Short answer: a lens's refractive index measures how efficiently it bends light — a higher index number means the lens can achieve the same optical correction with less material, resulting in a thinner, lighter, and (for minus prescriptions) less magnifying lens. Standard plastic lenses sit around 1.50 index; high-index options run 1.60, 1.67, or 1.74, and the higher the prescription, the more dramatic the difference in the finished lens.
Refractive index is a physical property of the lens material describing how much it bends (refracts) light passing through it. A higher-index material bends light more per unit of thickness, which means the lens can be shaped thinner and still achieve the same corrective power — this is why a high-index lens ground to the exact same prescription as a standard lens comes out noticeably thinner and lighter at the edges (for minus/myopic prescriptions) or center (for plus/hyperopic prescriptions).
At low prescriptions — roughly under -2.00 or +2.00 — the thickness difference between a standard 1.50 index lens and a high-index lens is barely noticeable, and most opticians won't push the upgrade. Past about -4.00 or +4.00, the difference becomes visually obvious: standard lenses start showing a thick, unflattering edge (for minus prescriptions) that noticeably magnifies or shrinks the wearer's eyes behind the lens. This is the point where high-index genuinely changes both comfort and cosmetics, not just marginally improves them.
Higher index isn't purely upside — index above 1.60 sacrifices a small amount of optical clarity (a property called Abbe value) compared to standard plastic, which can introduce faint chromatic fringing at the extreme edge of vision for some wearers. For most customers this trade-off is unnoticeable and easily worth the thickness and weight savings; it's worth mentioning to highly prescription-sensitive customers, but shouldn't discourage the recommendation for anyone with a genuinely strong prescription.
Beyond the cosmetic thinness, high-index lenses are meaningfully lighter than standard lenses at the same prescription strength — a real comfort benefit for anyone wearing glasses all day, and a factor that matters even before the lens looks obviously thick. Customers with strong prescriptions who complain about their glasses feeling heavy on the nose are frequently good candidates for a high-index upgrade even if they haven't specifically asked about lens thickness.
A 1.50/1.60/1.67 range covers the large majority of prescriptions walking through a typical optical counter; 1.74 is worth carrying for the smaller segment of very high prescriptions and premium-tier customers who want the thinnest possible result regardless of power. Position the upgrade clearly by prescription strength rather than as a blanket premium option — a customer at -1.50 gets almost no visible benefit and shouldn't be upsold on it.
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