Handbook for Sound Engineers

Transmission Techniques: Fiber Optics 453

The velocity of electromagnetic energy in free space
is generally called the speed of light, (186,000 mi/s
[300,000 km/s]). The equation clearly shows that the
higher the frequency, the shorter the wavelength.
Light travels slower in other media than a vacuum,
and different wavelengths travel at different speeds in
the same medium. When light passes from one medium

to another, it changes speed, causing a deflection of

light called refraction. A prism demonstrates this prin-

ciple. White light entering a prism is composed of all

colors which the prism refracts. Because each wave-

length changes speed differently, each is refracted

differently, therefore the light emerges from the prism

divided into the colors of the visible spectrum, as shown

in Fig.15-3.

The Particle of Light. Light and electrons both exhibit

wave- and particlelike traits. Albert Einstein theorized

that light could interact with electrons so that the light

itself might be considered as bundles of energy or

quanta (singular, quantum). This helped explain the

photoelectric effect.

In this concept, light rays are considered to be parti-

cles that have a zero rest mass called photons.

The energy contained in a photon depends on the

frequency of the light and is expressed in Planck’s Law,

as

(15-2)

where,

E is the energy in watts,

h is Planck’s constant, equal to 6.624 × 10^34

joule-second,

f is its frequency.

As can be seen from this equation, light energy is

directly related to frequency (or wavelength). As the

frequency increases, so does the energy, and vice versa.

Photon energy is proportional to frequency. Because

most of the interest in photon energy is in the part of the

spectrum measured in wavelength, a more useful equa-

tion which gives energy in electron volts when wave-

length is measured in micrometers (μm) is

.(15-3)

Treating light as both a wave and a particle aids

investigation of fiber optics. We switch back and forth

Figure 15-2. The electromagnetic spectrum.

1022

1021

1020

1019

1018

1017

1016

1015

1014

1013

1012

1011

1010

109

108

107

106

105

104

103

102

101

10

1 THz

1 GHz

1 MHz

1 kHz

Frequency–Hz

Cosmic rays

Gamma rays

X-rays

Ultraviolet light

Visible light

Radar

Tv & FM

Shortwave

radio

AM radio

Sound

Subsonic

Fiber optics

window

Radio frequencies

Wavelength–nm

frequency

speed of light

Wavelength =

Ultraviolet

Violet

Blue

400

450

490

Green

Yellow

Orange

Red

Infrared

580

620

750

800

850

1300

1550

550

Infrared light

Figure 15-3. Light prism.

Refraction

White

light

Refraction

Red

Orange

Yellow

Green

Blue

Violet

Ehf=

E in ev 1.2406

O in Pm

--------------------=