Chemistry Reference
In-Depth Information
is scattered by particles or air bubbles within the material and only the
remainder of the beam passes through. It is the scattering of light
that makes it impossible to see through translucent materials. Opaque
materials scatter and absorb all the light incident on them, blocking its
passage.
Light Interference
When the paths of two light beams cross at a given spot, they are said to
interfere
with each other; they either add light to or subtract light from
each other. If the two beams add together to form a stronger, more intense
beam, providing brighter light than either beam separately, the phenom-
enon is known as
constructive interference
. If the beams subtract from
each other, they give rise to a weaker, dim beam, or even to a region of
darkness. This latter phenomenon is known as
destructive interference
.
The interference of white light, for example, results in spectral colored
fringes. It is the interference of light that causes the iridescent colors often
seen in decayed archaeological glass.
Light Polarization
The electric field that creates the waves of a beam of ordinary light
oscil-
lates
(varies in strength) in many directions perpendicular to that of the
path of the beam (see Textbox 5), and ordinary light is said to be
unpo-
larized
. The phenomenon known as
light polarization
is caused by oscilla-
tions (variations in the strength) of the electric fields that make up a wave
of light. If the direction of the oscillations points in just one direction the
light is said to be polarized. Polarized light is used to characterize amino
acids and also to date animal remains (see Textbox 66).
Luminescence
Materials gain energy either when heated or when irradiated with some
form of energetic radiation, such as ultraviolet light or gamma rays. The
higher the temperature or the amount or radiation, the higher the energy
gained. When heated to high temperature most materials lose part of the
energy gained by emitting light; at about 450°C rocks, minerals, and
metals, for example, become
incandescent
, spontaneously losing part of
the energy gained and shining brightly.
Many forms of nonmetallic matter can also lose energy they gained
by processes other than becoming incandescent. Some materials previously
heated or irradiated with some form of energetic radiation may even lose
energy long before becoming incandescent and do so by a process known
as
luminescence
. Two forms of
luminescence
are generally distinguished,
fluorescence
and
phosphorescence
. If the luminescence is emitted at
the time of irradiation but ceases to be so immediately after the
