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Optics,
branch of physical science dealing with the propagation and behavior of light.
In a general sense, light is that part of the electromagnetic spectrum
that extends from X rays to microwaves and includes the radiant energy
that produces the sensation of vision (see Electromagnetic
Radiation; Spectrum; X
Ray). The study of optics is divided into geometrical optics and
physical optics, and these branches are discussed below.
II.
Nature of Light
Radiant
energy has a dual nature and obeys laws that may be explained in terms of
a stream of particles, or packets of energy, called photons, or in terms
of a train of transverse waves (see Photon;
Wave
Motion). The concept of photons is used to explain the interactions of
light and matter that result in a change in the form of energy, as in the
case of the photoelectric cell or luminescence.
The concept of transverse waves is usually used to explain the propagation
of light through various substances and some of the phenomena of image
formation. Geometrically, a simple transverse wave may be described by
points that oscillate in the same plane back and forth across an axis
perpendicular to the direction of oscillation such that at any instant of
time the envelope of these points is, for example, a sine function that
intersects the axis
. The wave front progresses, and the radiant energy travels along the axis.
The oscillating point may be considered to describe the vibration of the
electric component, or vector,
of the light wave. The magnetic component vibrates in a direction
perpendicular to that of the electric vector and to the axis. The magnetic
component is ineffective and may be ignored in the study of visible light.
The number of complete oscillations, or vibrations per second of a point on the light wave is known as the
frequency.
The wavelength is the linear distance parallel to the axis between two
points in the same phase, or occupying equivalent positions on the wave,
for example, the distance from maximum to maximum in the case of a sine
function representation. Differences in wavelength manifest themselves as
differences in color
in the visible spectrum. The visible range extends from about 350
nanometers (violet) to 750 nanometers (red), a nanometer being equal to a
billionth of a meter, or 4 × 10-8 in. White light is a mixture
of the visible wavelengths. No sharp boundaries exist between wavelength
regions, but 10 nanometers may be taken as the low wavelength limit for
ultraviolet radiation. Infrared
radiation, which includes heat energy, includes the wavelengths from
about 700 nanometers to approximately 1 mm. The velocity of an
electromagnetic wave is the product of the frequency and the wavelength.
In a vacuum this velocity is the same for all wavelengths. The velocity of
light in material substances is, with few exceptions, less than in a
vacuum. Also, in material substances this velocity is different for
different wavelengths, as a result of dispersion. The ratio of the
velocity of light in vacuum to the velocity of a particular wavelength of
light in a substance is known as the index of refraction of that substance
for the given wavelength. The index of refraction of a vacuum is equal to
1; that of air is 1.00029, but for most applications it is also taken to
be 1.
The
laws of reflection and refraction of light are usually derived using the
wave theory of light introduced by the Dutch mathematician, astronomer,
and physical scientist Christiaan Huygens. Huygens's principle states that
every point on an initial wave front may be considered as the source of
small, secondary spherical wavelets that spread out in all directions from
their centers with the same velocity, frequency, and wavelength as the
parent wave front. When the wavelets encounter another medium or object,
each point on the boundary becomes a source of two new sets of waves. The
reflected set travels back into the first medium, and the refracted set
enters the second medium. It is sometimes simpler and sufficient to
represent the propagation of light by rays rather than by waves. The ray
is the flow line, or direction of travel, of radiant energy, and the
assumption is made that light does not bend around corners. In geometrical
optics the wave theory of light is ignored and rays are traced through an
optical system by applying the laws of reflection and refraction.
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