Lens index is a number that describes how thick or thin your lenses are. The higher a lens' index is, the thinner it becomes. Higher prescriptions require higher index lenses, while lower prescriptions require lower index lenses.
What are high index lenses and what's the difference?
High index lenses are thinner, more powerful lenses. They're lightweight and stylish, but are mostly reserved for those with higher vision correction needs. While most prescriptions are compatible with more than one specific index, it's up to you to decide which one fits your personal needs!
The difference between your glasses' lens index can make a huge impact. Deciding which you should get depends on your prescription and your lifestyle. Thicker lenses are more affordable, but can only accommodate lower vision correction needs. Thinner lenses are lightweight (and more stylish!) and can accommodate MANY kinds of vision needs, including higher prescriptions. In terms of balancing price and quality, 1.59 index polycarbonate lenses accommodate most prescriptions and are extra durable.