One of the most useful upgrades to spectacle lenses which can be made is to make them thinner and lighter. There is a wide variety of thin, lightweight high-index eyeglass lenses, based on how efficiently they bend light. The light-bending ability of eyeglass lenses is determined by the “index of refraction” of the lens material. 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. High-index plastic lenses are now available in refractive indices, typically ranging from 1.53 to 1.74. Lenses with an index of refraction of 1.70 or higher typically are at least 50 percent thinner than conventional plastic lenses.
High index lenses are:
Thinner because of their ability to bend light more efficiently. High-index lenses for short sightedness have thinner edges than lenses with the same prescription power that are made of conventional plastic material. Similarly, high-index lenses for long sightedness have much thinner centres and therefore avoids the unsightly milk bottle bottoms appearance of normal lenses.
Lighter. Thinner edges require less lens material, which reduces the overall weight of the lenses. Lenses made of high-index plastic are lighter than the same lenses made in conventional plastic, so they’re more comfortable to wear.
Generally speaking, the higher the index, the higher the cost of the lenses. Your spectacle prescription determines what kind of high-index material you might benefit from, bearing in mind your needs and your budget.