Clear lenses are available in many types of materials.
CR-39 Plastic A plastic lenses are a good choice for a person that has a mild prescription.
Polycarbonate material has a higher index of refraction than plastic and is thinner and more impact-resistant with a built-in UV blocker. Since it is impact resistant, it can protect the eye against sudden impacts and is the safest lens material, especially recommended for children and for sports.
1.67 High index material has a higher index of refraction than polycarbonate and is recommended for prescriptions between 3.00 diopters and 4.50 diopters. Hi-index lenses reduce light transmission and require an anti-reflective coating.
1.74 High Index material is the thinnest lens material available. Recommended for prescriptions starting at 4.75 diopters and above. An anti-reflective coating is required due to reduced light transmissions.