3. Refractive index
The ratio of the velocity of light of a given
wavelength in air, to the velocity of light of
the same wavelength in refracting medium
n = velocity of light in air
velocity of light in medium
4. ABBE VALUE
It is reciprocal of dispersive power
It is denoted by “V”
V = _______1______
__nf – nc____
1 - nd
V = ____1 - nd________
nf - nc
6. SPECIFIC GRAVITY
the ratio of the density of material to the
density of water at 1⁰ centigrade
∫ = _____density of material______
density of water
7. Type of lens material
GLASS MATERIAL
1. crown glass
2. barium crown glass
3. flint
PLASTIC MATERIAL
1. CR 39
2. PMMA
3. Polycarbonate
4. Trivex
8. CROWN GLASS
Another name soda lime silica glass
composition
silica 70%
soda 25%
lime 15% (calcium carbonate)
trace of other material borax, potasium, antimony
, arsenic which improve the quality of glass by
acting as a refining agent
9. Refractive index 1.523
Abbe value 59
Specific gravity 2.5
Density 2.5 g/cm³
11. FLINT
COMPOSITION
PbO₂ = 80%
silicon = 30%
soda & potash = 8%
small percentage of arsenic & antimony
If increase the percentage of PbO₂ in
material increase refractive index as
well as dispersive power specific
gravity and density also increase
13. Plastic material
Polymethylmehtacralyate (PMMA) &
CR39 are principal plastic material that
emulate the crown glass
PROPERTIES PMMA CR39
REAFRACTIVE INDEX
1.49 1.498
ABEE VALUE
58 57.6
DENSITY 1.19 1.32
SPECIFIC GRAVITY 1.19 1.32
14. Trivex
New lens material, contain three good
lens characteristic
Also known as phoenix , trilogy
REFRACTIVE INDEX 1.532
ABBE VALUE 45
DENSITY 1.1
SPECIFIC GRAVITY 1.1
15. QUALITIES
Lightest weight among plastic material
Impact resistance (bullet prove)
Excellent Abbe value
Block UV ray
16. POLYCARBONATE
Alternate plastic material
Block ultra voilet rays
Refractive index 1.586
Abbe value 30
Density 120
Specific gravity