Optical Media and Indices of Refraction
A medium is any material that transmits light. Light travels at different speeds in different media. Light travels faster in a vacuum and slower through any material.
A medium’s refractive index (n) = speed of light in a vacuum (c)/speed of light in a particular medium (v).
Refractive indices are always equal to or greater than 1.0. The index tells us how much light has slowed down when entering a refractive media.
Denser media have higher n values; rarer media have smaller n values.
- Vacuum = 1.00
- Air is assumed to be 1.00
- Water, aqueous, vitreous = 1.33
- Averaged corneal refractive index used for keratometry = 1.3375
- Cornea = 1.37
- Crystalline lens = 1.42
- Plastic (CR-39) = 1.49
- Crown glass = 1.52
- Polycarbonate (higher index than glass or plastic) = 1.58
- Trivex = 1.53
- High index glasses = 1.6/1.7/1.8
- Titanium glass is now available with an index of 1.806. However, it is 2 ½ times heavier than CR-39.
With higher index lenses, chromatic aberration becomes a factor (chromatic aberration is discussed in Section 20).
Although higher index glass lenses are thinner, they have a higher specific gravity and so are considerably heavier than plastic, polycarbonate, Trivex or crown glass.
Because Polycarbonate lenses have a higher index of refraction than Trivex lenses, they are about 10% thinner than Trivex lenses. However, Trivex has a lower specific gravity than polycarbonate, making Trivex lenses about 10% lighter than polycarbonate lenses.
Trivex lenses are now considered to be the lens of choice, because of its greater safety and lighter weight.