STUDIES I N T H E EXPERIMENTAL TECHNIQUE O F PHOTOCHEMISTRY
V. Reflection Losses in the Optical System of the Hilger Ultra-Violet Monochromatic Illuminator BY n. N. RIDYARD AND D . w. G. STYLE
Introductory In earlier papers in this series' an account was given of the use of the Hilger Monochromatic Illuminator for the determination of energy distribution in light sources. I n the latter of these2 the authors considered the question of reflection at the mirror and quartz surfaces of the spectrometer. In the case of the mirror they gave figures for the reflections a t different wave-lengths, determined by a method, which although the only one available without special apparatus, was of very doubtful accuracy. In the case of reflections a t the quartz surfaces, it was stated that, as the change in reflection with wave-length is small, it can be neglected. This might be true for one surface at normal incidence; but in this instrument are several surfaces, with, in some cases, large angles of incidence, and correspondingly greater changes of reflection coefficient with wave-length. Thus tbe small change is repeated several times, with the result that it becomes very appreciable in total. Greater accuracy being now more important, this matter has been carefully investigated. Special apparatus has been constructed to measure the reflection of the mirror a t various wave-lengths, while the reflections a t the quartz surfaces have been calculated, using the formulae
for normal incidence, and
where I is the angle of incidence, R is the angle of refraction, for other cases. The total percentage of light passing the collimator slit of the instrument, which is transmitted t o the thermopile, has been calculated for a number of wave-lengths; and also factors by which galvanometer deflections should be multiplied in order to compare them with 579 pp, which is taken as standard for this purpose. Apparatus The mirror in the spectrometer consists of a quartz plate backed with a white metal-possibly tin amalgam. The angle of incidence to this metal in the instrument varies between 15' 32' a t 579pp and 13' 32' at 254pp. As a
J. Phys. Chem., 29, 39-57;713-726 (1925). Pp.722, 724.
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H. N. RIDYARD AND D. W. G . STYLE
such a small angle of incidence introduced numerous practical difficulties, the actual experimental work was carried out with the light incident a t 45‘ to the quartz-air surface, the resulting angles to the metal being 27’ 14’ at 579pp and 26’ 16’ a t 2 54pp. The apparatus employed is shown in Fig. I. The spectrometer H. J. with its associated thermopile K and Paschen galvanometer, were used to measure the relative intensities of the direct and reflected beams. A triangular brass frame was supported a t the base by two legs, adjustable in length, and carrying at their lower ends small wheels; and at the apex by a pivot placed vertically below the optical axis H - J of the collimator of the spectrometer. The frame could thus rotate over a smooth metal surface between the stops LL, through an angle of go’. This frame carried a quartz mercury lamp B (the supports of which are not shown), and, on the pivot side of this, a copper screen, on which is fixed a small diaphragm D, adjustable in position. I t also carried a lens E of 12.5 cms. focal length, mounted upon a slide to enable the lens to be focussed for parallel light at any wavelength. This slide consisted of a brass carrier sliding upon two silver-steel bars, these being supported by a plate, which was attached to the frame by a screw passing through a slot, thus allowing a lateral and rotary movement of the slide. Vertical adjustment of the lens was provided for in the carrier. A focussing scale waii attached to the frame. Immediately above the pivot was placed the mirror mounting A. This is shown in greater detail in the small diagram (Fig. I). The mirror M was clamped on the foremost of two brass plates N N by four small clips 00. These plates were held together by two screws PP, and kept apart by two others Q. These screws enabled the angle of the mirror to be adjusted. The rear plate was supported above the level of the frame by two bars RR, through the lower end of which passed a silver steel spindle S, running in bearings fixed upon a plate attached to the frame. This arrangement permitted the mirror to be swung up and down without any side-play. When in a vertical position the back of the plates supporting the mirror rested against a screw V, which passed through a stout vertical pillar W, and could be secured by the lock nut x. The top of W was, of course, below the level of the mirror itself. Two strings attached to these plates and to the bench, one of them passing through a hole in W, caused the plates to be drawn firmly back against the screw V as the frame swung into the ‘reflected’position, and to be lowered when it was moved into the ‘direct’ position. After leaving the mirror the light was concentrated upon the slit H by another lens G, which was fixed in position. This form of apparatus was evolved after considerable experimental work. The two surfaces of the mirror cause a number of images, three of which arp important. In earlier work attempts were made to separate these and measure the principal one, due to one reflection from the metal, with corrections
EXPERIMENTAL TECHNIQUE OF PHOTOCHEMISTRY
I I
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/ I
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H. N. RIDYARD AND D. W. G. SmLE
for back reflections from the quartz. Results obtained thus afforded rough confirmation of the final results. I n the apparatus as described, experiment showed that these images were not appreciably separated a t the collimator slit. It was found to be very important to have the light incident upon the mirror as exactly parallel as possible.
Adjustment During preliminary measurements, some considerable difficulty was encountered in making the necessary adjustments. The method finally adopted is given below. The height of the two legs of the frame were adjusted until the frame was level. The height of the mercury lamp B was then fixed, so that the centre of the arc tube was approximately on the optical axis of the collimator H - J when in the ‘direct’ position. A ground glass screen, suitably divided horizontally and vertically, was placed behind the collimator lens J, where it received an image of the slit H. The diaphragm D was then adjusted until this image was exactly central upon the screen. The lens E was next adjusted, and simultaneously the position of the slide F, so that in whatever position the lens was placed upon the slide, the image was still central upon the screen, while at the same time the image of D upon the collimator slit was symmetrically placed; showing that the slide was parallel to the optical axis, and the lens a t the right height, etc. The frame was then moved into the ‘reflected’ position, and the mirror adjusted. This was done by means of the screw V, controlling the position of the mirror relative to the axis of rotation of the frame, and the screws PP, Q, controlling the angle at which it is inclined to the optical axis of the apparatus. When the images on the slit and the ground glass screen were againcentral, as described above, the mirror was considered correctly placed. Finally the lens G wa.s fixed, using the same criteria.
Method Measurements of the relative intensities of the two beams were made by taking galvanometer deflections in pairs, alternately in the ‘direct’ and ‘reflected’ positions, until a satisfactory series of figures had been cbtained. The lamp was unaffected by the movement. In order to test the absolute accuracy obtainable, measurements were made of the reflecting power of a single quartz-air surface, obtained by grinding one side of an optically flat quartz plate, and coating the ground surface with a suspension of lamp black in Canada balsam; the polished surface being used for the reflection. The results obtained are compared below with the calculated figures.
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Wave-Length
Reflected Ray (observedj
Reflected Ray (Calc. from Formula 2)
5.51% 5.24%
5.66% 5.68% 5.78%
5.72%
5.84%
5.87% 5.78% 6 . 1 Sc
5 92% 6,0870
70
6.33%
6.2
Results Two mirrors were examined, the one not mounted on the frame being used in the spectrometer. The results are given in Table I.
TABLE I Kave-Length
Mirror A
579 546 436 40 5
67.2% 67.2 65.7 66.4
67.1 66.1
365 313
67.0 65.2
65.3
265
64.9 62. I
254
67.5%
Mirror B
Working Mean
66.17~ 66.0 68.9 66.2
67% 67 67 66. j
67.7 69.0 64.8 64.0
67 66 65 63
There is thus a small difference between the two mirrors, but as these were supplied at times several years apart,, it seems that the ‘working mean’ given is likely to give results accurate to 1% in most cases. The reflection losses a t the quartz surfaces in the spectrograph were then calculated, using the formulae above, and treating the lens surfaces as plane. The optical system is shown on Fig. 2 . It will be seen from this that every angle in the system is defined by the deviation, and the prism is in the position of minimum deviation. A list of refractive indices, taken chiefly from Rubens’ figures,’ together with the semi-deviations calculated from these, was given in Messrs. Hilgers’ notes on the instrument. From these figures the various angles of incidence and refraction, and the corresponding percentage reflections, were obtained. The values for the mercury lines were found by interpolation, and those in the Ultra-Violet are given with the calculated figures in Table 11. lRubens: Ann. Physik, (3) 60, 434 (1897).
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H. N. RIDYARD AND D. W. G. STYLE
FIG.2 Diagram showing Passage of Light through Hilger Spectograph.
TABLE I1 Reflections a t Quartz Surfaces WaveLength (PPI
198.81 231.25 248 2 54 265 247.67 2 80
303 313 317.98 358.18 365 404.58 434.09 486.16 534.96 589.32 656.33
Refractive Index (n).
.65070 1.61402 I ,6008 1.5972 1.5917 1.58750 I . 5852 1.5772 1.5741 1.57290 I . 56400 I . 5629 1.55706 1,55387 1.54961 I . 54663 1.54415 I . 54181 I
SemiDeviation @/a).
17' 26" 23' 48' 17''
25'
22'32'
15"
Reflections (%) Normal Incidence. I = 45". I = 30'
6.03 5.52
5.35 5.28 5.21 5.15 5.12
5.01
21'
51' 18'' 26' 39"
7' 34'' 51" 47' 14" 39' IO" 32' 26'' 26' 6''
20'58'
4.98 4.96 4.84 4.82 4.75 4.70 4.65 4.61 4.57 4.54
7.18 6.64 6.45 6.39 6.31 6.26 6.23 6.11 6.08 6.06 5.93 5.91 5.83 5.78 5.72 5.68 5.65 5.61
- D/2. 9.04 8.17 7.91 7.84 7.71 7.62 7.55 7.35 7.27 7.24 7 .OI 6.98 6.84 6.76 6.65 6.58 6.52 6.46
!=d,; 6.05 5.56 5.41
5.36 5.28 5.20 5.19 5.08 5.03 5.01
4.88 4.88 4.81 4.77 4.71 4.67 4.64 4.61
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It was stated earlier that the experimental work was carried out a t a different angle from that used in the spectrograph. There are no data available to enable a correction to be applied in the case of the metal, but it was pointed out that the difference in angle of incidence a t the metal surface was less than the difference in the case of the quartz-air surface. The reflection coefficient was corrected for this latter, but was found to be only -0.1% in each case. The true reflection of the metallic surface was found to be 0.6% less than the percentages given for the mirror, except for z54pp,which was 6 2 ~ 2 % ~ and 4 5 MP, 64.3%. From the various results in these tables, the total percentage of light transmitted by the spectrometer was calculated, taking the system as having 4 plane surfaces, 2 prism surfaces, and the mirror. From these figures, the factors by which it is necessary to multiply galvanometer deflections in order to reduce all lines to the basis of 579 pp, were obtained. These percentages and factors are given in Table 111.
TABLE I11 Wave-LengtL 2
Transmission
5 4PP
265 313 365 405 436 546 579
43.1% 44.6 46.2 47~5 47.5 48.0 48.4 48.5
Factor I . 13 I .09 1.05 1.02
I .02 1.01
I .oo 1.00
summary The reflection of the Mercury Lines by the mirror in the Hilger U. V. Monochromatic Illuminator has been studied experimentally. The reflection losses a t the various quartz surfaces in this instrument have been calculated. From these figures the total transmissions of the mercury lines have been calculated, together with factors to reduce results obtained with the instrument to a common basis. The writers wish to express their deep gratitude to Professor A. J. Allmand for continual help and encouragement in the course of this work. They also wish to thank Mr. W. E. Williams, of the Physics Staff of this College, for most helpful advice; and Messrs. Adam Hilger for the loan of one of the two qirrors used in this work. One of us (D.W.G.S.) is indebted to the Department of Scientific and Industrial Research, for a grant while a Student in Training. Chemical Department, University of London, Kings College. January IS, 1928.
