knowing the refractive index helps identify minerals and assess the quality of gemstones.

1. REFRACTIVE INDICES
Under Guidance Of- Presented by-
Prof. R.K. Trivedi Kokil Rabha
Reg No.Y18251013

2. • Introduction
• Properties of light
• Refraction
• Snell’s law of refraction
• Expression for refractive index
• Refractive index in minerals
• Determination of refractive indices
• Application of refractive indices

3. Light is an electromagnetic radiation within a
certain portion of electromagnetic spectrum.
Light usually refers to the visible light,
which is the visible spectrum that is visible to
the human eye.

5. Light has various properties. Some of
them are:
•Reflection
•Refraction
•Dispersion

6.  Refraction is the change in the
direction of light when it travels
from one medium to another.
 It occurs as the speed of light
changes in different media.
 Reason for the increase or decrease
in velocity is the presence of
particles in various media.

7. It states that,
The sine of angle of incidence
bears a constant ratio to the sine
of angle of refraction.
or
The ratio of angle of incidence to
the angle of refraction is a
constant.
This constant is known as
“REFRACTIVE INDEX”

8. According to Snell’s law,

9. The refractive index has a constant value for every substance. This is
because the speed of light varies in different media. When light rays
enter a mineral, there is also a change in velocity and hence each
mineral has a characteristic RI which helps in identifying them.
REFRACTIVE INDEX OF ISOTROPIC MINERALS
1) In isotropic minerals, light travels with the same velocity in all
directions
2)The optical properties are thus same in all directions.
3) So, they have only a single value of RI.
Minerals belonging to isometric system are isotropic in nature
and thus have single refractive index.
Eg: halite, garnet etc.

10. • In anisotropic minerals, the light doesn’t travel
with the same velocity in all the directions.
• Thus, the optical properties are not same in all
the directions.
• So, they have 2 or more refractive indices.
Minerals belonging to tetragonal and
hexagonal system will have two refractive
indices. Minerals belonging to orthorhombic,
monoclinic and triclinic systems have even 3
different values of RI.

11. Vacuum - 1
Air - 1.0003
Pure water - 1.33
Glass - 1.5
Diamond - 2.417
Calcite - 1.486 to 1.740
Corundum - 1.759 to 1.772
Gypsum - 1.519 to 1.531

12. 1) Immersion method- The specimen is immersed in
a liquid of known refractive index. The specimen is
immersed in liquids until a correct match is found.
This method is employed for transparent mineral
grains.
The liquids commonly used to give a range of
refractive indices of common rock forming minerals
are Clove oil (1.54), Methylene iodide(1.74) etc.

14. 2) Becke Method-
When the contact between two minerals or a
mineral and a liquid is focus under conditions of
vertical illumination, a thin white line is seen at
the contact,towards the substance of higher
refractive index. This effect is known as Becke’s
effect and the line is called Becke line.

15. 3) Shadow Method
When the contact between two minerals or a
mineral and a liquid is focus, a shadow is
seen due to spreading of oblique illuminated
rays.

16. 4) Refractometer
These are devices used to
measure refractive indices of
minerals. Several models are
available but all work on the
principle of total internal
reflection and critical angle and
gives RI value on the scale.

17. • It gives idea about the behaviour of light.
• Refractive index has a direct relation with the
lustre of a mineral. The higher the refractive
index, the higher is the shine of the mineral.
• Determining RI helps in mineral identification.
• Checking the purity of gemstones.

18. • Dana, Textbook of mineralogy.
• P.F Karr, Mineral optics.
• P.O. Alexander, A handbook of
minerals,crystals,rocks and ores.
• www.googlesearch.com