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, if we hear u sound from a tuning fork, a piano string, a metal bell, etc., it is a simple matter to prove that they are vibrating, and that, when tin- motion ceases, the sound does also. Again, it is a familiar fact that some time elapses between the instants when tin- vibration begins and the sound is heard, and bet... |
etc., are noises. We can study the nature of the vibrations of a body, as has been explained on page 319, and it is found that a musical note is always due to a periodic vibration ; a noise, to an extremely complex motion, consisting of differ- ent vibrations which differ slightly in frequency and which are rapidly da... |
differ in their frequencies or in their amplitudes, the corresponding notes are different ; but the ditVerenees in phase between the coni| -nts may be different in the two complex vibrations or may be the same; they have no influence on the note. (This is seen to be in ace. n.l with ohm's law, as stated above.) Two di... |
ear, the roaring that is heard is due to the resonance of the inclosed air produced by certain sounds in tin- room. The sound may be varied by partly closing the opening of the sht'll or vase. The passage leading from the outside of the head into the eardrum forms a small resonator, and its action is often noticed whe... |
observer are at rest relatively; or, more generally, we assign a number equal to the number of waves reaching the ear in one second. (See Doppler's Prin- ciple, page 345.) This number is called the "pitch" of the sound. Similarly, the pitch of a complex note is denned to be the pitch of its fundamental. The loudness o... |
play the part of reso- nators for musical notes. Other branches of the auditory nerve end in another cavity under conditions which have led several scientists to believe that their function was to respond to noises. In any case it is easy to trace a mechanical con- nection between the waves in the air and the nerve en... |
notes heard. All stringed instruments, with a few exceptions, have the strings stretched between pegs which are fastened to a wooden board or box. Owing to the vibrations of the strings, and the resulting motion of the pegs, this board is made to vibrate; and, since these vibrations are "forced," they imitate more or ... |
aking small changes a pipe may be " tuned" accurately. If the pipe is not of uniform cross section, but conical, there are marked differences produced — the position of the nodes and loops is affected ; as is shown by the difference between an organ note and one produced by a horn. The action of "stops" or "pistons" in... |
a strip of brass fastened at one end, called the "reed." When pressure in the wind chest is sufficient, this spring door is pushed open, a 1.1 r passes, and the pressure being ':! 1\. the Nprin^ returns and. o\\ in^ to its inertia. continues to vibrate in its own natural period. In this manner a series of puffs «.i de... |
the form of instrument pictured are not perpendicular to the plate, but are inclined slightly, so that the axis of the passage con- Fio. 192 a. -The sidered as a vector has a component parallel to tlu of the circle through the holes at that point. Immediately over tins fixed plate is a movable on*. \\hi< -h can rotate... |
same bellows may be arranged one over the other with their movable plates on the same shaft and facing each other; this enables one to produce two sounds whose pitches have a known ratio, that is, are at a known "inter- val" apart. This instrument was invented by the German physicist Seebeck, and was improved by Cagni... |
is what may be called " mechanical" refraction. True refraction, similar to that observed in Optics, may, however, be produced. Tyndall made a lens out of a soap bubble filled with nitrous oxide gas, which had all the properties with air waves that an ordinary glass lens has for ether waves. A few photographs of pulse... |
predict with great exactness the dura- tion of reverberation in a hall when its dimensions and the materials used in its construction are known. The acoustic property of a hall depends upon other things than reverberation, for its shape may be such as to focus the iraYee at {.articular points, etc. "Sounding boards" w... |
the same time. This fact has nothing to do with the state of civilization or of musical cultivation ; it is a property of the human ear. It was recognized by the Greeks before the days of Aristotle (probably as long ago as 500 B.C.) that, if two stretched strings of the same size and material and under the same tensio... |
V; those in the octave above this, 4^, 4 0, 4P, 4 #, -, 8N; etc. The notes selected for the octave below the original one have the pitches J JV, JO, J P, J (?, •••, JVi etc. So, in defining a scale, it is necessary to choose only the pitches of the keynote and the notes in its octave. The Diatonic Scale. — The " diaton... |
sym- bols have been given the notes in the various octaves of dif- ferent scales. For instance, " Ut3" is sometimes used as the name of the key tone above described. So it is seen that the pitch corresponding to a certain name or symbol in musical notation is not definite. Thus the note called Ut4 had a pitch less tha... |
experiment* on \\ln.-h Hrlmholtx established his theory of harmony, and also a complete explanation of musical lustrum* Mt- ;iti<l scales. RAY i I vols. London. 1886. i * is a mathematical tren • 1 1 contains • \ i > < 1 1 > experimental knowledge of vibrations, waves, and musical notes. LIGHT CHAPTER XXV GENERAL PHEN... |
away from a source, and those which are contracting toward a point. It is a familiar fact that an ordinary reading lens, or magnifier, may produce an image of the sun upon some opaque screen — thus acting as a " burn- ing glass " ; this means that the plane waves reaching the lens from any point of the sun are changed... |
ether are entirely absent. In general, it will be observed that there are apparently two or more sets of fringes superimposed, each set having a definite color. It is possible, however, to secure such a colored source, that the bands are all of one color, separated by the dark fringes. (This is approximately the case ... |
on page 292, by various means; and they have been observed as long as 25,000 pp, i.e. 0.025 cm. (Electrical oscillations produce waves in the ether, which are as a rule very long ; but, using minute conductors, waves as short as 0.6 cm. have been obtained. There is thus a gap between 0.6 cm. and 0.025 cm. which has no... |
shadow is that which we have called tin; "geometrical" shadow, if wo neglect diffraction phenomena, as We Shall f,,r the time beiO I f, however, the SOUrce is large, object^, representing the on • tureen rG\y »n opaque,ik|, | tl;iMi(i MI. ;m i]lllmm;lted piece of paper, the shadow phenomena are evidently not quite so ... |
ed. Thus if there is a small opening at O in the screen, and A is a point of an illuminated figure, there will be a cone of light from A passing through the opening. If this meets a screen, it will make at the point /* a bright './•:\/:/,M/. i'in-:.\n.Mi-:\.\ LK.IIT i-JT spot, which will hsivi- tin- >ha|>«- of tli«- op... |
opaque owing to absorption. If we can look through an object and see sources of light 428 LIU / IT on the other side sharply defined, we say that it is "trans- parent." Here, again, the body may be transparent to some waves and not to others. Waves, then, when incident upon a transparent body, will under ordinary cond... |
glass, etc., of the third. All these bodies which reflect light diffusively, diffuse it also when they transmit it ; that is, they are translucent. If these foreign particles in a transparent medium are very minute, it may happen that the shorter waves only are affected, while the longer ones will be transmitted throu... |
taken for a dis- turbance to pass from 01 to P along this line is less than along any other line. (Huygens proved this.) Therefore 01 is the pole of P on the incident wave front ; and by constructing Fresnel's zones around Ov we are led at once to Huygens's solution. The disturbance at P is due directly to that at 0; ... |
do long ones. (In the pure ether all waves, so far as we kno\v. have the same velocity.) This kind of medium is said to have ordinary or " normal " disper- sion. In other media, it may happen that some waves are refracted more than others which have a shorter wave length; they are said to have "anomalous dispersion" (... |
reflection or refraction, we may have either of two conditions : the centre of the reflected or refracted waves may be in the medium in which the waves are advancing, i.e. the waves converge to a "focus"; or the centre of the waves may be in the other medium, i.e. the waves will diverge away from their centre. (Of cou... |
shortly that in all eases a pencil which is normal to a surface produces by reflection or refraction a homocentric pencil; and in nearly all cases an oblique pencil produces,in astigmatic one. Properties of a Focus from the Standpoint of Waves. — Since the locus of the points readied by the dis- turbances at any one i... |
as spherical; and, if Q is the quantity of light emitted by the source in a unit of time, assuming that it radiates uniformly in all directions, the amount falling on a portion of the spherical surface of area A at a distance r is H tne illuminated surface is small and is oblique to the direction of propagation, let i... |
to our eyes like a luminous disk of uniform brightness. If the area of the oblique surface is A, and if it is inclined at such an angle that a perpendicular to it makes the angle N with a line drawn to the eye, A cos N is the area of the projec- tion of this surface perpendicular to this line ; and a surface of this a... |
to have the angles of inclination the same). In Bunsen's photometer, a screen consisting of white unglazed paper, in the centre of which there is a small round or star-shaped grease spot, is placed between the two sources. Looking at this screen from either side, any portion is illu- minated by the transmitted light f... |
a ray has been deduced (see page 431). We will now consider several special illustrations. /N, Plane Waves Incident upon a Plane Mirror. — This is the case already discussed on page 367, and needs no further treatment here. There is one illustration of it, however, which may be described. It is that of a plane mirror ... |
, after reflection at other points of the surface, have directions which pass through 0', provided the surface is only slightly curved, and that only a small portion of the mirror around M is used. For, since the line PC bisects the angle PO-.PO' = C or, putting Jlo = u, MO' = v, MC = r, PO'.PO' = u-r:r-v. If, however,... |
* and the virtual is therefore a point bi- secting th«- liii.-r.i/. Thin is called the " principal focus." Thus a normal pencil gives rte to a homocentrio one; and it may be shown, by following the same method as was used for concave mirror.'-, that an oblique pencil produces an astigmatic one by reflection. 454 Plane... |
NT and so ^ > N^ ; that is, the refracted ray is bent in closer to the normal than is the incident ray. This is the case illustrated in the cut. On the other hand, if V|<VP -ZV1<-ZV3; and tin- ivfrartrd ray is In-lit away t'nmi tin- normal. This 456 456 LIGHT may be illustrated by the cut, if the arrows indicating the... |
depends upon this. 458 Lid I IT Special Cases. — The refracting matter is generally made into a figure with regular geometrical surfaces. There are three cases of special interest : (1) a " plate," which is a figure bounded in part by two parallel planes ; (2) a " prism," which is a figure bounded in part by two non-p... |
by a prism. It is evident from the geometry of the figure that the angle b« -tween the two normals drawn to the two surfaces equals the angle of the prism ; and that the following relations are trii.- : Further, JVj and NT and NJ and NJ are connected by the ivt'rart i«»n I'nrinula; and so the value of D, the "de-. " a... |
and ° 4 2 Fw.314. — Formation of Image* of a point source 0 by a plane B01 its prolonga- tion backward until •nrfkoe: (1) when n>l ; (2) when n<l.,• •> •, j,-T it meets the normal OA. There are two cases to be con- sidered, depending upon whether the velocity of waves in the first medium is greater or less than that i... |
air. (n' for water is approximately -; OA \ so 00l is. J A method is thus offered for the measure- ment of w, since both OA and 00l can be measured. If the pencil of rays from 0 is oblique, it forms by refraction an astigmatic pencil. Thus, ^ Fw. 116. — Fnrin.itl..n of focal line* at A\ and A', by the refraction of an... |
A is x, the value of u is — x. If n' is the index of refraction of the first medium with reference to the second, n' = - ; and the general formula becomesn't; u n'r & u l=£, or! =!' Conversely, if a pencil of rays in the second medium is converging apparently toward a point C? in the first nn-dium, tlu-y will actually ... |
of the lens, because the centre of the first surface of the lens lies on this side. The refraction REFRACTION 471 at the second surface is from the lens out into the air ; so the formula is — = — h — —, where u2 and v2 are positive if the v2 "2 r2 points to which they refer are to the left of the lens. But, if the len... |
, if PR is large compared with RQ, we have, on expansion by the binomial theorem and neglecting small terms, 2 PR 2 PS 2 PR* PR-PQ = - Therefore, or. In the formula, as given above, for a lens, then, 2 PS 2 P R'O f f -S 1 CT? Consequently, ± + ^ = („ - 1)(I + I). If the incident ray makes a sufficiently small angle wit... |
the lens as to become parallel tn its axis. Conversely, we can have plane waves incident upon the lens from the other side, which will con- verge to a point at a distance/ from it. There are thus two •I7»i ( LIGHT principal foci ; one on each side of the lens, and at the same distance / from it, if the lens is thin. T... |
the rays from any point source : A ray parallel to the axis is deflected so as to pass through the principal focus on the other side of the lens ; a ray passing through the principal focus on the " incident side " of the lens will emerge on the other side parallel to the axis ; a ray meeting the lens at the point wher... |
If this angle is small. is the focal length, the length of the line Q(^ <«quals the | / l.y this nngl.-, for the an (PC/*,) an- 1 C<"\',) an- t-.jnal. If these two rays com*' from t\\o| 480 LIGHT on the edge of the distant object, its image will be bounded by the two points Q and Ql ; and the linear dimensions of this... |
thinnest at their centres are diverging. We can also deduce graphically the position of the image of any point source, because we know the effect of the lens upon three ravs : a ray parallel to the axis emerges in such a direction that if pro- longed backward it would ^ F Fio. 282. — Special case: the Incident rays ar... |
i intersecting the focal plane at Fliu AI draw through A the line AQSi QS is the continuation of the incident ray. For, if there were two parallel rays PQ and BC, they would diverge, on emerging, as if they came from A. F, \S Fio. 286. — Construction for the refraction of any ray PQ by a diverging lens. Spherical Aberr... |
deduced by considering a simple case. Let AOB be the cross section of a converging lens by a plane through the axis, and let 0' be the image of 0, a point on the axis. 0' is the image of 0 because the disturb- ances along the 0' different rays from the latter reach the former in the same time, and so are in Fio.287.-D... |
; and f* or, Therefore, the focal length y2 = FIG. 289.— Formation of an image by a combination of two thin lenses. A special case of this is when the two lenses are placed close together, i.e. when h = 0. Then the focal length equals 0' ; or, calling it /, 1 = 1 + -i- /l-r/2 / /I /2 The reciprocal of the focal length... |
I',. /! is different from lv So, as the waves pass from one medium into another, their wave length changes.) In the case of ordinary dispersion the shorter waves are refracted more than the long ones ; or, in other words, the index of refraction varies in an inverse manner from the wave length. Methods for the study o... |
waves chosen are those which act most intensely upon a photographic plate. Achromatism. — Since a lens can be considered as made up of prisms, we shall first show how one prism may be so chosen as to neutralize the dispersive action of another prism for two given trains of waves. In speaking of the effect of a prism, ... |
..r. as is seen from geometry, the angle of incidence <»t the ray £s is, 492 LIGHT under these conditions, (N -f D). Therefore a ray incident upon the second prism at the angle I1 is dispersed by it ; and two of its components, ^ and Z2, fall upon the first prism, are refracted by it, and emerge parallel to each other,... |
if one were to look through the other with the eye accommodated for an infinitely distant object, as is the case when one is using a telescope. Therefore it is better to make the distance of the lenses apart slightly less, although by so doing the chromatic aberra- tion is not exactly corrected. Thus, if k = $/j, the ... |
the first, no mirror or lens is used ; in the second, some form of mirror is the essential feature ; in the last, some prism or lens, or a combination of prisms or lenses. " Pin-hole " Instruments These have been described in Chapter XXV, and nothing further need be said here. They all depend upon the "recti- ir propa... |
form of eyepiece is a converg- ing lens, but a Ramsden or Huygens eyepiece is ordinarily used. In doing this the eyepiece is so placed that the image to be viewed comes at its principal focus (or just inside it), and it forms a virtual magnified image at apparently an infinite distance. Reflecting Telescope. — In all ... |
. therefnre, the great ad \, intake of having a teles< whose concave minor has a long focal length. 500 LIGHT The limit to the resolving power of a telescope is deter- mined, as we have seen on page 487, by the diameter of the mirror ; the larger it is, so much the greater is this power. A large mirror also gathers mor... |
teleobjective." Projection Lantern. — This instrument consists of a lamp, or a strong source of light, which by means of lenses illumi- nates a drawing or photograph on glass, or some object which is transparent in parts, and of a converging system of lenses which throws a real image of this illuminated object upon a s... |
. Dutch Telescope (or Galileo's telescope). — This telescope is free from the disadvantage of the astronomical instrument FIG. 252. —Dutch telescope. to which reference has just been made. It consists of a converging lens, forming the object glass, and a diverging <U>TK AL INSTRUMENTS 503 I- us so placed that the princ... |
which this image is formed is arbitrary: but it is generally chosen as about 2"> om. (or 1" in. > fn>ni the eye, because this U t: < <• at which j.copl,- with normal eyes hold an object in «>rder to see it most distinctly. 504 LIGHT b. Compound microscope. — The magnification secured by a single lens is not great ; an... |
can turn around it as an axle ; they carry metal tubes which lie in a plane perpendicular to the axis of the instrument and are thus movable in this plane. One of these tubes is a simple form of astronomical telescope; the other carries at one end an achromatic converging lens whusf focal length is that of the tube, a... |
— |- mined ; for n = ^ (See Ames and Bliss, Manual of sin- Experiments in Physics, pages 459-475.) OPTICAL M->T/,T.I//-;.\ :,«»; If the laws of reflection are to be studied, ordinary white light may be used to illuminate the slit; but, if the phe- nomena of refraction are to be observed, special precautions must In- ta... |
fall upon one face of a prism, they suffer refraction and emerge 608 509 on the other side, diverging apparently from a virtual image of the source. They do not, therefore, of themselves come to a focus ; but, if a converging lens is introduced, the rays may be focused upon a screen. This real image is due to the virt... |
front has been limited by this means to the shape of these apertures, the final image is not a line. These apertures are the edges of the lenses, of the prism, and of any diaphragms there may be in the tubes. There is, therefore, a diffraction pattern produced, exactly as described for a lens on page 484. The image of... |
, as they leave the prism they enter the telescope which is focused for plane waves ; and a series of colored images of the slit is seen 1)\ the observer looking in the eyepiece. Measurement of Dispersion. —If the eyepiece is removed, and the images formed by the object glass are focused sharply on an opaque screen, an... |
by him. He measured — by methods to be discussed later — the wave lengths of the waves which correspond to these lines ; and to the most prominent of them he gave names, in the form of letters. (Thus the A line is at the limit of vision in the red end of the spectrum ; B and 0 are also in the red ; Dl and D2 are in th... |
greater. So, if a prism is made of this substance, it will deviate the long waves near the ab- sorption band more than the short ones ; and the resulting S8iS3Sg§§2gg§8S!Sg 2.3'5>p'^>« oooSoo pc-t-V-t- WAVE LENGTHS IN fJ.fJL FIG. 2GO. — Dispersion curve of cyanine. spectrum will appear, in general terms, as if it were... |
the drop, suffer reflection once and be refracted out, as shown in the cut, in which 0 is the centre of a drop, OS is the direction of the sun, and AB, BC, CD, DE represent the portions of a ray. The deviation of the ray is shown in the cut by the angle (GFE), where F is the intersection of the prolongations of the in... |
angles of minimum de- viation for the different colors, which give rise to a red bow at the angle 51° and a violet one at 54°, these bows being sharply defined on their lower edges. CHAPTER XXXI INTERFERENCE OF LIGHT Interference Fringes. — The general phenomena of interfer- ence of waves were described in Chapter XXI... |
linn. This slit, if illuminated, will emit waves, which will suffer refraction and deviation by the two hal\«-s of the prism. If the slit is at 0, as sho\\ n in the cut, one half will form a virtual image at 0,, 522 LIGHT the other at 02. So the waves as they emerge from the biprism will come apparently from the two s... |
of the brilliant color effects by such thin films of transparent matter as soap bubbles, films of oil on water, layers of air between two pieces of nearly parallel glass, etc. These colors are due to the interference of the trains of waves which suffer reflection directly at one surface of the lil in with the waves wh... |
out through the upper surface, while the rest is either directly, or after two or more reflections, refracted out through the lower surface. By far the greater amount of the light is transmitted, owing to the poor reflect- ing power of the film. There will then be colors visible if one looks at either of the two surfa... |
of r in the formula is so small that cos r = 1 ; and the condition for complete interference is that 2 nh = ml, which is the same for all these face. INTERFERENCE OF LIGHT 527 pairs of rays. If the thickness of the film at the point C satisfies this condition, and if the eye is focused on the film, C will be a dark po... |
; It be a screen placed in tin- />rincipal focus of the lens. Draw from C, the centre of the lens, a line CE at random. Then, all rays falling upon the lens parallel to this line \\ill be brought to a focus at E. The point source 0 is emitting rays in all directions; one of them, OP,, is parallel to / '/•;. ThN ray aft... |
a slit, or opening be- tween two scratches, serves as a source of waves, and sends out rays in all directions. Let 630 PIG. 274,— Transmission diffraction grating. DIFFRACTION 531 a converging lens be placed with its axis perpendicular to the plane of the grating, and let a screen be placed in its focal plane. In the ... |
sin ^ = 0, or ^V0=0; that is along a line through the point A in the cut where 532 LIGHT the axis of the lens meets the screen; this is called the u central image " ; the next maximum, at the angle Nv is called the "first spectrum," etc. It is evident that there are maxima also on the other side of A, corresponding to... |
for, if this is true, the rays from the first and //th. the second and the (n+ l)th, etc., will interfere completely. The con- < N dition for a minimum at Pl is, then. //'/ sin NI = nml — -• illaily, for a minimum point on the other side of P, the vain.- \\onhl be nml + ^- ) The condition for a maxi- mum at P is, Ilrl... |
, and if 0 is the centre of curvature of this surface, draw a circle with 0(7 as a diameter which is tangent to the grating surface at C. Then, if the point source S is at any point of this circle, the spectral images will be formed at points P on this same circle. If the source is emitting white light, there will be a... |
axis are called "uniaxal" ; the others, which have two axes, are called " biaxal." All crystals which belong to the cubical system, so called, are single refracting; those that belong to the pyramidal or second system are doubly refracting and uniaxal ; the other crystals are doubly refracting and biaxal. Any ordinary... |
incident upon such a compound plate, it will, on entering it, be broken up into two beams, ordinary and extraordinary, which will have different directions ; the former will suffer total reflection at the section of balsam, the latter will be transmitted and will emerge at the end of the plate. The sides of the plate ... |
is true n. ral ; l.ut llnvgens observed that as the tube contain- ing the second |>rism Was turned around its axis of figure, there were f t mns, 90° apart, dm inir a complete revo- lution «.f the tube, in which only two pencils emerged ; and in one of tlieqp positions the two emerging pencils coincide 647 548 LIGHT i... |
A, 00 = A cos N, OE = A sin N. Similarly, if there is a vibration along CB of ampli- tude A, we have E = A, EO = A sin N, EE=A cos JV. We must consider the light received from ordinary sources, such as the sun, flames, etc., as being made up of vibrations in all directions in the wave front, because it is not polarize... |
; while, if the principal section is at right angles to the plane of incidence, only the extraordinary rays are transmitted. For positions of the plate between these two, both rays are transmitted, but with different intensities except for the position halfway between. The direction of the vibration of the rays reflec... |
just said, that if the angle (BA /') is the polari/mg angle, {CAD) is a right angle. Therefore the angle of refraction {DAP') equals (CAM) ; so, calling tin- angle of incidence N, (DAP') = *-N. The index of refraction n satisfies the emiat inn n s= 8in \ _'; and hence, ut the polarizing angle, n = - -—= tan N\ or, the... |
56 LIGHT with the observed position of the plane of polarization, it is proved that the vibrations are at right angles to it. (See page 554.) Interference of Plane Polarized Waves. — Fresnel and Arago performed by means of two piles of plates a most ingenious experiment to determine whether the vibrations in the waves ... |
will bo extin- guished. (They suffer total reflection at the surface win i. the two halves of the last nicol are cemented together.) Other cases, in which the two nicols are not crossed, m ax- be found discussed in advanced text-books. So, if white light is used, certain waves will be absent in the transmitted light, ... |
of the nicol through which the light is incident upon the plate. POL A UIZA T1ON If the amplitude of this plane polarized light is OP, those of the two beams transmitted by the plate are OP1 and OP2. Whatever the difference of phase between them introduced by the plate, these two vibrations will combine to form an ell... |
right angles to it. Detection of circularly or elliptically Polarized Light. - If circularly polarized light is incident upon a Fresnel's rhomb or upon a quarter wave plate, it will emerge plane polarized, for the effect of these pieces of apparatus is to in- troduce a difference of phase of a quartet* of a period bet... |
done, the second nicol must be turned on its axis of figure through a definite angle before the light is again extinguished. This angle varies directly with the thickness of the substance, and is different for waves of different wave lengths, being much _ri' ater for the short waves than for the long ones. This last p... |
ur that all optically active sub- stances were made up entirely or in part of certain crystals which had a "hemihedral" form ; of which there are for any substance two possible states. These two are symmetrical with reference to a plane, like the two hands of any individ- ual. Thus right-handed quartz has imbedded in i... |
MWII as the " Kerr effect," and will l>e found fully described in any advanced text-book. CHAPTER XXXV VELOCITY OF LIGHT THE first experiments to determine whether, like sound, light traveled with a measurable velocity were performed by Galileo. They consisted in having one observer flash a light which was seen by a se... |
eelipse should take place, there was found to be a differ- ence of 996 sec. between this calculated instant and the observed one. Roemer saw that these facts could all be explained if the assumption were made, that it takes time for the propagation of light across space. As the earth is receding from Jupiter, light ha... |
small, we may write AE = ES x N, or t™ AE 6.378 x 108 x 360 x 60 x 60 27TX8.79 = 1.4966 x 1018cm. The diameter of the earth's orbit is, then, twice this, or 2.993 x 1018 cm. Method of Bradley. — Another method by which the veloc- ity of light could be determined was discovered by Bradley, the great English astronomer,... |
"path of the light" is along the line AC. The angle through which the telescope has to be turned when it is pointed approximately perpendicular to the path of the earth in its orbit, is called the "constant of aberration." (Its value according to recent astronomical observations is slightly less than 20.5 sec. of arc.... |
2 7) in the time - — — ; and the velocity of light '_' nN is, then, 2 D divided by - J—, or * M fwAw 572 LIGHT In one of Fizeau's original experiments, the distance from La to L3 was 8.633 Kin. or 8.633 x 108 cm. ; the toothed wheel had 720 teeth ; and it was found that the first obscuration of the reflected light occ... |
to Fig. 298, there is a source of light at P ; a plate of glass at p ; a revolving mirror at m, whose axis of rotation is perpendicular to the plane of the paper ; a converging lens.L, which focuses upon a concave mirror S the light from P reflected at m ; the centre of curvature of this mirror S is at m. For a suitab... |
:>74 LIGHT Foucault increased the effective distance D by having the light reflected several times back and forth between five mirrors before it was finally returned to the revolving mirror ; but in no case did he obtain a very large displacement P'P". Michelson, however, by changing the arrangement of the apparatus w... |
in water, and thus olitaim-d two displaced images.) Michelson showed the same for water and for carbon bisulphide; and he also proved directly that in these substances red light travels more rapidly than blue. CHAPTER XXXVI RADIATION AND ABSORPTION SPECTRA Discovery by Newton of Nature of White Light. In the year 1672... |
to the vibrations of certain parts inside their mole- cules. If the body when placed in a darkened room can be seen by the eye, it is said to give off light. The ordinary method of making a body luminous is to raise its tempera- ture ; thus a body may be exposed to a hot flame, such as one from a Bun sen burner, or it... |
at the substance from one side. This phenomenon was first observed by Herschel and Brewster, but was first thoroughly investigated by the late Sir George Stokes. He showed that in all cases observed by him, the fluorescent light was of a wave length longer than that of the waves whose absorption caused the fluor- esce... |
there are no gaps in them. In the latter only certain isolated trains of waves are emitted, thus forming separate "lines." Investigations show that all solids and liquids — with possibly a few exceptions — emit continuous spectra ; while all gases and vapors emit discontinuous ones. (This is obviously what one would e... |
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