Sunday, May 21, 2006


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) = 1.59
- 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 or crown glass. Polycarbonate lenses continue to be the lens of choice, because of its greater safety and lighter weight.

As light goes from a vacuum to a medium, the light waves slow down slightly. The denser the medium, the slower they move.
Object vergence V = n/u
Image vergence V’ =n’/u’
n = index of refraction for where the light is coming from
n’ = index of refraction for where the light is going to
u = object distance
u’ = image distance

Question: If light of wavelength 460nm encounters the interface of a new medium with an index refraction of 1.24, find the reduced wavelength of the new medium.

Answer: When light encounters a denser medium, the frequency remains constant. Therefore, the speed of light is reduced compared to that inside a vacuum, and thus, the wavelength must be reduced to maintain c = wn. Vergence is inversely related to wavelength and is thus, increased. The light rays may emerge with the same frequency, wavelength, or be reflected or refracted. To calculate the wavelength in the new medium: wm = w/n = 460/1.24 = 371 nm


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