Chinese Magic mirror

Creating an impression at 2Love Coffee House, Clapham Junction

coffee, cake menu, Clapham Junction, monmouth coffee

The menu at 2Love in Clapham Junction and some of the coffee making equipment in the window.

There is a lot of coffee paraphernalia on display in the windows at 2Love Coffee House on St John’s Road near Clapham Junction. Reusable cups, filters, moka pots, Chemex’s etc. Stepping inside, a piano greets you while the counter is on the left. The wall behind the counter is lined with jars of different sorts of tea while the coffee menu is on a blackboard close to the window. Coffee is roasted by Monmouth and is also available to purchase for brewing at home. Moreover, the number of re-usable cups on display meant that I have to admit to a touch of reusable cup envy when I saw the variety of glass cups on sale, have I used my cup enough to justify a second*? One great feature about this café was the care that they have taken to specify the allergens in their cakes on the blackboard, it is a considerate touch for people with allergies. Although we didn’t enjoy a cake on this occasion, it is great to know that I can!

There is definitely a musical feel to the café, with statues of musicians on shelves around the shop and pictures of different singers on each of the walls. Although we managed to find a table, it was rather crowded with the amount of chatter and distractions in the café initially challenging my assumption that all cafés can offer a space to contemplate and consider connections. However, this brief doubt in the idea behind Bean Thinking did not last long. The change in direction started with our discussion over an Americano and a fruit juice: can there be a justification for not eating certain meats if you are not already vegetarian/vegan and if so, what is it? This didn’t seem to go down too well with the table adjacent to us. On the wall behind our table was a metal picture of a horse drawn cart where the figures had been raised out of the picture to form a 3D image. It was reminiscent of the patterns given for stone rubbing as a child. But it was also reminiscent of something else, something that shines a light on an area of manufacturing as well as, perhaps, our conversation about the ethics of meat eating.

Not quite a mirror at 2 Love

3D Metal picture, musician statue and poster at 2Love Coffee House, who is the fairest of them all?

It concerns Chinese (or Japanese) magic mirrors. Known about for millennia (and not just in China, Aulus Gellius (c125 – approx 180 AD) wrote of them in the second century¹), they are slightly convex mirrors made of bronze. One surface appears to be an ordinary mirror but on the reverse surface, images of mythology or special Chinese characters are cast in relief. A Nature paper of 1879 describes why they were considered ‘magical’:

“If a polished surface is looked at directly, it acts as an ordinary mirror, reflecting the objects in front of it, but giving, of course, no indication whatever of the raised patterns on the back; if however a bright light be reflected by the smooth face of the mirror onto a screen, there is seen on this screen an image formed of bright lines on a dark background more or less perfectly representing the pattern on the back of the mirror, which is altogether hidden from the light”.¹

You can see photos of such mirrors and their reflections here but how would such an image be produced? Apart from magic, the first explanations for the effect focussed on it being trickery on the part of the makers of these mirrors. Perhaps the image was patterned onto the front of the mirror using more dense (or less dense) material, covered with a thin layer of lead or tin and highly polished so that you would never notice it by looking at your reflection only by shining light at it? Maybe there was other trickery involved on the part of the mirror makers to deceive us into thinking we could see through the mirror to the back. Later researchers wondered if these mirrors really existed at all as few could be found when they searched for them amongst Japanese mirror workshops. And yet a few mirrors with this magic quality were found and subject to study in the late nineteenth century.

window display 2Love

How much is that cup in the window?
Some of the reusable cups on sale at 2Love coffee house.

The results showed that the image was not formed if projected too close to the mirror but only if the screen were held some distance away from the mirror’s surface. Moreover careful optical experiments showed that the image was formed by the surface of the mirror having thicker regions that were less convex than the rest of the mirror so that these reflected the light differently². Although the image at the back of the mirror had been cast and not stamped on the back, the stresses and strains formed by the pattern on the metal somehow propagated through the (thin) mirrors and produced distortions on the surface of the mirror. Even when highly polished, these minute distortions in curvature remained causing the reflection of the ‘magic’ image under certain lighting conditions.

The theory describing the optics behind the magic mirrors was described as a ‘beautiful fact’ in a fairly recent mathematical description. But exactly how the stress of the pattern at the back gets transferred to the surface of the mirror remains to be understood³. Nonetheless, the fact that imperfections on one side of a material can be revealed by the projected reflections from the surface of the other, a process known as “Makyoh imaging”, is now used to check the integrity of semiconductor wafers before they are used in the manufacturing of devices. A piece of physics based neither on magic, nor on trickery, that is useful for our computer based lifestyles.

When faced with something that seemed improbable, it is interesting that our first explanations were based on magic, deceit on the part of the one who made it or distrust of the phenomenon altogether. It was only by carefully studying something that was too easily dismissed that the beautiful physics and industrial relevance of the property was revealed. For me this has pertinence to the question of our own investigation into what we think about the world. Do we place too much weight in our judgement of what we do not understand merely based on our own experience of how things are? Do we need to look more carefully at what we thought we knew? Great pondering points for a visit to a café and confirmation that, provided you have good coffee and a nice chair to sit on, contemplation directions can be found no matter how popular the venue.

2Love coffee is at 89 St John’s Road, Clapham Junction, SW11 1QY

¹ “The Mirror of Japan and its Magic Quality” Nature, April 10 1879, p 559

² “The Magic Mirror of Japan, Part 1”, WE Ayrton and John Perry, Proc. Royal. Soc, 28, 127 (1878-79)

³ “Oriental Magic Mirrors and the Laplacian Image”, MV Berry, Euro. J. Phys. 27, 109 (2006)

*Although there are differences depending on what you take into account, lifecycle analysis done here, here and here suggest a break-even point of disposable to reusable cups from 15 to 100 re-uses. However, if you consider that part of the solution to our environmental problems involves breaking the consumerist mindset then perhaps, if it ain’t broke, no need to replace it.

 

It’s a kind of magic at Kaffeine

Kaffeine_neonIt’s nearly 7 years since Kaffeine first opened its doors on Great Titchfield St, but Kaffeine on Eastcastle Street is a new addition having opened just over a year ago. We visited the younger Kaffeine a couple of weekends ago when looking for a coffee in the Oxford St area. Along with an iced coffee, an Americano and a long black, we had a raspberry/cashew slice and a slice of banana bread. It was a relief to find that Kaffeine had a good nut policy so I could confidently enjoy my banana bread, knowing it was nut-free, while a friend devoured the cashew containing slice. The staff were attentive and friendly and there was plenty of space inside to sit and chat while taking in the surroundings. In this regard, it was nice to see this same point being made on Kaffeine’s  own website where it says that you can “…sit at the high stools at the massive sun filled front windows and watch the world go by”.  With the accompanying coffee, what more could you want? The coffee was, of course, very good (beans from Square Mile), and it was great to see that part of the philosophy behind Kaffeine is to take “the art and science and practice of making espresso coffee to a whole new level”. It’s always a pleasure to see those three distinct, but essential, elements combined. I do however remain unconvinced that many could tell the difference (in a blind taste test) between an Americano and a long black.

wood and slate with glass, Kaffeine

The table top at Kaffeine, Eastcastle St.

Complimentary mint-infused water was on offer at the back of the cafe and, although this made the Daily Grind last week, there was just too much to notice at Kaffeine to make this the subject of the cafe-physics review.  Indeed, from the perspective of anyone who wants to slow down and notice things in a café, Kaffeine is brilliant. This cafe-physics review could have been about so many different things. There were the weights holding the door open in a pulley system. The trademark neon sign. The compact-ness of the cashew/raspberry slice or the reflectivity of the copper on the side of the counter. I was in cafe-physics review heaven! So many different mental alleyways to run down and explore as the different bits of physics came into view. From pulleys to Archimedes, cakes to ceramics science, copper to atoms or to the odd puzzle about the colour of gold, all these will have to wait for another time. This time, what struck me was not what could be seen but what could be felt.

Far from going into a subjective piece about the ambience of the cafe, I mean this statement far more literally. The table, with the wooden grain, felt rough. In the middle of the table, a piece of slate had a surface that was more smooth and then, on the walls behind us, highly glazed tiles were very smooth indeed. What do we mean by rough or smooth, how rough is rough, how smooth is smooth and what has it to do with the “magic mirrors” of Japan?

reflective tiles but not really flat yet

Smooth tiled wall at Kaffeine

The wooden bit of the table for instance has a surface that undulates with a height of the order of about a millimetre. The slate is far smoother but the surface would still be rough, probably on a length scale tens to hundred microns or so (about the size of espresso to medium grind coffee). The tiles are a lot smoother than both the wood or the slate but they are still not so smooth that they could be considered flat on an atomic scale. To be flat on an atomic scale, the surface would have to have a height variation 100 000 times smaller than the smallest particles in an espresso grind*. While some crystals can, naturally, have ‘faces’ that are this smooth, the semiconductor industry needs to be able to achieve this level of flatness routinely to provide the electronics for your smart phones, computers and even perhaps the electronic scales that are used to help you make your coffee.

copper mirror Kaffeine Eastcastle St

The mirror-like copper clad counter.

In ordinary life however, perhaps we think that a smooth surface is like that of a mirror. So it is worth taking a look at an odd type of mirror for which very small variations on the surface cause a very strange effect: the “Magic mirrors” of the far East. Typically made of bronze, these mirrors have been manufactured for nearly 2000 years. On the back surface of the mirror is an artwork (perhaps signs of the zodiac or other religious symbols) which is in relief. The front of the mirror meanwhile is highly polished but slightly convex. Looking directly at the front surface of the mirror, there is no visible sign of the image on the back. Maybe you don’t find this surprising, the mirror is solid bronze after all and we can’t see through solid metal. However, if you were to take a step back, shine light on the front of the mirror and look at the reflection of the mirror projected onto the wall, the image at the back can clearly be seen, there in the reflection (you can see photos of this effect here).

Initially this phenomenon was dismissed as ‘trickery’ but subsequent, careful, study showed that small deviations from perfect curvature on the reflecting surface were enough to cause the effect. Although the mirrors were cast and then polished, nonetheless, stresses and strains from the pattern on the back  had propagated through the atomic structure that forms the metal and resulted in tiny, invisible to the eye, changes on the front surface of the mirror. Sometimes it does appear that looking at things in a different light can really change our impression of what something is.

Kaffeine can be found at 15 Eastcastle St. W1T 3AY

* Scaling to coffee grind size approximate but based on measurement of grind size reported here.