You are whizzing along at 186,000 miles per second and it slams into an eyeglass lens, enters the lens, and eventually exits the lens.

We need to examine each of those moments in detail.

When that ray is whizzing along and finally hits an eyeglass lens one of three things might happen.

For a ray to enter the lens it must be at the proper angle.

The angle of the approaching ray is called the “angle of incidence” and you need to know that.

We will cover reflection and internal reflection later.


What happens next depends on what the lens is made of or the “material” it is made of.

In optics lens materials are expressed by the letter “n’.

Each of the materials that eyeglass lenses are made from have an index of refraction.

The index of refraction tells us how much a given material will slow down and change the direction of a ray of light passing through it. The higher the index or “n” the thinner a lens can be and produce the same power. Common index numbers include, 1.498, 1.523, 1.586, 1.60, 1.67, and 1.74.

The index of refraction is a numerical representation of how much a specific material will slow the speed of light when it is traveling through it.

The amount of reduction is based on a relationship between the speed of light in air and the speed of light in the given material.

A material with a higher index of refraction will slow the light more than one with a lower index. If the material is in the form of a prism (a lens), not only will the light slow, it will also bend. The higher the index the more bending will take place in less material. Less material translates into a lens with less thickness, which is cosmetically appealing to customers.

The index of refraction, ā€˜nā€˜ is based on a formula, which divides the speed of light in air by the speed of light in a given medium. The speed of light in air is 186,000 miles per second. For the purposes of dispensing, all ā€œnā€ (index) values are calculated based on this fact. Of all the optical formulas this one is pretty basic.

n = index of refraction

n = speed of light in air/ speed of light in a material

For example:

Recall that the index of refraction in crown glass is 1.523.

We can know that for sure if we know the speed at which light travels through the material crown glass.

If I tell you that light travels through crown glass at 122,127 mile per second (mps) then you can plug it into the formula and find the index of refraction:

n = speed of light in crown glass

Divided by the speed of light in air


n = 186,000 / 122,127 = 1.523

Crown glass has an index of refraction of 1.523

To use the formula in reverse or when you have the index of refraction but need the speed of light in the material divide 186,000 by the index you have.

Speed of light when passing through some common lens materials

CR-39 = 1.498

Speed of light in CR-39 = 124,165

186,000 / 1.498 = 124,165

186,000 / 124,165 = 1.498

Trivex = 1.532

Speed of light in Trivex = 121,409

Polycarbonate = 1.586

Speed of light in polycarbonate = 117,276

High index plastic = 1.66

Speed of light in high index plastic 1.66 = 112,048

Note how as the index goes up the speed goes down.