-
- from: Everitt, Charles. "Refraction." The
Encyclopedia Britannica, 11th Ed. vol. 23. The Encyclopedia
Britannica Company, New York (1911) pp 26-27.
-
- © 1998 R. Paselk
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-
-
- Refraction by a Prism. - In optics a prism is a piece of transparent material
bounded by two plane faces which meet at a definite angle, called
the refracting angle of the prism, in a straight line called
the edge of the prism; a section perpendicular to the edge is
called a principal section. Parallel rays, refracted successively
at the two faces, emerge from the prism as a system of parallel
rays, but the direction is altered by an amount called the deviation.
The deviation depends on the angles of incidence and emergence;
but, since the course of a ray may always be reversed, there
must be a stationary value, either a maximum or minimum, when
the ray traverses the prism symmetrically, i.e. when the
angles of incidence and emergence are equal. As a matter of fact,
it is a minimum, and the position is called the angle of minimum
deviation. The relation between the minimum deviation D, the
angle of the prism i, and the refractive index u
is found as follows. Let in fig. 2, PQRS be the course of the
- ray through the prism: the internal angles
each
equal 1/2 i and the angles of incidence and emergence
are
each equal and connected with
by Snell's law,
i.e.
. Also the deviation D is
. Hence
.
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- Refractometers.-Instruments
for determining the refractive indices of media are termed refractometers.
-
- The simplest are really spectrometers, consisting
of a glass prism, usually hollow and fitted with accurately parallel
glass sides, mounted on a table which carries a fixed collimation
tube and a movable observing tube, the motion of the latter being
recorded on a graduated circle. The collimation tube has a narrow
adiustable slit at its outer end and a lens at the nearer end,
so that the light leaves the tube as a parallel beam. The refracting
angle of the prism, i in our previous notation, is determined
by placing the prism with its refracting edge towards the collimator,
and observing when the reflections of the slit in the two prism
faces coincide with the cross-wires in the observing telescope;
half the angle between these two positions gives i. To
determine the position of minimum deviation, or D, the prism
is removed, and the observing telescope is brought into line
with the slit; in this position the graduation is read. The prism
is replaced, and the telescope moved until it catches the refracted
rays. The prism is now turned about a vertical axis until a position
is found when the telescope has to be moved towards the collimator
in order to catch the rays; this operation sets the prism at
the angle of minimum deviation. The refractive index u
is calculated from the formula given above.
-
- More readily manipulated and of superior
accuracy are refractometers depending on total reflection. The
Abbe refractometer (fig. 3) essentially consists of a double
Abbe prism AB to contain the substance to be experimented with;
and a telescope F to observe the border line of the total reflection.
The prisms, which are right-angled and made of the same flint
glass, are mounted in a hinged frame such that the lower prism,
which is used for purposes of illumination, can be locked so
that the hypothenuse faces are distant by about 0.15 mm., or
rotated away from the upper prism. The double prism is used in
examining liquids, a few drops being placed between prisms; the
single prism is used when solids or plastic bodies are employed.
The mount is capable of rotation about a horizontal axis by an
alidade J. The telescope is provided with a reticule, which can
be brought into exact coincidence with the observed border line,
and is rigidly fastened to a sector S graduated directly in refractive
indices. The reading is effected by a lens L. Beneath the prisms
is a mirror for reflecting light
- into the apparatus. To use the apparatus,
the liquid having been inserted between the prisms, or the solid
attached by its own adhesiveness or by a drop of monobromnaphthalene
to the upper prism, the prism case is rotated until the field
of view consists of a light and dark portion, and the border
line is now brought into coincidence with the reticule of the
telescope. In using a lamp or daylight this border is coloured
, and hence a compensator, consisting of two equal Amici prisms,
is placed between the objective and the prisms. These Amici prisms
can be rotated, in opposite directions, until they produce a
dispersion opposite in sign to that originally seen, and hence
the border line now appears perfectly sharp and colourless. When
at zero the alidade corresponds to a refractive index of 1.3,
and any other reading gives the corresponding index correct to
about 2 units in the 4th decimal place. Since temperature markedly
affects the refractive index, this apparatus is provided with
a device for heating the prisms. Figs. 4 and 5 show the course
of the rays when a solid and liquid
- are being experimented with. Dr R. Wollny's
butter refractometer, also made by Zeiss, is constructed similarly
to Abbe's form, with the exception that the prism casing is rigidly
attached to the telescope, and the observation made by noting
the point where the border line intersects an appropriately graduated
scale in the focal plane of the telescope objective, fractions
being read by a micrometer screw attached to the objective. This
apparatus is also provided with an arrangement for heating.
-
- This method of reading is also employed in
Zeiss's "dipping refractometer " (fig. 6). This instrument
consists of a telescope R having at its lower end a prism P with
a refracting angle of 63° above which and below the objective
is a movable compensator A for purposes of annulling the dispersion
about the border line. In the focal plane of the objective 0
there is a scale Sc, exact reading being made by a micrometer
Z. If a large quantity of liquid be
- available it is sufficient to dip the refractometer
perpendicularly into a beaker containing the liquid and to transmit
light into the instrument by means of a mirror. If only a smaller
quantity be available, it is enclosed in a metal beaker M, which
forms an extension of the instrument, and the liquid is retained
there by a plate D. The instrument is now placed in a trough
B, containing water and having one side of ground glass G; light
is reflected into the refractometer by means of a mirror S outside
this trough. An accuracy Of 3.7 units in the 5th decimal place
is obtainable.
-
- The Pulfrich refractometer is also largely
used, especially for liquids. It consists essentially of a right-angled
glass prism placed on a metal foundation with the faces at right
angles horizontal and vertical, the hypothenuse face being on
the support. The horizontal face is fitted with a small cylindrical
vessel to hold the liquid. Light is led to the prism at grazing
incidence by means of a collimator, and is refracted through
the vertical face, the deviation being observed by a telescope
rotating about a graduated circle. From this the refractive index
is readily calculated if the refractive index of the prism for
the light used be known: a fact supplied by the maker. The instrument
is also available for determining the refractive index of isotropic
solids. A little of the solid is placed in the vessel and a mixture
of monobromnaphthalene and acetone (in which the solid must be
insoluble) is added and adjustment made by adding either one
or other liquid until the border line appears sharp, i.e.
until the liquid has the same index as the solid.
-
- The Herbert Smith refractometer (fig. 7)
is especially suitable for determining the refractive index of
gems, a constant which is
- most valuable in distinguishing the precious
stones. It consists of a hemisphere of very dense glass, having
its plane surface fixed at a certain angle to the axis of the
instrument. Light is admitted by a window on the under side,
which is inclined at the same angle, but in the opposite sense,
to the axis. The light on emerging from the hemisphere is received
by a convex lens, in the focal plane of which is a scale graduated
to read directly in refractive indices. The light then traverses
a positive eye-piece. To use the instrument for a gem, a few
drops of methylene iodide (the refractive index of which may
be raised to 1.800 by dissolving sulphur in it) are placed on
the plane surface of the hemisphere and a facet of the stone
then brought into contact with the surface. If monochromatic
light be used (i.e. the D line of the sodium flame) the
field is sharply divided into a light and a dark portion, and
the position of the line of demarcation on the scale immediately
gives the refractive index. It is necessary for the liquid to
have a higher refractive index than the crystal, and also that
there is close contact between the facet and the lens. The range
of the instrument is between 1.400 and 1.760, the results being
correct to two units in the third decimal place if sodium light
be used. (C. E.*)
-
- © R. Paselk
- Last modified 22 July 2000
