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686
INTERFERENCE OF LIGHT


is wholly dependent upon their phase-relation, and it is of interest to inquire what we are to expect from the composition of a large number (11) of equal vibrations of amplitude unity, and of arbitrary phases. The intensity of the resultant will of course depend upon the precise manner in which the phases are distributed, and may vary from ni to zero. But is there a definite intensity which becomes more and more probable as n is increased without limit? The nature of the question here raised is well illustrated by the special case in which the possible phases are restricted to two opposile phases. We may then conveniently discard the idea of phase, and regard the amplitudes as at random positive or negative. If all the signs are the same, the intensity is nz; if, on the other hand, there are as many positive as negative, the result is zero. But, although the intensity may range from o to n2, the smaller values are much more probable than the greater.

The simplest part of the problem relates to what is called in the theory of probabilities the “expectation " of intensity, that is, the mean intensity to be expected after a great number of trials, in each of which the phases are taken at random. The chance that all the vibrations are positive is 2'", and thus the expectation of intensity corresponding to this contingency is '2“".n2. In like manner the expectation corresponding to the number of positive vibrations being ill-I) is

2'".n.(n-2)2,

and so on. The whole expectation of intensity is thus n(n

é '1.n2 -l-n.(n -2)2+~-% (n -4)'l

+n$n-l)(n-2)

1 2 3 (n-6)2+ . . (I).

Now the sum of the (n-1-I) terms of this series is simply n, as may be proved by comparison of coefficients of x2 in the equivalent forms

(e'+e“')"=2"(1+§ x2-l- . .f)"

=enz { 7le(n-2)z+ e(n-4)z+

The expectation of intensity is therefore n, and this whether n be great or small.

The same conclusion holds good when the phases are unrestricted. From (4), § 2, if A=1,

P'3=n-l-22 cos (az-al) (2),

where under the sign of summation are to be included the cosines of the -;1I(7'l'I) differences of phase. When the phases are arbitrary, this sum is as likely to be positive as negative, and thus the mean value of P2 is n.

The reader must be on his guard here against a fallacy which hes misled some high authorities. /Ve have not proved that when ii is large there is any tendency for a single combination to give the intensity equal to n, but the quite different proposition that in a large number of trials, in each of which the phases are rearranged arbitrarily, the mean intensity will tend more and more to the value zz. It is true that even in a single combination there is no reason why any of the cosines in (2) should be positive rather than negative, and from this we may infer that when n is increased the sum of the terms tends to vanish in comparison with the number of terms. But, the number of terms being of the order nz, we can infer nothing as to the value of the sum of the series in com arison P

with 11.

Indeed it is not true that the intensity in a single combination approximates to n, when n is large. It can be proved (Phil Mag 1580, io, p. 73; 1899, 47, p. 246) that the probability of a resultant intermediate in amplitude between r and r+dr is %e"'” /"fdf "(3).

The probability of an amplitude less than r is thus "I 7

Q 0 €"°l"'d"=1“@"”"" <4>,

or, which is the same thing, the probability of an amplitude greater than r is

I-r* ln

P (5)

The accompanying table gives the probabilities of intensities less than the fractions of n named in the first column. For example, the probability of intensity less than n is ~632I. -05 i -0488 ' -so I -5506

-Io -0952 r~0o -6321

~2o '1813 | 1~5o ~7768

-40 I -3296 | 2-oo ' ~8647

-60 | -4512 | 3-oo J ~9502

It will be seen that, however great n

of considerable relative fluctuations

combinations.

may be, there is a fair chance

of intensity in consecutive

The mean intensity, expressed by

2 °°

Q O e“"/".r2.rdr,

is, as we have already seen, equal to n. It is with this mean intensity only that we are concerned in ordinary photometry. A source of light, such as a candle or even a soda Hame, may be regarded as composed of a very large number of luminous centres disposed throughout a very sensible space; and, even though it be true that the intensity at a particular point of a screen illuminated by it and at a particular moment of time is a matter of chance, further processes of averaging must be gone through before anything is arrived at of which our senses could ordinarily take cognizance. In the smallest interval of time during which the eye could be impressed, there would be opportunity for any number of rearrangements of phase, due either to motions of the particles or to irregularities in their modes of vibration. And even if we supposed that each luminous centre was fixed, and emitted perfectly regular vibrations, the manner of composition and consequent intensity would vary rapidly from point to point of the screen, and in ordinary cases the mean illumination over the smallest appreciable area would correspond to a thorough averaging of the phase-relationships. In this way the idea of the intensity of a luminous source, independently of any questions of phase, is seen to be justified, and we may properly say that two candles are twice as bright as one.

§ 5. Interference Fringes.-In Fresnel's fundamental experiment light from a point O (fig. 1) falls upon an isosceles prism of glass BCD, with the angle at C very little less than two right B.

gr' °°"°°"' ' "" Y C

0, """"""

n

FIG. I.,

angles.

sunlight enters a dark room, or, more conveniently, the image of the sun formed by a lens of short focus (1 or 2 in.). For actual experiment when, as usually happens, it is desirable to econornize light, the point may be replaced by a line of light perpendicular to the plane of the diagram, obtained either from a linear source, such as the filament of an incandescent electric lamp, or by admitting light through a narrow vertical slit. If homogeneous light be used, the light which passes through the prism will consist of two parts, diverging as if from points O1 and O2 symmetrically situated on opposite sides of the line CO. Suppose a sheet of paper to be placed at A with its plane perpendicular to the line OCA, and let us consider what illumination will be produced at different parts of this paper. As O1 and O2 are images of O, crests of waves must be supposed to start from them simultaneously. Hence they will arrive simultaneously at A, which is equidistant from them, and there they will reinforce one another. Thus there will be a bright band on the paper parallel to the edges of the prism. If P1 be chosen so that the difference between P1O2 and PIO1 is half a wave-length (i.e. half the distance between two successive crests), 'the two streams of light willeonstantly meet in such relative conditions as to destroy one another. Hence there will be a line of darkness on the paper, through P1, parallel to the edges of the prism. At Pg, where OQPQ exceeds O1P2 by a whole wave-length, we have another bright band; and at P3, where OZP3 exceeds OIP3 by a wave-length and a half, another dark band; and so on. Hence, as everything is symmetrical about the bright band through A, the screen will be illuminated by a series of bright and dark bands, gradually shading into one another. If the paper screen be moved parallel to itseif to or from the prism, the locus of all the successive positions of any one band will (by the nature of the curve) obviously be an hyperbola whose foci are O1 and O2. Thus the interval between any two bands will increase in a more rapid ratio than does the distance of the screen from the source of light. But the intensity of the bright bands diminishes rapidly as the screen moves farther off; so that, in order to measure their distance from A, it is bettefto substitute the eye (furnished with a convex lens) for the screen. If we thus measure the distance AP1 between A and the nearest bright band, measure also AO, and calculate (from the known material and form of the prism, and the distance CO) the distance OO2, it is obvious that we can deduce from them the lengths of O1P2 and O2P2. Their difference is the length of a wave of the homogeneous light experimented with. Though this is not the method actually employed for the purpose (as it admits of little precision), it has been 'thus fully explained here because it shows in a very simple way the possibility of measuring a wave-length. The difference between O1P1 and OZP1 becomes greater as AP; is greater. Thus it is clear that the bands are more widely separated the longer the wave-length of the homogeneous light employed. Hence

The source of light may be a pin-hole through which