Optics is the study of the nature and behavior of light. It can be divided into subdisciplines based on the type of model used to describe light. In physical optics, light is assumed to behave like a classical wave. In quantum optics, light is assumed to have both wave and particle properties. oParticles of light are called photons. In geometric optics, light is assumed to travel in a definite direction with relatively little diffraction. oThis behavior is known as rectilinear propagation. oThe path of propagation of a light wave is a geometric ray. The rays of geometric optics … are perpendicular to the wave fronts of physical optics. indicate the most probable path of the photons of quantum optics. A ray is the path of least action connecting two points in space and is also … the path of least time (the quickest path) the path of least distance (the shortest path) unique and therefore reversible The principle of reversibility states that light will follow exactly the same path if its direction of travel is reversed. Rays are … straight lines in empty euclidean space geodesics ingeneral relativisticspace-time The eye can see something only if a ray from the object reaches the eye. Interface An interface is the boundary between … otwo different media. otwo regions of a medium with different characteristics such as … density (which is often related to temperature) concentration of solute (salinity, for example) mechanical stress When an incident ray meets an interface it will be partially oreflected Reflected rays obey the law of reflection described in this sectionof this book. otransmitted Transmitted rays obey Snell's law, which is described in the next section of this book. oabsorbed Absorbed rays obey the law of conservation of energy. (The energy of the ray is not destroyed, but changes form.)
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Optics is the Study of the Nature and Behavior of Light
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8/12/2019 Optics is the Study of the Nature and Behavior of Light
It is sometimes possible to see over the horizon. The superior mirage or Fata Morgana.
"Fata Morgana (superior mirage), the Italian name of the enchanted, half sister of KingArthur. Italian writers and poets described these effects as seen over the straights of
Messina, between Italy and Sicily. Although the effects occur worldwide, the Italian
name sticks. (From Greenler's book?)"
Quote from somebody, "Under highly stable atmospheric conditions (typically on calm,clear nights), the radar beam can be refracted almost directly into the ground at some
distance from the radar, resulting in an area of intense-looking echoes. This "anomalous
propagation" phenomenon (commonly known as AP) is much less common than groundclutter. Certain sites situated at low elevations on coastlines regularly detect "sea return",
a phenomenon similar to ground clutter except that the echoes come from ocean waves."
dispersion
rainbow
8/12/2019 Optics is the Study of the Nature and Behavior of Light
A 22° halo with an upper tangent arc and two sun dogs
as seen in New York City. The sun is blocked by theauthor's bicycle gloved fist.
Dispersion is generally highest in solids and lowest in gases.
Dispersion is often measured in terms of the coefficient of dispersion, which is defined as thedifference between the refractive indices for for two prominent lines in the spectrum of hydrogen
— the blue F line at 486.1 nm and the red C line at 656.3 nm.
n f − nc
Another common measure of dispersion is the dispersive power.
n f − nc
n D − 1
where nD is the index of refraction for the yellow D line of sodium at 589.0 nm.
Use of a single number to quantify dispersion is rather misleading. Index and wavelength are notlinearly related. Dispersion is best quantified as the rate of change of index of refraction with
wavelength.
dn
d λ
For most transparent materials, a graph of index versus wavelength is curve with a few generalcharacteristics.
8/12/2019 Optics is the Study of the Nature and Behavior of Light
The index of refraction is larger for shorter wavelengths; thus, its slope is always
negative.
Dispersion (the rate of change of index with wavelength) is greater for shorterwavelengths; thus, the graph starts out steep and gradually levels off.
birefringence
Calcite is a common, transparent mineral. It can be found throughout the world, but some of the
best samples were originally found in Iceland. Pieces of this mineral are easily split (or cleaved
as the geologists say) into parallelogram-faced prismatic chunks. Nonmetallic minerals thatcleave easily were called spar in German and so calcite is sometimes also known as Iceland spar.
It is of little economic importance by itself (although it is a component of limestone, which is
used to make cement), but is of some scientific importance. It has been known for several
centuries that light transmitted through calcite takes two paths. This can best be seen by laying alarge crystal of it on a page of text. Every letter can be seen twice. This phenomena is known as
double refraction or birefringence.
8/12/2019 Optics is the Study of the Nature and Behavior of Light
A ray of light incident on a doubly refractive or birefringent material divides into two rays: an
ordinary ray (or o ray or ω [omega] ray) and an extraordinary ray (or e ray or ε [epsilon] ray). Asthe name implies, the o ray behaves in an "ordinary" way, following Snell's law without a
problem. The ratio of the sine of the angle of incidence to the sine of the angle of ordinary
refraction is a constant. The e ray gets its name because it does not obey this rule.
falls apart here …
Birefringence is only a property of solids
When the incident angle is just right, the o and e rays will follow the same path and the
birefringence cannot be seen geometrically. At all other angles, the the two rays will follow
different paths. Thus, the index of refraction for extraordinary rays is also a continuous function
8/12/2019 Optics is the Study of the Nature and Behavior of Light
of direction. The index of refraction for the ordinary ray is constant and is independent of
direction
The index of refraction for the extraordinary ray is a continuous function of direction. The index
of refraction for the ordinary ray is independent of direction. The two indices of refraction are
equal only in the direction of an optic axis.
The measure of birefringence (δ) [delta] is the difference between the indices of refraction of the
two rays.
δ = ne − no
In some materials (like calcite) ne < no and the birefringence is less than zero (that is, the e ray is
refracted less than the o ray) and the material is said to be optically negative. In other materials
(like quartz) the reverse is true and these materials are said to be optically positive. Materials thatdo not show birefringence are said to be isotropic (like diamond); that is, they behave the same
no mater what the alignment of the crystal is relative to the incident ray.
optical behavior comment examples
isotropic (linear) single refraction gases, liquids, glasses, diamond
uniaxial negativedouble refraction
e ray travels fastercalcite, tourmaline, sodium nitrate