The Refractive Index — Explained

If you’ll recall from our last blog, we dipped a little into the science behind choosing the right lens materials. Each material has something unique to offer and has its own properties, but one of the primary ways they can be deemed good enough to meet the standard of prescriptive lenses is by its placement on the refractive index.

What is the refractive index?

Remember that car mileage metaphor from before? That’s a very generalized explanation. Honestly, it goes even beyond eyewear! The refractive index is a measurement of how light travels through any given material.

A quick experiment at home: fill a drinking glass with water and then dip a pencil into it. You’ll notice by looking at the glass from the side how the pencil in the water seems disconnected from the pencil above the water. It’s the same pencil, but light bends in water, causing the pencil to look different. This is refraction!

Light is refracted differently in certain materials. The way it refracts in water is different from how it refracts in plastic or glass. It all depends on the material and the density of said material of what the light is hitting. The refractive index measures how efficient this refraction is. The higher the number on the index, the slower light travels through the medium, the more the light is bent, and ultimately – the more efficient the refraction is. For the use in eyewear, a higher score on the index means less material needs to be used to achieve a desired effect.

Normal Index = 1.48–1.53

  • Mid Index = 1.54–1.63

  • High Index = 1.64–1.73

  • Ultra High Index = 1.74+

It seems like a small change, going from 1.5 to 1.6. But that one-tenth decimal is the difference between having coke bottle glasses and something more modern and lightweight. This is especially nice for people who require a stronger prescription. Stronger prescriptions traditionally mean the patient needs thicker glasses in order to see 20⁄20. But with these improvements in the refractive index, someone with a strong prescription can achieve 20⁄20 vision with thinner and lighter glasses.

Why is it so important?

Like any technology, understanding and refining refractive index will only advance. The glasses we have today will seem archaic in the future, simply because technology is always improving. By understanding the refractive index, scientists can look for more ways eyewear can be innovated.

Not only that, but understanding the index means current eyewear lenses can be picked out by a patient and favored for their prescription. It allows for us to make better recommendations and more accurate readings for what a patient needs to gain perfect vision.

Do I need to have high index lenses?

First, higher doesn’t always mean the lens is better. The refractive index is only one part of the equation. Other things like Abbe values (the amount of distortion or aberrations) and optics play a vital role in determining if any given material will perform more or less than any other material.

Sometimes a higher index isn’t always the best solution, either. Sure, it’s good to know it exists, but high index lenses aren’t really necessary for most prescription lenses. CR-39, Polycarbonate, and Trivex all offer spectacular lens options without having a 1.60 score like our high index lenses do. It’s all about finding what fits best, without overdoing it.

So, no, you probably don’t need high index lenses. But when we examine your eyes and begin testing prescriptions, it’s good for us to know what materials are available for your ultimate comfort. Afterall, you’re going to be wearing your glasses all the time! If they aren’t comfortable, then something’s not right. But thanks to improvements in lens materials and having high index options, we can provide you with a best-fitting option that is both comfortable to wear and offer better vision.