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Crown Glass: A silicate glass containing oxides of sodium and potassium, used in compound lenses and spectacles; harder than flint glass, with low index and low dispersion.

Refractive Index: The index number compares the speed of light in a transparent medium with the speed of light in air. The higher the index number, the thinner a given lens will be.

Refraction: Refraction occurs when the energy of an incoming light wave matches the natural vibration frequency of the electrons in a material. The light wave penetrates deeply into the material, and causes small vibrations in the electrons. The electrons pass these vibrations on to the atoms in the material, and they send out light waves of the same frequency as the incoming wave. But this all takes time. The part of the wave inside the material slows down, while the part of the wave outside the object maintains its original frequency. This has the effect of bending the portion of the wave inside the object toward what is called the normal line, an imaginary straight line that runs perpendicular to the surface of the object. The deviation from the normal line of the light inside the object will be less than the deviation of the light before it entered the object. The amount of bending, or angle of refraction, of the light wave depends on how much the material slows down the light. Diamonds would not be so glittery if they did not slow down incoming light much more than, say, water does. Diamonds have a higher index of refraction than water, which is to say that they slow down light to a greater degree.

One interesting note about refraction is that light of different frequencies, or energies, will bend at slightly different angles. Let's compare violet light and red light when they enter a glass prism. Because violet light has more energy, it takes longer to interact with the glass. As such, it is slowed down to a greater extent than a wave of red light, and will be bent to a greater degree. This accounts for the order of the colors that we see in a rainbow. It is also what gives a diamond the rainbow fringes that make it so pleasing to the eye.

Specific Gravity: This is a measurement of density or weight of a material at 23°c divided by the same volume of deionized water, expressed in grams per cubic centimeter.

Abbe Number: The Abbe number of a lens material is the reciprocal of the medium's V value, or dispersive power, usually ranging between 20 and 60. Higher Abbe numbers indicate Less chromatic dispersion than materials with lower Abbe values.

Chromatic Dispersion: The separation of light into its component colors, as a prism disperses white light into a color band, or a rainbow effect.

Comfort is an important aspect of modern eyewear. When your thin, lightweight polycarbonate lenses are ordered in an attractive lightweight frame made from one of the new space-age metals or plastics, you will be wearing the lightest (and strongest) eyewear possible. Your eyecare provider will guide you in selecting one of these modern frames for the lightest, most comfortable eyewear possible.

Anyone working or playing outdoors, under fluorescent lamps or in front of a computer screen is exposed to ultraviolet radiation. Polycarbonate lenses include a special permanent filter that provides the maximum protection available against this hazard. These modern lenses block 99% of all potentially damaging ultraviolet radiation. There is no additional cost for this added protection.

Eyeglass lenses work to correct vision by bending light as it passes through the lens. The amount that the light must be bent in order to give you good vision is determined by the prescription that your eye doctor provides. The higher the number in the prescription, the stronger that prescription is, and the more the light must be bent in order to correct your vision. (For example, if your prescription is -5.00, you need a stronger lens than someone with a -2.00 prescription.) So people with stronger prescriptions traditionally have had thick lenses -- that is, their lenses provided more visual correction than the thin lenses worn by others.

But lens materials with a higher index of refraction (or IOR) allow light to pass more quickly through the lens, so less of the actual lens material needs to be used. That's why lenses with a higher index of refraction, or high index lenses, can be thinner than traditional lenses.

The Higher the Number, the Thinner the Lens 

High index lenses are available in either glass or plastic. Different manufacturers make different high index lenses, and what sets each lens apart from the others is its index of refraction. The higher the IOR, the denser the material. All things being equal, a 1.66 IOR material will result in a thinner and lighter-weight lens than a 1.57 IOR material will.

Generally, the higher the IOR, the higher the cost of the lenses will be. The thinnest, lightest-weight lenses are typically the most expensive. With high index lenses, it seems that the less you get, the more you pay.

Regardless of the index of refraction, plastic high index lenses tend to be lighter in weight than glass high index lenses, simply because plastic weighs less than glass to begin with.

Complicated mathematical formulas are used in conjunction with the optical prescription to determine the exact thickness of a lens in a particular frame for a particular patient. But generally, the way to make sure that the resulting lenses are as thin as possible is to use the highest-index lens material available and to keep the eyeglass frame as small and as round as possible.

Low Index	High Index