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CHURCH-REFRACTION 25 medium, it is found that the direction of the transmitted ray in the second medium is different from that of the incident ray; in other words, the ray is refracted or See also:

bent at the point of incidence. The See also:

LAWS

laws governing refraction are: (I) the refracted and incident rays are coplanar with the normal to the refracting surface at the point of incidence, and (2) the ratio of the sines of the angles between the normal and the incident and refracted rays is See also:

CONSTANT, BENJAMIN (1845-1902)

constant for the two See also:

MEDIA

media, but depends on the nature of the light employed, i.e. on its wave length. This constant is called the relative refractive See also:

INDEX

index of the second medium, and may be denoted by /sob, the suffix ab signifying that the light passes from medium a to medium b; similarly µba denotes the relative refractive index of a with regard to b. The See also:

ABSOLUTE (Lat. absolvere, to loose, set free)

absolute refractive index is the index when the first medium is a vacuum. Elementary phenomena in refraction, such as the apparent bending of a stick when partially immersed in See also:

WATER

water, were observed in very remote times, but the laws, as stated above, were first grasped in the I'i th See also:

CENTURY (from Lat. centuria, a division of a hundred men)

century by W. See also:

Snell and published by See also:

DESCARTES, RENE (1596-1650)

Descartes, the full importance of the dependence of the refractive index on the nature of the light employed being first thoroughly realized by See also:

Newton in his famous prismatic decomposition of See also:

white light into a coloured spectrum. Newton gave a theoretical See also:

INTERPRETATION (from Lat. interpretari, to expound, explain, inter pres, an agent, go-between, interpreter; inter, between, and the root pret-, possibly connected with that seen either in Greek 4 p4'ew, to speak, or irpa-rrecv, to do)

interpretation of these laws on the basis of his corpuscular theory, as did also See also:

Huygens on the wave theory (see LIGHT, II. Theory of). In this See also:

ARTICLE (from Lat. articulus, a joint)

article we only consider refractions at See also:

PLANE

plane surfaces, refraction at spherical surfaces being treated under See also:

LENS. The geometrical theory will be followed, the wave theory being treated in LIGHT, DIFFRACTION and DIs-PERSION.

Refraction at a Plane Surface.—Let LM (fig. I) be the surface dividing two homogeneous -media A and B ; let IO be a ray in the first medium incident on LM at 0, and let OR be the refracted ray. Draw the normal POQ. Then by Snell's See also:

law we have invariably See also:

sin IOP/sin QOR=µab. Hence if two of these quantities be given the third can be calculated. The commonest question is: Given the incident ray and the refractive index to construct the refracted ray. A See also:

SIMPLE

simple construction is to take along the incident ray 01, unit distance OC, and a distance OD equal to the refractive index in the same See also:

units. Draw CE perpendicular to LM, and draw an arc with centre 0 and See also:

RADIUS

radius OD, cutting CE in E. Then EO produced downwards is the refracted ray. The See also:

PROOF (in M. Eng. preove, proeve, preve, &°c., from O. Fr . prueve, proeve, &c., mod. preuve, Late. Lat. proba, probate, to prove, to test the goodness of anything, probus, good)

proof is See also:

LEFT

left to the reader. In the figure the given incident ray is assumed to be passing from a less dense to a denser medium, and it is seen by the construction or by examining the See also:

formula sin fl= sin a/a that for all values of a there is a corresponding value of f3. Consider the See also:

case when the light passes from a denser to a less dense medium.

In the See also:

EQUATION (from Lat. aequatio, aequare, to equalize)

equation sin 13= sin a/a we have in this case p<I. Now if sin a< a, we have sin a/a< I, and hence fl is real. If sin a=a, then sin $=1, i.e. $=9o°; in other words, the refracted ray in the second medium passes parallel to and grazes the bounding surface. The See also:

ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

angle of incidence, which is given by sin 8=1/a, is termed the See also:

CRITICAL (in alphabetical order of authors)

critical angle. For greater values it is obvious that sin a/a> I and there is no refraction into the second medium, the rays being totally reflected back into the first medium; this is called See also:

TOTAL

total See also:

INTERNAL

internal reflection. Images produced by Refraction at Plane Surfaces.—If a luminous point be situated in a medium separated from one of less density by a plane surface, the ray normal to this surface will be unrefracted, whilst the others will undergo - refraction according to their angles of emergence. If the rays in the less dense medium be produced into the denser medium, they envelop a See also:

CAUSTIC (Gr. rcavvraubs, burning)

caustic, but by restricting ourselves to a small See also:

AREA

area about the normal ray it is seen that they intersect this ray in a point which is the geometrical See also:

IMAGE (Lat. imago, perhaps from the same root as imitari, copy, imitate)

image of the luminous source. The position of this point can be easily determined. If l be the distance of the source below the by a lens L. Beneath the prisms is a See also:

MIRROR (through O. Fr. mirour, mod. miroir, from a sup-posed Late Lat. miratorium, from mirari, to admire)

mirror for reflecting light surface, 1' the distance of the image, and ,u the refractive index, then 1'=I/n. This theory provides a convenient method for determining the refractive index of a See also:

PLATE

plate.

A See also:

MICROMETER (from Gr. wcp6c, small, jArpov, a measure)

micrometer See also:

MICROSCOPE (Gr. µucp6s, small, asenreiv, to %view)

microscope, with See also:

VERTICAL (from Lat. vertex, highest point)

vertical See also:

motion, is focused on a scratch on the surface of its See also:

STAGE (Fr. 6/age; from Lat. stare, to stand)

stage; the plate, which has a See also:

FINE

fine scratch on its upper surface, is now introduced, and the microscope is successively focused on the scratch on the stage as viewed- through the plate, and on the scratch on the plate. The difference between the first and third readings gives the thickness of the plate, corresponding to 1 above, and between the second and third readings the See also:

DEPTH

depth of the image, corresponding to 1'. Refraction by a See also:

PRISM (Gr. rrpivµa, properly a thing sawn, Irpii"ety, to saw)

Prism.—In See also:

OPTICS

optics a prism is a piece of trans-See also:

PARENT, SIMON NAPOLEON (1855– )

parent material bounded by two plane faces which meet at a definite angle, called the refracting angle of the prism, in a straight See also:

LINE

line called the edge of the prism; a See also:

section perpendicular to the edge is called a See also:

PRINCIPAL

principal section. Parallel rays, refracted successively at the two faces, emerge from the prism as a See also:

SYSTEM

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 See also:

MATTER

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 p is found as follows. Let in fig. 2, PQRS be the course of the ray through the prism; the internal angles each equal and the angles of incidence and emergence ' are each equal and connected with 4' by Snell's law, i.e. sin ¢ =n sin 0'. Also the deviation D is 2 (0-0'). Hence µ=sin (p/sin =sine (D+i)/sinli. Refractomelers.—See also:

INSTRUMENTS

Instruments for determining the refractive indices of media are termed refractometers.

The simplest are really spectrometers, consisting of a See also:

glass prism, usually hollow and fitted with accurately parallel glass sides, mounted on a table which carries a fixed collimation See also:

TUBE (Lat. tuba)

tube and a movable observing tube, the motion of the latter being recorded on a graduated circle. The collimation tube has a narrow adjustable Flit at its See also:

OUTER

outer end and a lens at the nearer end, so that the light leaves the tube as a parallel See also:

beam. The refracting angle of the prism, i in our previous notation, is deter-See also:

MINED

mined 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 See also:

CROSS

cross-wires in the observing See also:

TELESCOPE

telescope; See also:

HALF

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 See also:

GRADUATION (see also GRADUATE)

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 See also:

AXIS (Lat. for " axle ")

axis until a position is found when the telescope has to be moved towards the collimator in See also:

order to catch the rays; this operation sets the prism at the angle of minimum deviation. The refractive index p is calculated from the formula given above. More readily manipulated and of See also:

SUPERIOR

superior accuracy are refractometers depending on total reflection. The See also:

ABBE (Serbo-Croatian Rai))

Abbe refractometer (fig. 3) essentially consists of a See also:

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 See also:

flint glass, are mounted in a hinged See also:

FRAME

frame such that the See also:

LOWER

lower prism, which is used for purposes of See also:

ILLUMINATION

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 the prisms; the single prism is used when solids or plastic bodies are employed.

The See also:

MOUNT, WILLIAM SIDNEY (1807-1868)

mount is capable of rotation about a See also:

HORIZONTAL

horizontal axis by an See also:

ALIDADE (from the Arab.)

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 See also:

READING

reading is effected 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 See also:

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 See also:

LAMP (from Gr. Xag ras, a torch, Xaµaav, to shine)

lamp or daylight this border is coloured, and hence a compensator, consisting of two equal See also:

AMICI, GIOVANNI BATTISTA (1786-1863)

Amici prisms, is placed between the See also:

OBJECTIVE, or OBJECT GLASS

objective and the prisms. These Amici prisms can be rotated, in opposite directions, until they produce a See also:

dispersion opposite in sign to that originally seen, and hence the border line now appears perfectly See also:

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 See also:

PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place. Since temperature markedly affects the refractive index, this apparatus is provided with a See also:

DEVICE

device for See also:

HEATING

heating the prisms. See also:

FIGS

Figs. 4 and 5 show the course of the rays when a solid and liquid are being experimented with. Dr R. Wollny's See also:

BUTTER (Lat. butyrum, Gr. 0obrvpov, apparently connected with $ous, cow, and rvpbs, cheese, but, according to the New English Dictionary, perhaps of Scythian origin)

butter refractometer, also made by Zeiss, is constructed similarly to Abbe's See also:

FORM (Lat. forma)

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 See also:

scale in the See also:

FOCAL

focal plane of the telescope objective, fractions being read by a micrometer See also:

SCREW (O.E. scrue, from O. Fr. escroue, mod. ecrou; ultimate origin uncertain; the word, or a similar one, appears in Teutonic languages, cf. Ger. Schraube, Dan. skrue, but Skeat, following Diaz, finds the origin in Lat. scrobs, a ditch, hole, particularl

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 See also:

INSTRUMENT (Lat. instrumentum, from insincere, to build up, furnish, arrange, prepare)

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. Ia 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 See also:

DIP (Old Eng. dyppan, connected with the common Teutonic root seen in " deep ")

dip the refractometer perpendicularly into a See also:

BEAKER (Scottish bicker, Lat. bicarium, Ger. Becher, a drinking-bowl)

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 See also:

metal beaker M, which forms an See also:

EXTENSION (Lat. ex, out ; tendere, to stretch)

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 See also:

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 See also:

FOUNDATION (Lat. fundatio, from fundare, to found)

foundation with the faces at right angles horizontal and vertical, the hypothenuse See also:

FACE (from Lat. fades, derived either from facere, to make, or from a root fa-, meaning " appear "; cf. Gr. cbatvstv)

face being on the support. The horizontal face is fitted with a small cylindrical See also:

VESSEL (O. Fr. vaissel, from a rare Lat. vascellum, dim. of vas, vase, urn)

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 See also:

ACETONE, or DIMETHYL KETONE

acetone (in which the solid must be insoluble) is added, and See also:

ADJUSTMENT (from late Lat. ad juxtare, derived from juxta, near, but early confounded with a supposed derivation from justus, right)

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 See also:

Herbert See also:

Smith refractometer (fig.

End of Article: REFRACTION (Lat. refringere, to break open or apart)

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