Natural Science in Archaeology

28 2 Properties of Minerals
go around corners, that it could interact to produce “beats” similar to those produced
by sound waves, and that sunlight could be separated into colors and then recombined
seemed to indicate that light traveled in “waves.” However, Sir Isaac Newton
proposed that light consisted of a series of particles, which, because they traveled in
straight lines, could be considered “rays”. Experiments conducted from Newton’s
time until the beginning of the twentieth century gave ever-increasing support to the
wave theory as opposed to the particle theory. In fact, it became the custom in science
to designate a particular color in terms of its “wavelength.” This system is still used.
In the latter part of the nineteenth century, James Clerk-Maxwell showed that
visible light as well as infrared and ultraviolet radiation are forms of electromagnetic
radiation (Fig. 2.3). All forms of electromagnetic radiation travel at a constant
speed in empty space, approximately 3 × 10 10 cms –1 . Types of radiation differ only in
their wavelengths (λ in Fig. 2.3).
Shortly after Clerk-Maxwell’s work, radio waves (with wavelengths longer than
infrared radiation) and X-rays (with wavelengths much shorter than ultraviolet)
were discovered. Thus the “electromagnetic spectrum” was greatly extended at both
ends of the range of visible light, and today it is known that visible light occupies
only a very narrow portion of that spectrum (Fig. 2.4). The only other portion of the
electromagnetic spectrum that humans can sense is the infrared, the region of heat
radiation. We feel the warmth of infrared radiation generated by particles colliding
in the hot gases of the sun, as well as the infrared radiation from a hot stove, electric
iron, and similar heat-producing implements.
At about the time radio waves and X-rays were discovered, Albert Einstein
created a “new physics” by relating electromagnetic energy to the mass concept
of matter and the velocity of light. We were back to light consisting of particles.
According to this concept, radiation comes in discrete packets, later called photons.
The more energetic photons correspond to radiation with short wavelengths; the
less energetic, to radiation with long wavelengths. Light, then, can be considered
either an electromagnetic wave with a specific wavelength or a stream of light particles
called photons, each possessing a specific amount, or quantum, of energy. The
narrow band of the electromagnetic spectrum occupied by visible light, or light that
registers on the average human eye, can be further broken down by wavelength into
colors (Fig. 2.5). The longest waves in the visible spectrum form the color we call
red; the shortest waves produce blue or violet; the remainder of the rainbow hues
falls in-between. Wavelengths of light recognized by the human eye range from
Fig. 2.3 The wave nature
of light