Krishnamurti, D.

Raman, C. V.

English

Publications of the IAS Fellows

"Proc. Indian Acad. Sci. A36 330-334 (1952) 
On the iridescence of potassium chlorate crystals- 
Part 111. Some general observations 
SIR C V RAMAN and D KRISHNAMURTI 
(Raman Research Institute, Bangalore) 
Received November 3, 1952 
1. Introduction 
In the two preceding parts we have discusked the optical behaviour of a few 
selected crystals which could be regarded as ideal or nearly ideal specimens 
exhibiting highly monochromatic iridescence. It is not without interest however 
to consider the subject from a wider standpoint. It is proposed in the present part 
to report briefly on the optical phenomena observed by us in our studies with a 
rather extensive collection of specimens. The availability of the material was due 
to the circumstance that some years ago a chemical manufacturing concern at 
Bangalore was engaged in the large-scale production of potassium chlorate. 
Arrangements were made with them for a systematic collection of the iridescent 
crystals that accasionally formed in the crystallising vats. 
A good way of exhibiting a collection of the iridescent crystals is to place the 
same on a flat circular disc of black glass held horizontally and which could be 
rotated about a vertical axis. The reflected colours can be viewed under the diffuse 
illumination provided by a large window. The variety and brilliance of the tints 
and their rapid change with the obliquity of observation then become strikingly 
evident; and the disappearance and the reappearance of the iridescence of each 
specimen twice in each revolution of the disc can also be exhibited. This 
arrangement, however, does not eliminate the dilution of the iridescence by the 
reflection of white light from the two surfaces of the crystal. This can only be 
secured by mounting the plates in Canada balsam between two prisms of glass. 
The liveliness of the colours is thereby vastly improved and the arrangement is 
also suitable for exhibiting the transmission colours in a very striking manner. It 
is also the most suitable arrangement for a critical spectroscopic study of the 
reflected or transmitted light. The simple technique of immersing the specimen in 
a watch glass containing xylene, the outside of which has been dead-blacked, 
however suffices for a rapid visual examination of numerous specimens. Indeed it 
may be used also for obtaining a photographic record of their spectra. 
C V RAMAN: SCIENTIFIC P A P E R S  IV 
2. Characters of the iridescence 
The majority of the specimens are rhombus-shaped tablets, but hexagonal- 
shaped plates are also not infrequently met with. An impressive feature is the fact 
that though the individual crystals exhibit the widest diversity of colours by 
reflection, such colour is usually quite uniform over the area of a particular 
crystal. Occasionally however, one finds plates which exhibit iridescence of one 
colour over part of the area and either no iridescence at all or else an iridescence of 
a different colour or intensity over the rest of it; the dividing line between the two 
parts in such cases is usually sharp but rather wavy. Straight boundaries are 
however occasionally observed. 
It is possible at first sight to believe that a particular specimen is non-iridescent 
and yet to discover on closer examination that it exhibits the characteristic 
reflections. This can happen, for instance, when the reflection is at the violet end of 
the spectrum at nearly normal incidence and moves out of the visible with oblique 
observation. It may also happen that the crystal has a series of monochromatic 
reflections in the visible spectrum the integrated effect of which is merely white 
light. There are cases also where the characteristic reflection moves into the 
visible spectrum and results in visible colour only when the crystal is viewed 
obliquely. 
A systematic examination of the specimens was made to find whether any of the 
tablets have stratifications so fine that the first order reflection appears only in the 
ultra-violet. These were looked for by photographing a continuous spectrum of 
white light transmitted obliquely through the specimen. Though many crystals 
were examined, no such case was met with. Prima facie there seems to be no 
reason why such fine stratifications cannot appear, and it is possible they were not 
found merely because they had been passed over when the material was collected. 
On the other hand, crystals were forthcoming with stratifications so widely 
spaced that thirteen successive orders of reflections are observed between 16600 
and 13200, This would mean that the order actually observed at the red end 
would be the twelfth, the first order being far out in the infra-red and that the 
actual spacing of the stratifications would be about 2.7 p. On the other hand, a 
crystal showing a first order in the violet say at 14200 would have a spacing of 
0 . 1 4 ~  
3. Variations of its spectral nature 
In plate I, figures 1 to 8 are illustrated the reflection spectra taken at nearly 
normal.lncidence of a series of eight crystals, beginning with a specimen which 
shows dngle reflection (probably a first order) at 14275, followed by others 
showirig a large number of reflection maxima up to thirteen. It is generally the 
case that if s 'crystal exhibits a series of monochromatic reflections, the 
IRIDESCENCE OF POTASSIUM CHLORATE CRYSTALS - 111 305 
monochromatism becomes sensibly more perfect in the higher order reflections 
seen in the ultra-violet. This feature is readily noticeable in several of the spectra 
reproduced in plate I. It is also generally characteristic of monoohromatic 
reflections that their intensity is far larger than that of the subsidiary maxima 
accompanying them, so much so that the latter are recorded only with prolonged 
exposures. We have however come across several cases in which a crystal gives a 
series of reflection maxima forming a regular sequence, but in which we do not 
observe the regularities referred to above. Some examples of this kind are 
reproduced as figures 6,7 and 8 in plate I. It will be seen in these three pictures 
that the reflection maxima are rather diffuse, each of them visibly exhibiting 
several components distinctly resolved from each other and that the structure of 
' the individual reflections is not always the same. Further, the subsidiary maxima 
may often attain a notable intensity and show a lack of regularity in their 
- distribution. This feature is very conspicuous in figure 8, plate I. A natural 
explanation of the facts observed is that the stratifications are not perfectly 
equidistant in these cases but are subject to errors in spacing which may be either 
random or periodic. 
Another interesting feature which is occasionally noticed is that some of the 
orders are unusually strong and others unusually weak in comparison with their 
neighbours, e.g., the reflection at 15800 appearing in figure 6 in plate I is very 
strong, whereas the order at 1 4850 is weaker than the orders on either side of it. 
Similarly in figure 4, plate I, the orders appearing at 14350 and at 14900 are 
far stronger than the orders appearing on either side at 13950 and 15600 
respectively. The latter is so weak that the principal maximum and-the subsidiary 
maxima on either side are of nearly comparable intensity. Another example of the 
same kind is the reflection appearing at 14130 in figure 5, plate I, which is 
much weaker than the orders at 13920 and 14380 respectively on either side. One 
more illustration is furnished by figure 7 in plate I in which the reflection 
appearing at 14475 is far stronger than the orders at 14050 and 15000 
respectively bn either side, the latter being extremely weak. A natural explanation 
for these facmis to be found in the different thicknesses of the alternate layers of 
the polysynthetically twinned crystaf. The ratio of these thicknesses would 
determine the relative intensity of the different orders of reflection. 
4. ~ u l t i ~ l ~ - ~ e r i o d i c  twinning 
In the course of our studies also, several examples were met with in which instead 
of a single series of sharply defined reflection maxima, regularly spaced groups of 
such reflection maxima were obtained, the successive orders exhibiting almost 
identically similar features. Four such spectra are reproduced as figures tp'4 in 
plate 11. The similarity of the successive groupings is particularly conspicuous 
in figure 4 in that plate, though it may be readily recognised in the others as well. 
306 c v R A M A N :  S C I E N T I F I C  P A P E R S  I V  
In figure 1 in plate I1 the principal feature observed is a strong band centred at 
16000 with other features on either side of it. The corresponding order in the 
violet is rather weak and exhibits a strong central band at 14050 and weak bands 
at 13820 and 14350 respectively which are hardly visible in the reproduction. 
Two alternative views may be put forward regarding the structures responsible 
for such spectra. One is that each specimen contains several separate layers of 
iridescent material, each of which gives its own series of monochromatic 
reflections of different orders. While this view cannot be summarily rejected, it is 
rather difficult to reconcile with all the observational facts and it would, in 
particular, require a considerable thickness of the crystal to contain all the 
separate lamellae required by such an explanation. Another view which appears 
to merit consideration is that the twinning in such crystals is multiply-periodic, in 
other words, that two or more sequences of equidistant planes exist side by side in 
the same layer. Such a structure would naturally give rise to complex groups of 
lines in the spectra instead of a single series of monochromatic reflections. 
Finally, mention should also be made of cases which depart still further from 
the ideal structures giving monochromatic reflections. They give spectra in which 
the bulk of the intensity is concentrated in particular regions of the spectrum but 
not in the manner characteristic of regularly ordered stratifications. Examples of 
spectra of @is type are reproduced as figures 5 to 8 in plate 11. 
5. Summary 
The paper describes the results of a study of an extensive collection of material. 
The spacing of the stratifications giving rise to monochromatic reflections has 
been found to range from 2.7 p to 0.14~.  The monochromatism becomes sensibly 
more perfect in the reflections of higher orders. Variations in intensity from order 
to order are also noticeable and these are ascribed to the alternate lamellae in the 
twinning being of different thickness. Instead of a single series of monochromatic 
reflections, regularly spaced groups of reflection maxima are also observed in 
some cases. The components in each group of such reflections may be wide apart 
or else may be closely spaced. The nature of the stratifications in these cases is 
discussed. 
References 
(In chronological order) 
Stokes G G, Proc. R. Soc., 1885,38, 174. 
Rayleigh I, Philos. Mag., 1888,26,241. 
Rayleigh I, Ibid., 1888, 26, 256. 
Wood R W, lbid., 1906, 12, 67. 
Rayleigh I, Proc. R. Soc., 1917, A93, 565. 
IRIDESCENCE OF POTASSIUM CHLORATE C R Y S T A L S  - I11 
Ramdas LA,  Proc. Indian Assoc. Cultiv. Sci., 1923, 8, 231. 
Rayleigh 11, Proc. R. Soc., 1923, A102, 668. 
Subrahmanian R V, Proc. Indian Acad. Sci., 1941, A13,467. 
Ramachandran G N, Ibid., 1942, A16,336. 
Ramachandran G N, Ibid,, 1944, AZO, 100. 
Cairo, Euler and Fricke, Abh. Braunschw, Wiss. Ges. 1951, 3, 21. 
C V RAMAN: SCIENTIFIC PAPERS I \  
Plate I 
Figurea 1-8 
Plate I1 


On the iridescence of potassium chlorate crystals. Part III. Some general observations

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On the iridescence of potassium chlorate crystals. Part III. Some general observations

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