What does the Lenses Index numbers mean?

The Lens Index number (1.57, 1.61, 1.67, 1.71 or 1.74) refers to the degree of refraction the lens provides. This means how much the light that enters the lens is refracted (bent) as it passes through the lens. Lens Index is also related to the thickness of the lens. The higher the index, the thinner the lens will be. People with higher prescriptions should choose high-index lenses.