Chemistry Reference
In-Depth Information
In addition to the minerals, there are also some rock-forming homoge-
neous materials that have neither the definite chemical composition nor
the distinctive crystal structure characteristic of minerals. Such materials
cannot, therefore, be considered as minerals and are known as
mineraloids
.
Obsidian
, for example, a natural material that has been widely used since pre-
historic times for making lithic tools and decorative objects, is a mineraloid.
Obsidian has neither a definite chemical composition nor a characteristic
crystal structure and is not, therefore, a mineral.
Copal
and
amber
are other
mineraloids that since antiquity have been treasured as semiprecious
gemstones
.
TEXTBOX
5
ELECTROMAGNETIC RADIATION; WAVES
Electromagnetic radiation
(EMR) is the generic term used to refer to all
forms of energy that, in the form of waves, travel through space and
matter at very high velocity.
Visible light
, to which the eyes of humans and
animals are sensitive, and
radio and television waves
, which provide much
of the electronic information available today, are the most familiar forms
of electromagnetic radiation. Less familiar, but no less important, forms of
electromagnetic radiation include
infrared
radiation (also known as
heat
waves
),
ultraviolet light
,
X-rays
, and
gamma rays
(see Textbox 13).
Waves
A
wave
is a form of movement, an oscillatory disturbance characterized by
repetitive patterns in fixed time intervals, that propagates through space
or matter without displacing mass, but energy (see Fig. 6). Ocean waves,
sound waves, and electromagnetic waves are common examples of waves.
There are two main types of waves:
longitudinal
and
transversal
waves.
When the oscillation takes place along their axis of propagation, the waves
are known as
longitudinal waves
; when the oscillation occurs across their
axis of propagation, the waves are known as
transverse waves
. Sound
waves are longitudinal. Water waves, in the seas and oceans, and the
waves sometimes seen on the strings of musical instruments are transverse
waves. Electromagnetic waves are also transverse waves.
All types of waves, whether longitudinal or transverse, can be accu-
rately described by their
wavelength
and
frequency
values (see Fig. 6),
which are mathematically related to each other by the expression
νλ =
c
,
where the Greek letter
λ
(lambda) is the wavelength, the Greek letter
ν
(nu) is the
frequency
of the wave, and
c
is the
velocity
of the wave.
