The refractive index (or index of refraction) of lens is a measure of how much the speed of light is reduced inside the material that lens made of. For example, typical glass has a refractive index of 1.5, which means that in glass, light travels at 1 / 1.5 = 0.67 times the speed of light in a vacuum. Two common properties of glass and other transparent materials are directly related to their refractive index:

First, light rays change direction, bended when they cross the interface from air to the material, an effect that is used in corrective (Ophthalmic) lenses to concentrate light in the focal point.

Second, light reflects partially from surfaces that have a refractive index different from that of their surroundings.

Benefits

The higher refractive index of the lens is the higher is angle in which light bended when passes trough this lens. Thus for the same focal distance (prescription) lenses made from material with higher refractive index are going to be flatter and thinner than lenses from Material with lower refractive index.

Different varieties of thinner, lighter high-index lenses are classified by how well they bend light. The ability to bend light is controlled by the material's "index of refraction," a ratio that compares the speed of light when it travels through air with the speed of light when it passes through a clear material. If a material bends light more, speed is slowed as well. So the higher the refractive index of a lens material, the thinner the lens that made of this material.



High-index lenses can be much thinner and lighter, even in a strong prescription.

For conventional plastic, the ratio (or "index") is 1.50. For glass, it's 1.52. Any lens material

Downside

The higher refractive index of the lens is the higher is necessity of Anti-reflective coating on this lens.