involves the use of a spectroscope, an instrument that disperses radiant
energy either through a prism or a diffraction grating so that the spectrum
can be observed and measured.
Using a spectroscope, it is possible to
isolate individual wavebands of color.
By rotating the prism, we can see
the colors change from red to yellow, from yellow to green, from green to
blue-green (cyan), and finally to blue.
c. Visible Portion of the Spectrum.
The radiant energy we are
primarily concerned with occupies a relatively small part of the overall
For the most part, we are concerned with
wavelengths that range from approximately 400 to 700 nanometers (nm). This
is the visible portion of the spectrum, the portion called light.
A nanometer is a metric measurement equal to 1 billionth of a meter.
It has the same value as the millimicron, which it replaces in
affects the sensory organs of the eye and produces the sensation of vision.
For this reason, other portions of the energy spectrum do not meet the
definition of visible light.
How often have you heard someone mention
infrared or ultraviolet light? They would have been correct, according to
our definition, to have said infrared or ultraviolet radiations.
a. Infrared and Ultraviolet Radiations.
The visible portion of the
electromagnetic spectrum is the part primarily responsible for photographic
exposure. Infrared (IR) and ultraviolet (UV) also play a significant part
in the field of photography. Infrared and ultraviolet radiations fall into
the spectrum immediately above and below the range of visible