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.