Dietrich Zawischa     Home/Contact      German version

Accidental Observations

Iridescent butterfly wings

The purple emperor (Apatura iris) is very rare in the region where I live. Recently I could spot one and even take photographs, but I could not see its iridescence, as it opened the wings only a few times for a very short moment (left image below). On the right image which I got some years ago, a smidge of the blue iridescence is seen.

  

Wing scales

Two layers of tiny scales cover the wings of butterflies like tiles on a roof. The lamellar scales are hollow; their underside is smooth, the upper lamina is finely sculptured.

       
Scanning electron microscopic (SEM) images of the scales on the wing of a peacock butterfly (Aglais io) with stepwise increased magnification. The last image in the row is a detail of the fourth one, the width is approximately 7 μm. (Click to enlarge!) Photos: SecretDisc, sources: Wikimedia commons a, b, c, d, License CC BY-SA 3.0

Red, orange, yellow, brown and black colour of the wings is due to pigmentation of the scales, blue, purple, and green with few exceptions are structural colours.

The scales of the purple emperor have been investigated by Pantelić et al.[1], the following images are from that paper:


   
Scanning and transmission electron microscopic images of iridescent scales of A. iris. Left: cover scale in dorsal view, middle: dorso-lateral view, right: cross section. Adapted with permission from [1], OSA.
            
   Cross section of a ridge
   of an iridescent scale
   of Morpho sp. (source).

Comparing the images one can see that the scales of Apatura iris follow the same general blueprint as those of the peacock butterfly, but the proportions are very different. The cross section of the ridges on the cover scales reminds of the shape of fir trees. Depending on the angles of viewing and illumination, iridescent multilayer interference is seen or simply remitted light, brown due to the pigmentation of the scales. But as the ridges are so closely spaced, the rays reflected interfere with their neighbours, next neighbours and so on, reducing the angular distribution of the reflected light. According to Pantelić et al.[1] with fixed illumination iridescence is seen only within a narrow angle (18°) and the spectral distribution of the reflected light has a maximum at 380 nm and width (FWHM) of 50 nm. Thus, most of the reflected radiation is in the ultraviolet region which the butterflies can see, but we can't.

The iridescent scales of the Morpho butterflies are very similar to those of Apatura. But why is their appearance so different? One reason is, of course, that the maximum reflectivity of Apatura is in the UV, and what we see is only the tail of the spectral distribution. Apparently, this is not the case with Morpho. (In Morpho species, not the cover scales, but the ground scales are responsible for the iridescence. M. rhetenor, however, has no cover scales.)

Secondly, the ribbons at the sides of the ridges are not opposed to each other, but stacked, see the image to the right, and adjacent ridges slightly and randomly differ in their height, so that interference between neighbours is averaged out with the consequence that reflection goes into a wider solid angle [2]. Cover scales are transparent with coarser sculpture, or missing.

Two images to compare Apatura iris with Morpho rhetenor:


     
Left: Apatura iris with clearly visible Iridescence. Photo: Rosenzweig, source: Wikimedia commons., License: CC BY-SA 3.0.
Right: Morpho rhetenor cacica from South America. Specimen from the Insektenmuseum in Wunstorf/Steinhude. The wingspan is approximately 15 cm.

Many species of butterflies have scales of similar shape, but in almost all cases the iridescence occurs in the ultraviolet region. The clouded yellow (Colias croceus) is an example.

References

[1] Dejan Pantelić, Srećko Ćurčić, Svetlana Savić-Šević, Aleksandra Korać, Aleksander Kovačević, Božidar Ćurčić, and Bojana Bokić, “High angular and spectral selectivity of purple emperor (Lepidoptera: Apatura iris and A. ilia) butterfly wings”. Opt. Express 19, 5817–5826 (2011). doi:10.1364/OE.19.005817

[2] Shuichi Kinoshita, “Structural Colors in the Realm of Nature”, World Scientific Publishing Co. Pte. Ltd., ISBN-13 978-981-270-783-3



Very strange interference colours on ice

pond


    

    A frozen bog pond somewhere in northern Norway showing a
    glitter path decorated with colourful interference fringes.

“I was skating on a random bog pond when I noticed these colorful arcs on both sides of the bright reflection of the sun. As you can see the sun was fairly low, and the ice was incredibly reflective, but even though perfect for skating it was of course full of small cracks and all kinds of features […] The phenomenon was visible while moving, without a camera, sunglasses or polarizing filters, and I even noticed it showing on an iphone video. … ”

This is what Mika-Pekka Markkanen wrote when he asked if I knew this phenomenon. But I never have seen anything like that, and I don't think that many people have.

Mika-Pekka is a keen photographer, see his instagram or fb-galleries, and had his camera and also a tripod ready when skating.


The colours can be explained by the interference of light reflected by very peculiar structures of the ice which probably have been caused by snow falling on the water and floating just before congelation. As the floating snow crystals protrude only slightly from the water surface, this may produce a fine sculpture of the ice, consisting of tiny ridges, bumps and pits. How the light is reflected by these will be discussed in some detail.


Click on the following thumbnails to see zoomed-in details and a frame of the video clip!

        

Photos © Mika-Pekka Markkanen, shown with permission.


Explanation attempt

Modeling the surface

The most prominent effect seen on the images is the bright glitter path running from the far shore towards the observer. If the ice were perfectly flat one should see the mirror image of the sun. Glitter paths are a quite common sight on water when the sun is low, and their shape depends on the size of the waves. The first image in the row below shows where the sun is reflected by surface elements inclined by 0° (one point), 3.5° (smaller contour), and 8°. (The height of the sun is 10° as is approximately the case in the photos). Comparing this with the photographs, we can conclude that the majority of glints was due to facets inclined by less than 3.5°, but up to 8° inclination occurred. However, randomly oriented facets with up to 8° inclination only produce glints within the 8° contour, even if second reflections of the rays are accounted for, and no rays come from the regions where the vivid colours occur. There must be surface elements so steep that they don't contribute to the glitter path, but can scatter rays already reflected once to larger angles. This is shown by the second figure below, assuming facets with 3.5° and with 60° inclination. Glints from light rays reflected first by gently inclined and then by steep facets are possible in the grey region. Note that the apex of this region is lower than the horizon. This compares well with the region where the coloured fringes occur in the photos. But as yet there is no colour.

     

Colours appear if there are glints due to interfering rays which slightly differ in the path lengths which the light has to travel. Interference may suppress parts of the spectrum while enhancing other parts. This occurs if there are smooth transitions between flat and steep regions, as shown in the third sketch in the row above. Two light paths are shown where the light comes from the left side after an assumed first reflection on a flat facet of the surface. Until they reach the yellow dashed line, the path lengths are equal, and after passing the orange dashed line, the distance to the observer's eye is also the same. But the distance AD is not the same as BC, and therefore destructive interference is possible.

The fourth sketch in the row shows the result of a model computation which reproduces some of the features seen in the photographs. (Click to enlarge!) It has been assumed that the light is first reflected by a slightly inclined facet and then hits the steep flank of a step as shown in the third image above.

While in the photographs the colours fade and vanish in the foreground, this is not the case in the simulation. The reason is simply that apart from the inclination of the reflecting facets, all geometrical details have been ignored, as well as the dependence of the reflectivity on the angle of the light's incidence.

The following parameters have been used: the shallow facets are inclined by an angle of θ = 6°, their orientation φ is random. The maximum inclination angle of the steep flanks relative to the shallow ones is Θmax = 60° (thus Θ is ranging from 0° to 60° due to the smoothed transitions), the cross section is sinusoidal as in the above sketch. The orientation of the flanks is related to the shallow facets in the following way: introducing a coordinate system fixed to the shallow facet, the z'-axis being orthogonal to the facet and pointing upwards, the x'-axis pointing in the direction of steepest descent, then the surface normal to the flank has polar coordinates Θ and Φ, with Φ restricted to Φ = ±150° only. The total height of the step is H = 10 μm.

The parameters used do not reflect the hexagonal symmetry of snow crystals, this is a serious flaw. But anyhow, this simulation cannot be the final answer to the problem, because the postulated height H of the steps is so small, about 0.01 mm. The widths of the interference fringes is determined by H; the smaller H, the wider the fringes. But if the light is reflected by such narrow facets, diffraction effects cannot be ignored and simple geometric optics (as used here) is only a crude approximation at best. A correct calculation is, however, not a simple task …


The Poisson spot

In 1818 the French Academy of Sciences launched a competition to explain the properties of light, where Siméon Denis Poisson was one of the members of the judging committee. The civil engineer Augustin-Jean Fresnel entered this competition by submitting a new wave theory of light. Poisson, being a supporter of the particle theory of light, thought that he had found a flaw when he argued that a consequence of Fresnel's theory was that there would exist an on-axis bright spot in the shadow of a circular obstacle (see Wikipedia).

However, the head of the committee, Dominique-François-Jean Arago decided to do the experiment and found the bright spot (wich therefore is also called Arago spot), with the consequence that Fresnel won the competition. Arago later pointed out that the phenomenon had already been described in 1715 by Delisle [1] and in 1723 by Maraldi [2].

In view of the experimental possibilities of that time – dependent on the sun as light source – this achievement is admirable. With today's artificial light sources and digital cameras, the proof is much easier, as seen from numerous images found in the web. This encouraged me to try the experiment with as simple means as possible, to see the effect with my own eyes.

The material

As light sources a small LED torch and a small laser pointer in combination with a convex lens with 10 cm focal length have been used; shadow giving objects were a neodymium-magnet sphere of 5 mm diameter and pins with spherical heads of about 3.5 mm thickness. Small wooden blocks and a cork served as supports, a sheet of paper taped to the wall was the screen. Two tables. Photos were taken with a amall digital camera mounted on a tripod.

LED torch, Torch with diaphragm made from aluminium adhesive foil, laser pointer with lens, magnetic sphere on top of a pin, pins with round heads

Experimental set-up

The distance from the light source to the object was 3.40 m, that from the object to the screen was 1.30 m. Using only the torch without diaphragm, the shadows were too diffuse to see the central light spot, therefore a pinhole diaphragm (ca 1 mmø) was put in front of it.

The press key of the laser pointer was fixed with adhesive film. The lens in front of it makes a light cone from the laser beam.

The results

Poisson spot Poisson's spot   Arago spot Asago's spot

The spots are not easily seen with bare eyes, but on the photographs they are obvious. (Click to enlarge!)



Fake Poisson spots

There are other possibilities to produce bright spots within shadows. Browsing images and videos in the web I found the following example, which is too beautiful to be true, namely a CD with central hole and transparent part covered.

Fake poisson spot Fake Arago spot
A CD, middle part covered with aluminium adhesive tape, and its shadow – the shadow, taken with longer exposure time – rim of the CD with millimetre paper background at 15 mm distance – the same, focussed to the background
   

The illumination of the centre is not due to Fresnel diffraction but rather to refraction and reflection. The rim of the disc is transparent. The image to the right shows the transformation of a beam of light which passes the outermost part of the disk. The beam of a laser pointer was directed through the rim on the left side “at 9 o'clock” in approximately the same direction as the rays from the torch. The major part is refracted slightly outwards – note the bright ring surrounding the shadow in the left image above –, but another part is reflected at the slightly cambered outer surface and lightens the shadow region along a radial line. In the centre all these lines meet, producing a brighter spot.



Fresnel diffraction

Even with today's means it is not easy to demonstrate the Poisson spot, a small bright spot in the middle of the shadow of a sphere or a circular disk. Arago reportedly has discovered the spot using a tiny disk of 2 mm diameter.

With a steel ball of 24 mmø and an LED torch or a small laser pointer I could not make the spot visible. If, however, one only wants to see that light is diffracted into the shadow region, there is an easier way. If your eye is in the shadow of the ball, you will see part of its rim brightened by the diffracted light. And if the eye (or the lens of the camera) is in the centre of the shadow, the whole rim is bright.

  

Left pair of pictures: an LED torch behind a steel ball of 24 mm, right pair: the beam of a laser pointer is widened by a magnifying glass and lightens the ball from behind.

Similar pictures can be obtained by a coin with unruffled rim like the Euro five cent (with 21.2 mm diameter).

Fresnel diffraction can be seen occasionally on photographs when bright “points” are out of focus [1], [2]. Such patterns are tiny on the sensor of the camera. But larger patterns are possible: the paradoxical counterpart of the Poisson spot, a dark speck in the middle of the bright patch of light behind a circular aperture, is easily demonstrated with a small laser pointer and a magnifying glass and can be clearly seen with bare eyes.

Fresnel diffraction at pinhole Fresnel diffraction at pinhole   computed diffraction pattern computed diffraction pattern

Left pair: diffraction patterns of a small, not exactly circular hole (1 mmø) pierced into an aluminium foil. The centre can be dark or bright, depending on the distances of light source and screen.

Right pair: theoretical diffraction patterns for a circular pinhole; image widths 2 cm. Parameters: aperture 1 mm, wave length λ=650 nm, distance from diaphragm to screen 1.5 m; distance from the point-source to the diaphragm 22.5 cm and 15 cm. (The point source is the focus of the magnifying glass in front of the laser.)



Unexpected stripes – elliptic polarisation

The glasses used for viewing 3D movies have already been discussed here [1]. I use one pair as a handy and cheap analyzer of circular [2] and, turned backwards, linear polarisation. When, at sunset, I was looking for the polarisation of the skylight in different directions and, inadvertently, held the goggles not reverted, I was surprised to see stripes on the glass of my garret window.

Left: the sun is setting in the south-west, in south-east direction the skylight is polarised vertically. Turned around, the goggles in this position let pass only horizontally polarised light, the glasses therefore appear dark.

Middle: Held in normal position, the goggles show stripes on the glass of the inclined garret window. (Only left-handed light passes the left glass, only right-handed the right one.)

Right: the left lens held immediately in front of the camera. As the window is double-glazed, there is some moiré from the superposition of the patterns of both panes. In addition, there are wide stripes perpendicular to the narrow ones.

I noticed that the stripes remain stationary when changing the viewing angle; this rules out interferences like those discussed earlier [3] as a reason.

Thus, the stripes must be due to the properties of the panes, presumably periodically changing compressive stresses. Thermally toughened safety glass! At present for the inner panes of garret windows laminated safety glass is used, but here in this older one, obviously, both panes are of the toughened type.

From internal stress glass becomes birefringent. The incoming linearly polarised light is split into parts oscillating in directions orthogonal to each other, and behind the pane there is a phase shift between these parts. If this shift is exactly λ/4, the light is circularly polarised, as has been described earlier [1], in general elliptical polarisation results which may be considered as superposition of circular and linear polarisation.


Goethe's black and white crosses

Goethe's essays on colour science contain many interesting and accurately described observations. Here an addendum from 1820 to his treatise on chromatics (“Zur Farbenlehre”) is considered (digitized by Google), an essay on “entoptic” colours, colours arising from double refraction and polarisation of light as we understand it now. Goethe repeats and extends experiments of Seebeck and of Brewster with considerable effort. Specially prepared small plates of glass are used to mirror the light of the blue sky, showing black or white crosses. Blackened glass plates are used as mirrors to enhance the visibility of the effects in various ways.

Preparation of the entoptic glass:
„man zerschneide eine mäßig starke Spiegelscheibe in mehrere anderthalbzöllige Quadrate, diese durchglühe man und verkühle sie geschwind. Was davon bei dieser Behandlung nicht zerspringt ist nun fähig entoptische Farben hervorzubringen.“
cut a moderately thick plate of mirror glass into several squares of one and a half inch, heat them until they glow, and then rapidly cool them. Those pieces which are not shattered by this treatment are now able to produce entoptic colours.

What did Goethe see?

The simplest experiment – the glass square lies horizontally on a dark ground and reflects the blue sky. Depending on the direction ov viewing, dark or bright crosses can be observed. This simplest observation is more difficult to explain than the more sophisticated experiments which we therefore consider first.

The black mirrors polarise the light or serve to detect polarisation. Goethe was obviously not aware of that, writing that they serve to reduce brightness. Today we have more convenient methods: the computer's LED monitor screen is a source of linearly polarised light, to analyse this, polarising sunglasses, goggles for 3D-movies (held in reversed position) or, of course, a camera polarising filter may be used.

The treatment of the glass squares produces strong internal tensions, making the glass birefringent in a pattern which under most circumstances is invisible for the naked eye, but can be seen if the plate is placed between polarising filters. For the more elaborate experiments, Goethe used an “entoptic” glass cube. Will a glass die work as well?

A glass die (a) on black ground, (b) in transmitted light, (c) between crossed polarisers, and (d) between parallel polarisers. Goethe's cube must have looked like that!

Today, to see this effect, no specially prepared glass specimens are necessary, instead, objects made of acrylic glass or other transparent synthetic resins made by injection moulding may be used. Beakers or round lids are very suitable as their inner tensions and orientation of the filiform molecules are nearly rotationally symmetric.

A plastic beaker in front of the white screen of my computer, phothgraphed with a polarising filter. To the left the polarising filter blocks the light from the screen (“crossed polarisers”) and a dark cross is seen; to the right the polarising direction of the filter is the same as that of the screen and a white cross appears.

Due to the rotational symmetry of the pattern of birefringence, light that enters the material is split into one part polarised in the radial direction and another part polarised tangentially. After passing the polarising filter, what remains interferes and thus parts of the spectrum are suppressed, others enhanced. But if the impinging light oscillates in radial or tangential direction already before entering the material, there is no splitting, as the other component is zero, and there is no interference. This happens along two diameters orthogonal to each other which form a dark or a bright cross, depending on the orientation of the analysing filter. (The colours arising from interference have been dealt with in a special section.)

Of course, the pattern of internal tensions of Goethe's glass squares was not rotationally symmetric, but the symmetries of the square produce quite similar patterns.

Goethe describes his experiments correctly, but in his interpretations and attempted explanations he is ensnared by his ideas on the origin of colours.

Now consider the simplest experiment where no black mirror is used. The glass square lies horizontally on dark ground and mirrors the blue sky. The blue sky is sunlight scattered by air molecules (Rayleigh scattering), and is polarised with the electric vector at right angle to the sun's rays. We consider the case that the electric field oscillates in the plane spanned by the incoming ray and the perpendicular to the glass (p-polarisation), and the incoming ray's angle is close to the Brewster angle. In this case, there is little reflection at the surface, the light refracted into the glass is split into two rays (ordinary and extraordinary ray). The reflection at the bottom surface of the glass acts like a polarising filter because of the near Brewster angle, and so the situation is that of a birefringent medium between crossed polarisers. The first polariser is Rayleigh scattering, the second one the reflection near Brewster's angle. Colours and the black cross appear. If the incoming light is polarised horizontally (s-polarisation) the bright cross can be seen. The following two photos of a plastic lid reflecting the white computer screen have been taken without polarising filter; the colours are the same as seen with the bare eye.

With a bit of good will in the left picture the black cross can be seen, and in the right one the white cross. As the light from the screen is polarised obliquely, I had to position my “entoptic” plastic lid obliquely too to demonstrate this effect.

We see the dark cross if p-polarisation prevails, the bright cross in the opposite case.

Goethe was fascinated by the colours arising in crystals, mica sheets, or glass between polarising black mirrors, as expressed in a poem (no translation). Here is another plastic beaker:


Colourful Black II: Iridescent Vinyl

  

The coloured reflections on compact discs are well known and an everyday experience. But similar colours could also be seen on the old vinyl discs, though only at grazing incidence of the light. To the right there is a photograph taken in about 1960.


  

When I was going to abuse an old vinyl (which I couldn't play any more) to serve as disc of a Wimshurst machine, I was surprised to see faint colours on it in the spotlight. Not at all grazing incidence, instead lit and viewed from almost perpendicular directions.

The colours bear some slight resemblance to Quetelet fringes as well as to those of thin films. The fringes seem to be circular. My first idea therefore was to look for light paths with slightly differing lengths, possibly with two reflections in the grooves, in analogy to the Quetelet mechanism. There are such paths as shown in the sketches at the end of this short section. The two rays coming from the same groove cannot be resolved by the eye, so the waves are superposed on the same spot of the retina (or the camera's sensor) and interfere to produce the colours well known from soap bubbles, oil films on wet pavements, generally from two-beam interference.

Using a small LED torch I tried to reproduce the effect with other old vinyls. It turned out that under similar conditions the results with different discs were quite different, presumably because of the shapes of the grooves' cross sections. Below are some examples.

The last picture in the row shows the record where I first saw the effect (which survived the unsuccessful test as a Wimshuest machine disc with minor injuries). Click on the images for larger versions! The sequence of colours is quite different from that seen in soap films or other examples of two-beam interference which means that the physics is not so simple: the grooves are so narrow that diffraction effects must not be ignored. Instead of accounting only for the paths where the light is deflected by specular reflection, we should, in the spirit of Huygens and Fresnel, consider the elementary waves scattered from all points along the cross section of the groove, scattered once more and finally forming the outgoing wave.

We have seen that small surface elements of black paper glisten in different colours, and as the surface structure there is irregular, the pattern of coloured specks is irregular too. Here now the surface is structured by slightly wiggly grooves with constant cross section. If at a certain point some colour is reflected at given illumination and observation angles, along the groove the colour changes only slowly, and the neighbouring grooves, having the same cross section and viewed and illuminated at almost the same angles, are likely to show nearly the same colour too. Therefore there are “regular” fringes instead of chaotic speckles.

Geometric optical visualization of light scattered by a groove with parabolic cross section. An incoming bunch of parallel beams is reflected by the black, but glossy material. The complete drawing to the left may be too confusing, therefore it is split: the middle image shows only the incoming light and the rays after one reflection, the right one only the rays outgoing after a second reflection. There are three bundles of diverging rays, and for small observation angles (as measured from the normal to the disc) there are parallel interfering beams. The path length difference of the rays outgoing after only one reflection (middle picture) to the twice reflected ones is presumably too large to produce noticeable interference effects.

Colourful Black

A sheet of black cardboard in the sunlight. It looks inconspicuously, black or, more precisely, dark grey. But when looking at it closely in the direction to the sun, a pattern of colourful tiny specks can be seen. The image to the right is an enlarged detail of the left one. At the bottom the millimetre-scale of a set square.

I remember that once I have read about this phenomenen in Goethe's “Colour Theory”. This book, scanned and digitized, is now accessible in the web in the “Deutsches Text-Archiv”, here is the citation:

373.
      Etching a piece of polished silver by aqua fortis such that the copper contained therein gets dissolved and the surface becomes rough, and then observing the mirror image of the sun in this plate, one will see it glittering from every infinitely small protruding point and the surface of the plate will appear vividly coloured. Similarly, a piece of black non-glossy paper exposed to the sun, if observed carefully, shows its smallest parts shining in most vivid colours.

Of course, a correct explanation is not to be expected from Goethe. Diffraction and interference produce this effect, as has been discussed in detail in the corresponding section.

The dark brown fur of the dog in the next image is shimmering. The effect is much the same. Colours are seen where the gloss of the fur is slightly out of focus, and pink and green prevail.



A similar observation has also been described accurately by Goethe:


      
367.
       If a thin string of steel is taken off from the reel, curling according to its elasticity, and is looked at in daylight near a window, the windings and loops will be seen bright, but not glossy nor colourful. If, however, the sun comes out, the brightness shrinks to a point, the eye beholds a tiny glossy image of the sun which, if looked at closely, shows no colour. But stepping back and looking from some distance at the glistening, one can see many small varicoloured images of the sun. Though green and pink seem to be most abundant, the other colours reveal themselves to more careful examination.

(The text images have been derived from the scans of the Deutsches Textarchiv. Goethe, Johann Wolfgang von: Zur Farbenlehre. Bd. 1. Tübingen, 1810.)

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