light: There is no easy way to define
“light” scientifically without utilizing
physics. Light can be thought of as
consisting of particles called photons
or as a continuous wave. Because it
acts like both a particle and a wave,
scientists often refer to “the dual
nature of light.” Astronomers are
generally most concerned with light’s
wave nature. All parts of the
electromagnetic spectrum can be
referred to as “light,” although the
term most often refers to the optical
or visible wavelength portion of the
electromagnetic spectrum.