What Is Refractive Index? In a Sentence: How Much a Lens “Bends” Light
You’ve probably heard an optometrist ask: “Do you want the 1.60 or the 1.67 lens?” That string of numbers is the refractive index.
The principle isn’t complicated. When light travels from air into a denser transparent material like a lens, it slows down and changes direction — that’s called refraction. Refractive index is simply a number that measures how strongly a material bends light — the higher the number, the stronger the bending power.
What does that have to do with your glasses? A lens’s job, at its core, is to bend light so it focuses precisely on your retina. The stronger a material’s bending power, the shallower the curve and the thinner the lens needed for the same prescription. So for the same -6.00D prescription, a 1.74 lens will be noticeably thinner and lighter than a 1.50 lens.
Remember it this way: refractive index is a material’s “bending score” — the higher the score, the thinner the lens at the same prescription.
Why Do Higher Prescriptions Usually Call for Higher Refractive Index?
A low prescription lens isn’t thick to begin with — 1.50 or 1.56 is usually more than enough. The trouble starts as the prescription climbs: a low-index material at a high prescription produces a noticeably thick edge. Myopic lenses are naturally “thin in the center, thick at the edge,” and the higher the prescription, the more that edge looks like the bottom of a glass bottle.
Switching to a higher-index material (1.67, 1.74) lets you achieve the same prescription with a shallower curve, so the whole lens gets thinner. This makes both weight and appearance much easier to manage. To be clear, this is a result of geometry and material physics — it’s not that the lens gained some special new function.
Is Higher Refractive Index Always Better? Meet the Abbe Number
Higher refractive index does come with a trade-off worth knowing about: the Abbe number, which reflects a lens’s degree of chromatic dispersion.
What is dispersion? White light is actually a mix of different colors, and each color bends at a slightly different angle. When a lens bends light, these colors get pulled apart slightly — in more noticeable cases, you’ll see a faint colored fringe around the edges of objects, streetlights, or high-contrast boundaries. The lower the Abbe number, the more noticeable the dispersion; the higher the Abbe number, the cleaner the edges.
Here’s the catch: materials with a higher refractive index usually have a lower Abbe number. So choosing a lens isn’t just “go with the highest index available” — it’s about striking a balance between thinness and dispersion that fits your prescription and visual habits. That’s also why we won’t just tell you “more expensive and thinner is always better” and leave it at that.
Common Refractive Index Comparison Table
| Refractive Index | Common Name | Relative Thickness at Same Prescription | Approximate Abbe Number (Dispersion) | Typical Prescription Range |
|---|---|---|---|---|
| 1.50 | Standard, CR-39 | Thickest | ~58, least dispersion | Low prescriptions |
| 1.56 | Mid-index | Thicker | ~36–40 | Low-to-mid prescriptions |
| 1.60 | High-index, thin | Medium | ~41–42 | Mid prescriptions |
| 1.67 | Ultra-thin | Thin | ~31–32 | Mid-to-high prescriptions |
| 1.74 | Ultimate thin | Thinnest | ~33 | High prescriptions |
The “prescription range” column shows typical pairings to give you a general idea — which index actually suits you depends on your prescription, frame size, pupillary distance, and visual needs, best assessed by a licensed optometrist.
Two things are worth remembering from this table. One: thickness decreases steadily as refractive index goes up — that part is intuitive. The other is easier to miss: Abbe number doesn’t simply get lower as refractive index goes up — notice that 1.67 actually has a lower Abbe number than some of the others in this list. So judging “how clean the vision will look” purely by how high the refractive index number is can lead you astray.
Does Refractive Index Affect Light Transmission or Image Quality?
Refractive index, as a spec, governs thickness and dispersion. Whether a lens is “bright” or “reflective” is a separate matter, handled by anti-reflective coating — a different process applied to the lens surface, unrelated to refractive index. That’s why you’ll see specs written as “1.67 + anti-reflective coating” — the first part describes the material, the second the surface treatment; each does its own job.
Common Misunderstandings / FAQ
Q: Does a higher refractive index mean clearer vision?
No. Refractive index determines how thin a lens is at a given prescription — it has no direct relationship to clarity. Clarity mainly depends on how accurately the prescription is fitted, along with lens design and coating. A higher-index lens is thinner, lighter, and looks better, but usually has a lower Abbe number and slightly more dispersion.
Q: Do I need to pay extra for 1.74 if I have a low prescription?
Usually not. A low prescription in 1.50 or 1.56 already isn’t thick, so paying extra for 1.74 buys you a very marginal thickness difference — and you may end up with slightly more dispersion from the lower Abbe number. Refractive index is a spec that matters more the higher your prescription is.
Q: How much difference is there really between 1.67 and 1.74?
1.74 is currently the highest refractive index commonly available, slightly thinner and lighter than 1.67, mainly aimed at minimizing bulk for high prescriptions. The two have similar Abbe numbers, so the difference comes down to that last bit of thickness. Whether it’s worth it depends on how much thinness matters to you and your budget.
Refractive index is probably the most fundamental — and most often glossed over — term in getting glasses. Understand it, and you won’t be swayed by the numbers alone when choosing a lens. Come to Beyond Visual Optometry in Xinzhuang or Banqiao to feel the thickness difference between refractive indices for yourself, with an optometrist explaining exactly where the differences matter for your prescription. Our lens page also has the full rundown of refractive index specs and brand options.
(Beyond Visual Optometry carries ZEISS, HOYA, and other lens brands — come compare the different refractive indices in person.)