Interference Bands Research Articles

Interference of R ntgen Radiation (Preliminary Account)

The authors have made a preliminary investigation of the R?ntgen radiation proceeding from a crystal of rock salt (which is of the simple cubical form) when a pencil of R?ntgen radiation is incident in a direction nearly grazing one of the three sets of mutually perpendicular cleavage planes. Reflection of X-rays by the Cleavage Planes. - Using a very narrow pencil of radiation, it was seen that the principal secondary pencil was one obeying the laws of reflection from the cleavage planes. A pencil diverging in all directions from a point source produced a corresponding reflected pencil of radiation converging to a line focus after reflection from a set of parallel cleavage planes. The quality of the radiation forming the secondary pencils was shown both by the photographic and by the ionisation method to be, not the fluorescent X-radiation, but of the kind previously described as scattered X-radiation. It was approximately of the same penetrating power as the primary radiation, and was approximately homogeneous, having traversed 5 mm. of rock salt in the case investigated. Interference Fringe Systems. - A diverging pencil of radiation was directed on to a crystal so that various portions were incident on the cleavage planes at different angles. A photographic plate showed the relative intensity of the corresponding reflected radiations. It was seen that the intensity of the reflected pencil varied periodically with varying angle of incidence, the maximum being separated by intervals corresponding to approximately equal increments in the value of cos ?, where ? was the angle of incidence on the reflecting planes. Such a series of maxima may be explained by interference of the pencils reflected from equal spaced parallel planes, the maxima being spectra of various orders. The wave-length, calculated on the assumption that these are planes passing through corresponding portions of molecules in the planes of cleavage, and that a molecule is simply NaCl, is found to be 0.6 ? 10-9 cm. If the molecule be more complex, the calculated wave-length would be greater. This value thus agrees remarkably well with the value (between 1 and 2 ? 10-9 cm.) calculated from the velocity of ejection of electrons by this X-radiation, taking this to behave as ultra-violet light of short wave-length. Using a more extended source the above spectral lines became indistinct and disappeared, but there appeared a periodic variation in intensity, the band-width being about four times the distance between the above lines. Such a system of interference bands would be produced by a second plane of reflection within the molecules in a position dividing the distance between the other planes in the ratio 1: 3 approximately. All the evidence considered indicates that these effects are due to interference. Various crystals were used, and one was turned through a right angle so that another system of planes acted as reflecting planes. The only experiment made up to the time of writing on the effect of varying the penetrating power showed a 25 per cent. smaller band-width with a radiation of increased penetrating power. Further experiments are, however, being made. Finally, there can be little doubt that the fringe systems are interference fringe systems. That the smaller system is a series of spectra of different orders and the other an interference band system seems probable; this theory certainly explains the results observed up to the time of writing.

An optical method of measuring vapour pressures: Vapour pressure and apparent superheating of solid bromine

In the course of an investigation of the refractivity of gaseous bromine conducted with a Jamin refractometer it was observed that, as the temperature changed, very slight variations of the density of the vapour could be quickly and accurately recorded. This fact suggested that an optical method would be useful for measuring vapour pressures or densities at low pressures, especially in the case of substances which attack mercury. The following observations on the vapour of bromine serve as an illustration of the possibilities of the method, and have brought to light an interesting phenomenon, which is described below. A thin glass bulb, containing 1 or 2 c. c. of Kahlbaum’s pure bromine, was sealed to a tube communicating both with the pump and the refractometer tube. The temperature of the bromine was reduced to —80°C., at which its vapour pressure is negligible, and the bulb was completely evacuated, so as to remove all traces of hydrobromic acid which might have formed by the action of the bromine on the tap-grease since the previous experiment. The refractometer tube was separately evacuated, and the two were put in connection by opening a tap. The cooling bath was then slowly brought up to atmospheric temperature by stages, while an observer counted the interference bands which crossed the field of view of the refractometer.

Note on oscillatory interference bands and some practical applications

Note on oscillatory interference bands and some practical applications

I. A wave-length comparator for standards of length: An instrument for fine measurement in wave-lengths of light. - With an appendix on the use of wave-length rulings as defining lines on standards of length

The following is a brief account of a new apparatus for fine measurement in wavelengths of light, designed primarily as a comparator for the measurement in wavelengths of the difference between a standard of length, either a line or an end measure bar—the Imperial Standard Yard, for instance—and any duplicate or similar bar proposed to be employed as a derived standard. The instrument is also, however, the most perfect instrument yet devised for measurement in wavelengths in general, and performs its functions so admirably as to render it highly desirable that a description should now be published concerning it. It has been constructed to the designs and under the supervision of the author for the Standards Department of the Board of Trade, and this account of it is communicated to the Royal Society with the permission of the President of the Board of Trade. The principle underlying the instrument is that of the author’s interferometer, which has also proved so successful in its application, in the interference dilatometer, to the determination of the thermal expansion of small bodies by the Fizeau method, and in the elasmometer, to the measurement of the elastic bending of a small plate or bar under a given weight applied at the centre. The essence of the interferometer is that homogeneous light, of a definite wave-length, corresponding to a single spectrum line—isolated with the aid of a constant-deviation prism from the spectrum derived from a cadmium or hydrogen Geissler tube, or a mercury lamp—is directed by an autocollimation method, ensuring identity of path of the incident and reflected rays, normally upon two absolutely plane surfaces, arranged close to each other, and nearly, but not absolutely, parallel; the two reflected rays give rise, by their interference, to rectilinear dark interference bands on a brilliantly illuminated background in the colour corresponding to the selected wave-length. In the instrument now described, one of these two reflecting surfaces concerned in the production of the interference bands is carried by, and moves absolutely with, one of the two microscopes employed to focus the fiducial marks, or "defining lines", determinative of the length of the standard, the other surface being absolutely fixed. The movement of either of the surfaces with respect to the other causes the interference bands to move, and the extent of the movement of the surface is equal to half the wave-length of the light employed for every interference band that moves past a reference mark carried by the fixed surface. The movement of the microscope parallel to itself and to the length of the standard bar is thus measured by counting the number of bands and the initial and final fractions of a band which are observed to pass the reference spot during the movement, and multiplying that number by the half wave-length of the light radiation used in the production of the bands. It is only necessary, therefore, in order to compare the lengths of two bars, (1) to place the bar of known length, say, the Imperial Standard Yard, under the two microscopes so that the two defining lines are adjusted in each case between the pair of parallel spider-lines carried by each of the micrometer eye-pieces; (2) to replace the standard by the copy to be tested, so that the defining line near one end is similarly adjusted under the corresponding microscope, then, if the other defining mark is not also automatically adjusted under the second microscope which carries the interferometer glass surface, as it should be if it is an exact copy, (3) to traverse that microscope until it is so adjusted, and (4) to observe and count the number of interference bands which move past the reference spot during the process. The product of this number into the half wave-length of the light used to produce the bands thus obviously affords the difference between the two lengths included between the defining marks on the two bars.

Standard Measurement in Wavelengths of Light

THE employment of the principle of the interference of two rays of monochromatic light, derived from the same source, one retarded behind the other by having to traverse a longer path, for the production of rectilinear interference bands constituting a scale of half-wave-lengths, has now been brought to such perfection that this highly refined scale may be used for the measurement of short distances or small movements of any description whatsoever. The accuracy is absolute to the tenth part of a scale division, the twentieth part of a wave-length of light, and is actually measurable with the most ordinary micrometer to the one-hundredth of a scale division, corresponding to the two-hundredth part of a wave-length. Now a wave-length of even the grossest radiations employed, those of red light, derived from either cadmium vapour (0.0006438 mm.) or hydrogen (0.0006562 mm.), is a forty-thousandth of an inch, so that the measurable unit is an eight-millionth part of an inch.

The use of wave-length rulings as a defining lines on standards of length

The delicacy of the method of measurement in wave-lengths described in the preceding communication calls for a corresponding refinement in the engraved lines, which form the defining lines of the length of a standard yard or metre or other line-measure bar. The defining lines on the Imperial Standard Yard are sharp-edged, but contain the equivalent of 40 interference bands of red light in their thickness, and the Benoît defining lines of the platinum-iridium copy made in 1902 are not only very ragged-edged out contain 15 interference bands in their thickness. The author has been in communication with Mr. J. H. Grayson, of Melbourne, whose fine rulings have recently evoked such interest among microscopists, and after a ling investigation has found that wonderfully satisfactory rulings on the scale of 40,000 to the inch can be made on polished speculum metal, covered with a thin cover-glass. Now the forty-thousandth of an inch is a single wave-length of red light (for H α = 1/38710 inch, and Cd red = 1/39459 inch), so that the interval between any adjacent pair of these lines is equivalent to only two interference bands. The thickness of each line, which is absolutely sharp-edged, is less than a single interference band. The author has therefore devised a defining mark in these rulings, which he terms a "Tutton location signal," to distinguish it from the "Benoît defining line." It consists of five such parallel lines spaced one forty-thousandth of an inch apart, with a pair of strong "finder" lines outside them and parallel to them, and another pair of similar finder lines perpendicularly transverse to them, to indicate a central part the lines for use. The central line of the five fine Grayson ruling is the defining line.

Societies and Academies

Societies and Academies

The refractive index and dispersion of light in argon and helium

The initial object of this research was to find the dispersion of light in the monatomic gases argon and helium, but as it was necessary to know the absolute value of the refractive index with considerable accuracy, determinations of the refractive index have also been made. Their relative refractive powers (air = 1 ) have been found by Rayleigh, and Ramsay and Travers, using coloured interference fringes, but no determinations of the absolute refractive index for light of given wave-length have hitherto been made. The interferometer method due to Jamin was used. A horizontal beam of parallel white light was incident on the first Jamin plate. The two reflected beams traversed two brass tubes of equal length closed by equal thicknesses of worked plane glass. Each brass tube had a small side tube attached which led to apparatus for altering the density in the tube and measuring its pressure. After reflection at the second Jamin plate, the recombined beam was focussed on the slit of a spectrometer, the spectrum obtained with a plane diffraction grating was examined through the telescope of the spectrometer and was seen crossed by bands of maximum and minimum intensity. At the same time, by means of a small reflecting prism, a portion of the slit was illuminated by the light from a Plücker tube containing hydrogen, helium, and mercury vapour. Thus one observed simultaneously the interference bands and certain standard lines of known wave-length with any one of which the cross wire of the telescope could be brought into coincidence.

Societies and Academies

Societies and Academies

On photographic evidence of the objective reality of combination tones

In the following paper we propose to describe a series of photographs which prove the objective reality of difference and summation tones. The work was suggested to us by Professor Rücker, and we have used the method of detecting these tones which has been described. Rücker and Edser in the ‘Philosophical Magazine’ for April, 1895 the resonator they employed was a tuning fork. On to one prong the fork was fixed a mirror, which was made one of a system by which Michelson’s interference bands were produced.

Apparatus Illustrating Michelson's Method of Obtaining Interference Bands

IN the American Journal of Science for August, 1881, Captain Michelson described an ingenious method for producing interference bands, used by him in determining the relative motion of the earth and the luminiferous ether. Light from a lamp at a falls on a thinly silvered mirror b, where it divides into two rays—one, b c, reflected from the thinly silvered surface, is reflected back to b by the plane mirror c; the other ray traverses the glass plate b, and falls on the plane mirror d, whence it is reflected back to b. Here both rays reunite, and pass onward toward e. The mirrors d and c are silvered and polished on their front surfaces. By this means bands, similar to Newton's rings, are obtained between the mirror c and the reflection of d in b; the retardation of one ray with respect to the other being the length bc-bd; f is an unsilvered piece of plane glass, cut from the same piece as b, and placed in the ray b c, so as to compensate for the ray bd passing twicethrough b; otherwise, owing to the dispersive power of the glass in b different wave-lengths of light in the ray b d would be unequally retarded in comparison with the same wave-lengths in the ray b c. If the path b c be now equalised in length with the path b d, and if, moreover, the piece of glass f be exactly equal and parallel with b, the central band will be black, owing to the ray d b being reflected from the rarer medium, whilst a b is reflected from the denser medium.

Interference Bands and their Applications1

Interference Bands and their Applications1

XLVII. On the interference bands of approximately homogeneous light; in a letter to Prof. A. Michelson

XLVII. <i>On the interference bands of approximately homogeneous light; in a letter to</i> Prof. A. Michelson

On the theory of certain bands seen in the spectrum

The principal object of the author in this communication is to point out some practical applications of the interference bands recently discovered by Professor Powell, the theory of which was considered by the author in the paper to which the present is a supplement. The bands seem specially adapted to the determination of the dispersion in media which cannot be procured in sufficient purity to exhibit the fixed lines of the spectrum. The ordinary experiments of interference allow of the determination of refractive indices with great precision; but in attempting to determine in this way the dispersion of the retarding plate employed, there is the want of a definite object to observe in connection with the different parts of the spectrum. In Professor Powell’s experiment, the wire of the telescope, placed in coincidence with one of the fixed lines of the spectrum previously to the insertion of the retarding plate into the fluid, marks the place of the fixed line, and so affords a definite object to observe when the retarding plate is inserted into the fluid, and the spectrum is consequently traversed by bands of interference. The practical applications considered by the author are principally four. In the first, the variation of the refractive index of the plate in passing from one fixed line to another is determined, the absolute refractive index for some one fixed line being supposed accurately known. The observation consists in counting the number of bands seen between two fixed lines of the spectrum, the fractions of a band-interval at the two extremities being measured or estimated.