Heavy in weight Less impact resistance (Gets easily broken)

Hi Index lenses
If the patient has a very high powered correction, then lenses will look thicker at edge in case of minus lenses and at centre in case of plus lensesTo reduce thickness, titanium oxide is added to glass material and this type of lens is known as a Hi Index lens. They are available in 3 different refractive indices in glass material

1.7 Index
30% thinner than regular glass lens (1.523), also lighter than regular glass.
1.8 Index
40% thinner than regular glass & 10% thinner than 1.7 Hi Index glass
1.9 Index
50% thinner than regular glass, 20% thinner than 1.7 Hi Index glass & 10% thinner than 1.8 Hi Index glass.

Plastic
CR-39 is the most commonly used plastic material with refractive index of 1.498
Benefits
Light weight than glass, 40% light weight than regular glass
Better impact resistance than glass
Doesn't fog up as easily as glass lens.
Can easily be tinted to any colour
Disadvantages.
Gets scratched easily [but this can be reduced by Scratch Resistant Coating (SRC)]
Hi Index Plastic lenses
1.56 Index
15% thinner than normal plastic (1.498)
1.61 Index
20% thinner & lighter than normal plastic
1.67 Index
35% thinner & 30% lighter than normal plastic
1.74 Index
45% thinner than normal plastic; 10% thinner than 1.67; 55% lighter than 1.7 Glass Hi Index

Polycarbonate
Classified as thermoplastic with refractive index of 1.586, is extremely soft & flexible material.
Benefits:
100% U.V. protection
20% thinner than traditional plastic lens.
38% flatter than normal plastic lens because of aspheric design
43% lightweight than normal plastic lens.
Highest impact resistance (Unbreakable)