Page 220 - Essentials of physical chemistry

182 Essentials of Physical Chemistry

y, ε
λ
Electric field

x

Magnetic field
z, B
FIGURE 9.1 The classical understanding of light as an electromagnetic wave. Make special note of an
oscillating magnetic field as well as an oscillating electric field. In this representation, the fields are restricted to
a single plane and would be said to be ‘‘plane polarized.’’

log (l)cm <--- þ----þ----þ----þ----þ----þ----þ----þ----þ----þ----þ----þ----þ----þ----þ----þ--->
4 3 2 1 0 1 2 3 4 5 6 7 8 9 10 11
Range Radio Microwave Infrared Visible Ultraviolet X-ray g-ray

Other discoveries were being made in Physics in the late 1800s which seemed unrelated to
spectra at first. James Clerk Maxwell (1831–1879), a Scottish physicist and mathematician, pub-
lished several revolutionary papers uniting the theory of electric and magnetic fields in 1861–1862.
Maxwell noted that light might be an electromagnetic wave (Figure 9.1). Soon thereafter, Heinrich
Rudolf Hertz (1847–1894) was the first investigator to send and receive radio waves over a short
distance at Karlsruhe Polytechnic in Germany in 1865. Hertz demonstrated that radio waves could
be diffracted just as can light waves and generally confirmed Maxwell’s prediction that light is an
electromagnetic wave. For our purpose, we need to understand that the visible range of light is only
a narrow range of wavelengths and that other forms of radiation differ only in their wavelength.
Maxwell’s equations and Hertz’s experiments led to a wave explanation with a very fast, but finite,
propagation speed of close to 3 10 10 cm=s. Today this speed, ‘‘c,’’ is known more accurately than
many of the other physical constants. In fact, the other constants are measured in terms of ‘‘c’’ that is
10
now defined as 299,792,458 m=s ¼ 2.99792458 10 cm=s. The wavelength of a wave is the
length of the wave for one full cycle and the frequency of light is related to the wavelength by the
constant speed of their product.

c ¼ ln

So,
c
l
n ¼
and

c
n
l ¼
or

1 n
l ¼ n ¼ c