City of Light: The Story of Fiber Optics

24 CITY OF LIGHT
Total Internal Reflection:
What makes a diamond sparkle
The same effect that makes a diamond sparkle guides light along a jet of water
or an optical fiber. It also helps to create the rainbow. It’s called total internal
reflection, and it’s something we rarely recognize. Total internal reflection has
been known for hundreds of years; light guiding is a more recent discovery.
Total internal reflection is a side effect of refraction, the bending of light that
passes from one transparent material into another. Lenses depend on refraction,
for example, to focus the words from this page onto the back of your eye or to
focus a movie film image onto a screen at the front of the theater.
Refraction occurs because the speed of light in transparent materials is less
than the universal speed limit of 299,792 kilometers per second in a vacuum.
In air, light is just a little bit slower, but in glass it slows to about 200,000
kilometers per second. In diamond, light is even slower, about 125,000
kilometers per second.
This slowing down causes light to bend as it passes from air into glass (figure
A) or vice versa. The light waves keep oscillating at the same frequency, but
they are closer together in slower materials. The degree of bending depends on
the refractive index, which is the speed of light in vacuum divided by the speed
in the material. The refractive index of air is 1.0003, of pure water 1.33, of
ordinary glass about 1.5, and of diamond about 2.4.
The amount of refraction at the surface depends on the difference in refractive
index; the bigger the difference, the more refraction. Light rays passing from air
Fig. A: Refraction of light waves going from glass into air (peaks are shown).
→