The study of radioactivity, beginning with Antoine-Henri Becquerel’s dis-
covery in 1896, was another problem relating to atomic structure. In fact, ra-
dioactivity was another enigma not explained by classical mechanics. Studies
showed that atoms spontaneously gave off three distinct types of radiation, of
which two were eventually shown to be particles of matter. The alpha particle
() was identical to a doubly ionized helium atom, and the beta particle
() was identical to an electron. [The third type of radiation, gamma ()
radiation, is a form of electromagnetic radiation.] However, no known
chemical process could eject particles from atoms in the manner indicated by
radioactivity.
9.6 The Photoelectric Effect
In 1887 Heinrich Hertz, who is better known for his discovery of radio waves,
noticed in his investigations of evacuated tubes that when light was shined on
a piece of metal in a vacuum, various electrical effects were produced. Given
that the electron was yet to be discovered, an explanation was not forthcom-
ing. After the discovery of the electron, however, reinvestigation of this phe-
nomenon by other scientists, especially the Hungarian-German physicist
Philipp Eduard Anton von Lenard, indicated that the metals were indeed emit-
ting electrons upon illumination. Ultraviolet light was the best light to use, and
in a series of experiments several interesting trends were noticed. First, the fre-
quency of light used to illuminate the metal made a difference. Below a certain
frequency, called the threshold frequency,no electrons were given off; above that
certain frequency, electrons were emitted. Second and more inexplicable, a
greater intensity of light did not cause electrons to come off at greater speed,
it increased the number of electrons that were emitted. However, a shorter
wavelength (that is, a higher frequency) of light did cause the electrons to come
off at greater speeds. This was unusual, for the modern theory of waves (espe-
cially sound waves) suggested that the intensity was directly related to the en-
ergy of the wave. Since light is a wave, a greater intensity of light should have
a greater energy. The emitted electrons, however, did not come off at any
greater kinetic energy when the intensity of the light was increased. The kinetic
energy (equal to ^12 mv^2 ) of the electrons did increase when the frequencyof the
light was increased. The current understanding of light, waves, and electrons
did not supply any reasonable justification for these results.
9.7 The Nature of Light
Since the time of Newton, the question “What is light?’’ has been debated,
mostly because of conflicting evidence. Some evidence showed that light acted
as a particle, and some evidence indicated that light acted as a wave. However,
Thomas Young’s double-slit experiment in 1801 (Figure 9.10) demonstrated
conclusively the diffraction patterns caused by constructive and destructive in-
terference of light. It seemed clear that light was a wave of extremely small
wavelength, about 4000–7000 Å depending on the color of the light. (One
angstrom, 1 Å, equals 10^10 meters. Anders Jonas Ångström was a Swedish
physicist and astronomer.)
After the introduction of the spectroscope, scientists began studying the in-
teraction of light and matter to understand how light was emitted and ab-
sorbed by bodies of matter. Solid bodies heated to glowing emitted a continu-
ous spectrum composed of all wavelengths of light. The intensities of the
9.7 The Nature of Light 253