Cr39 1.499 Invisible Bifocal Hc 70/28mm Optical Lens

What is Lens Index?

The lens index refers to the index of refraction (otherwise known as refractive index) of lens material for eyewear. It is a relative measurement number that describes how efficiently the material bends light. Light refraction will depend on how quick light itself passes through the lens. 

Refractive Index

Lens materials are classified on their refractive index. This refractive index is the ratio of the speed of light when it travels through air to the speed of light when it passes through the lens material. It is an indication of how much light is bent as it travels through the lens. Light is refracted, or bent, at the front surface of the lens, then again as it exits the lens.A denser material bends light more, so not as much material is needed to achieve the same refractive effect as a less dense material. Hence the lens can be made thinner, and also lighter.

With regular eyeglasses lenses, the center of the glasses is thinner and the outer edges are thicker to facilitate refraction which is what makes prescription glasses work! High index lenses have a higher index of refraction than regular lenses, which means they don't need to be as thick around the edges to be effective.

High-index lenses mean that the lens itself can be both thinner and lighter. This allows your glasses to be as fashionable and comfortable as possible. High-index lenses are particularly beneficial if you have a strong eyeglass prescription for nearsightedness, farsightedness, or astigmatism. However, even those with a low eyeglass prescription can benefit from high index lenses.