refractive index

A Story with many layers, Clapham Junction

Story Coffee St John's Hill Clapham

The doorway to Story, or a story depending on how you look at it.

A “ghost sign” above the door to Story Coffee on St John’s Hill ensures that you know that you have arrived at the correct place. “Peterkin Custard, Self-Raising Flour – Corn Flour, can be obtained here”, only now it is coffee rather than custard that is sold in the shop beneath. The sign is an indicator to the many tales that could be discerned while exploring the coffee within. I had had a couple of attempts to visit Story Coffee (thwarted for a variety of reasons) before Brian’s Coffee Spot’s review appeared a couple of days after one of my attempted visits. Suitably re-motivated, another trip was attempted (address checked, closing times checked) and this time we were in luck. Although a pour over is listed on the menu, sadly this was not available on our visit and so I enjoyed a lovely long black instead (Red Brick, Square Mile) while looking at the cakes on offer. There was plenty of seating in which to shelter from the rain outside and many things to notice in this friendly café. In addition to the cakes and lunch menu, a box on the counter housed “eat grub” protein bars, protein bars made of cricket powder. Are insects the future for humans to eat protein sustainably?

glass jar at Story

Through a glass darkly?
The distortions produced by the refractive indices of air, water and glass and the shape of the glass produces interesting effects on our view through it.

The tables were well arranged for people to sit chatting while enjoying their beverages and it is always an excellent thing (from a personal point of view) to encounter a café with a no laptop (or tablet) at the tables policy. Complementary tap water was available in jugs placed on each table while it was also nice to note that Story branded re-usable cups were on sale from the counter. Many things we noted can be seen in the gallery pictures in the review on Brian’s Coffee Spot: the funky fans, the egg shaped light shades, the light introduced by the large glass window panes (though it was a much fairer day on Brian’s visit than on ours). Each had its contribution to a thought train, the way the glass water jar bent the light coming through, the concept of a Prandtl boundary layer in fluids (and its connection to both fans and coffee cups). Moreover there were hexagons, which for someone who has worked on the periphery of the graphene craze, are always thought provoking.

Apart from hexagons decorating the top of the stools, there were hexagons lining the counter made of cut logs, each showing the rings from the tree that was felled. Rather than a flat surface, these hexagons were made to be different thicknesses on the wall, rather like the hexagonal columns of the Giant’s Causeway. It is a subtle thing that may have implications for the space that is otherwise surrounded by flat, solid, walls. Such spaces can become echo-y and yet, the music and conversation in Story was not overly distracting presumably because features such as the uneven hexagonal wall reflected the sound waves such that they destructively interfered rather than echoed around the room.

every tree tells a story, but which story

A macroscopic crystal of hexagonally cut logs forms the side of the counter.

Each log in the hexagonal decoration was cut with its cross-section showing a number of tree rings. We know that we can age a tree by counting the rings (though each of these would be underestimated as they have been trimmed into hexagons post-drying), but what more do the tree rings, and the trees themselves have to tell us? The rings are caused by the rapid growth of large cells during spring followed by a slower growth of smaller cells as the year progresses. But this method of growth means that the cut logs have more to tell us than just their age. The spacing between the rings can tell of the weather the tree experienced during that year, were there many years of drought for example? Such clues, from the relative density of the tree rings, can help researchers learn about the climate in previous centuries, but conversely, reading the climate report in the rings can indicate in which year a tree was felled and so the age of a building for example.

coffee at Story

Many stories start with a coffee.

And then there is more, trees will grow at an average rate per year so that, as a rough guide, the circumference of a mature (but not old) tree increases by 2.5cm per year¹. There is therefore something in the idea that you can have a good guess at how old a tree is by hugging it. But this assumes that the tree is growing in its optimum conditions, far enough from any neighbouring trees so as not to be crowded into growing more slowly. So the absolute density of tree rings must also give a clue as to whether this tree was in a dense forest or an open clearing. Which is reminiscent of something else that living trees can tell you if you listen to them closely enough: trees will grow so that their leaves are exposed to the maximum amount of light. For us in the UK, this means that the crown of a tree will frequently tip towards the south (where the Sun is most often) and there will be more leaf growth (and consequently more branches) in a southerly direction². But again, we only see this if the tree has room to grow on its own, without the crowding, and competition, of too many neighbours. A solitary tree helps us to know which direction we are walking in.

empty coffee cup Story St John's Hill

While many coffees could also tell a story. It depends on how you read them.

Which all points to the idea that there are many stories being told all around us all of the time, the ones we hear depend on what we choose to pay attention to. So what about the story behind the ghost sign above the door? The Peterkin custard company was a venture by J. Arthur Rank in an attempt to start a milling company in the mould of his father’s (Rank Hovis McDougall, later bought by Premier Foods). The company failed and Rank went on to form the Rank Organisation that was responsible for many films made throughout the 40s and 50s as well as running a chain of cinemas around the UK. Truly a sign concealing many stories.


Story Coffee is at 115 St John’s Hill, SW11 1SZ

¹Collins complete guide to British Trees, Collins, 2007

²The Walker’s Guide to Outdoor Clues and Signs, Tristan Gooley, Hodder and Stoughton, 2014





Bend it like sugar at Muni, Fulham Road

Muni Coffee, near Chelsea and Westminster hospital

Muni Coffee on Fulham Road

The area around Fulham Road and Chelsea & Westminster hospital is one that has long been fairly empty of speciality coffee establishments. Then, in June this year, Muni opened up on Fulham Road (just over 200 m from the main entrance of the hospital, in case you are visiting and looking for a good café nearby). Muni’s website emphasises its social mission, knowing the farmers they trade with by name and introducing Filipino coffee to the UK. Inside, there are plenty of tables (with more outside if you are visiting in warmer times). There is a menu on the wall behind the counter to your right as you enter, but I missed the listing of the Pandan iced tea (which would have been very interesting to try) as I was obviously not paying enough attention and instead opted for my default trying-a-new-cafe coffee, a black Americano.

My sometimes companion in these reviews had a soya hot chocolate while I was very confident to enjoy one of the (lovely) salted caramel brownies because Muni lists all the ingredients for all of their cakes on a tablet device at the counter and so I was encouraged to double-check the ingredients list to see that there was nothing vaguely nut-related in it. A very good feature and this cafe definitely gets a tick in the “cafes with good nut knowledge” category on the right (as well as the new allergy-friendly category). As mentioned, the coffee is imported directly from the farmers in the Philippines, and roasted by Muni in North London. The black Americano I tried was fruity and flavoursome, while the beans I purchased and prepared later using a V60 produced a sweet and floral brew, perhaps with blueberry notes (but with no tasting notes on the packet, I’d be interested to see if others agree with me on this, please let me know in the comments section below).

coffee cake Muni

Coffee and nut-free salted caramel brownie at Muni

On the ceiling, wooden beams had cracked and aged creating a lovely aesthetic and taking me on a thought trail that involved aeroplane engines and heat process treatments. But then I noticed something else. As it was getting dark, the cars passing by on the busy Fulham Road were mostly using their headlights and this meant that, every so often, the edges of the windows around the door changed these headlights into a spectrum of colour. Flashes of blue, red and green as each car passed. It reminded me of Newton’s experiments in which he used a prism to first separate sunlight into its various colours before recombining it with another prism into white light. An effect that led me to think about an instrument that has been advertised as a tool to creating better coffee: the coffee refractometer.

Some of the same physics links Newton’s prism with the coffee refractometer. Perhaps you remember “Snell’s law” from school. The equation describes how much deviation light experiences as it passes from one medium (air) to another medium (glass or coffee). Light travels at different speeds through different media and the refractive index can be thought of as an indicator of the degree to which each medium slows down the light.

the door at Muni

The window at the side of the door at Muni. Rainbows of colour were produced by the headlights of cars as they went by.

For the prism, the important detail is that light is composed of many colours (which means in this context, many wavelengths) and not all wavelengths are slowed to the same degree. This means that the refractive index of the glass prism is slightly different for red light than it is for blue. Consequently, the spectrum opens up as the white light travels through the prism.

For the coffee refractometer, the important point is slightly different. Water containing dissolved solids has a slightly different refractive index than pure water. Measuring the deviation of a light beam through a drop of coffee therefore gives an idea of the concentration of “total dissolved solids” and so a guide to the extraction of coffee from the grind that you have achieved. The difference in refractive index is however quite small, if the measurements here can be relied upon, while water has a refractive index of 1.333 (at 20ºC), a well extracted coffee showed a refractive index of 1.335. We can calculate how much difference this makes to the angle that the light is deflected: Assuming light enters the drop at an angle of 30º, the angle that light is refracted in water is 22.03º, while in the coffee it is 22.00º. A small effect that would be quite difficult to measure unless you had a refractometer.

However, there is an ingredient in some people’s coffee that bends light enormously: sugar (though I do hope that no one reading this uses it in the quantities needed for the experiment below). The refractive index of water is very dependent on the total concentration of dissolved sugar it contains. Therefore you can do a really cool experiment in which a sugar solution (which has more concentrated sugar at the bottom than the top) can be seen to bend the path of a laser beam. All the equipment can be easily found at home (or purchased for not too much from hardware/office equipment shops). Let me know if you try the experiment how you get along (and if you decide to try using a refractometer to enhance your coffee brewing experience). The video was shared on youtube by the Amateur Astronomical Spectroscopy group (CAOS).

Muni coffee is at 166 Fulham Road, SW10 9PR. Just around the corner on Drayton Gardens, is the blue plaque for Rosalind Franklin who used to live at an address there.


Can you see me? At 123 Gasing, KL

Coffee at 123 Gasing

Latte, Long black and chocolate muffin at 123 Gasing, PJ, KL

There are times when you can sit and observe things for quite a while before noticing the physics that becomes a cafe-physics review. There are other occasions when the subject of the review is staring you in the face indeed, it is practically there written for you, on a noticeboard in black and white. Such was the case at 123 Gasing, a cosy and quirkily decorated cafe located, strangely enough at 123 Jalan Gasing (ie. Gasing Road), in PJ, Kuala Lumpur. We enjoyed a lovely breakfast of scrambled egg, long black and a latte (along with a very rich chocolate muffin). The coffee is from Degayo (according to Malaysian Flavours) which means that it is practically a local food product (originating as it does from neighbouring Indonesia). Coffee with minimal ‘food miles’. The only point of regret about our time at 123 Gasing was that we didn’t manage to spend longer there.

decoration at 123 Gasing

Birds on the wall at 123 Gasing.

It is the decoration that strikes you as you look around this cafe. A couple of painted birds sit on top of an electrical wire, prompting the question “why do birds not get electrocuted when they sit on a wire?”. Another question painted to a notice board on the wall asks “what is it that we need that we cannot see or feel?” (answer at the end of this post). Yet it was another thought on another noticeboard that prompted this cafe physics review. That thought suggested invisibility (see picture below).

The idea of invisibility has fascinated story tellers and philosophers for millennia. Trying to render objects invisible is, understandably, very desirable for the military and the defence industry. Although we have always had access to camouflage and deception, it is only relatively recently that it has become feasible to talk about invisibility cloaks as a real possibility.

A sign at 123 Gasing

Am I invisible?

What has moved “invisibility cloaks” into the realm of reality has been the advent of a field called “metamaterials”. As the name suggests, metamaterials are not materials that occur naturally but materials that we manufacture. Combinations of different materials or repeating patterns of a specific material that interact with light in a way that the material itself would not do. The classic example is a so-called split-ring resonator (SRR). These are rings (that were first made with copper) which have a slice cut out of them. Many such rings are arranged in a repeating, lattice pattern. Due to the engineered pattern of the copper, these lattices interact with light in a way that ordinary copper does not (for details click here). Specifically metamaterials can be engineered to bend light around objects so that it appears that the object is not there.

In order to work, the artificial structures (e.g. the copper rings) must be smaller than the wavelength of light that is to be ‘bent’. This means that microwaves (which have a wavelength ranging from a few cm to a few m) can be manipulated far more easily than visible light (with a maximum wavelength of 700 nm, or about 1/100th of the size of a grain of espresso grind). At first sight this may seem disappointing until we remember that even devices that only work with microwaves would have a clear application for the defence industry (radar).

already disturbed

Hopefully not a comment on current scientific funding

There are many ethical and philosophical questions that follow from the fact that it is now within our reach to render some objects invisible. It is not a scientific question as to whether we should do it, the scientific question is whether we can. Where science and ethics collide though is in the funding issue. A subject such as this with obvious applications receives far more funding than fields that advance our understanding but do not enhance our weaponry. Indeed, one of the researchers involved in this field describes how he was “offered large sums of money (almost on the spot)” when he spoke of the potentials of the “Harry Potter project”¹. Something that is alien to those of us who work in less fashionable subject areas where funding is a constant struggle. Government funded science quickly becomes dominated by a quest for application and technology. In effect we bypass the ethical questions of whether we should do this because it is this that will get funded. Science that is not driven by obvious applications will not get funded.

Is this what we want? Should the humanities and philosophy play a role in helping to determine what research is beneficial for society and so which research receives funding? Should ethical considerations play a part in funding considerations, or should scientific research all be about the devices that we can use? It is certainly something to ponder while sipping on our long blacks.

Answer to the question “what is it that we need that we cannot see or feel? Answer in 1990 – Air, answer in 2000 – Wi-fi (though personally I think maybe this should be the answer in 2015, the given answer of “2000” was still a bit early for widespread wifi).

Further reading and [1]: “The Physics of Invisibility” Martin Beech, New York, Springer, 2012

Patterns in a tea cup

light patterns on the bottom of a tea cup

Looking into my peppermint tea. Dancing filaments of light are just visible

Have I been unfair to tea drinkers? It has been pointed out to me on more than one occasion recently that tea is also a good source of science in a cup. So, last week, I drank a large amount of tea and started gazing into my (peppermint) tea cup. I watched as dancing lines of light played on the bottom of the cup. Never staying in one position for long, the filaments moved around, snaking across the tea cup. You can possibly see them in the picture on the right, although you would get a better view of them if you watched them dance yourself in a cup of freshly made tea. Similar lines can often be seen at the bottom of the swimming pool. Such lines of light must be caused by something in the water (or tea) bending the light from the surface into concentrated patches on the bottom. But are the two effects, though visually similar, caused by the same thing? And, what can this possibly have to do with forensic science and drug dealers?

straw, water, glass

When light travels from one medium to another (e.g. air to water) it gets bent by refraction

When light passes from air into a transparent medium (eg. into tea) it gets ‘bent’, in a process called refraction. This is why a spoon (or straw) put into a glass of water looks bent when viewed from the side (see picture). The amount that the light bends is dependent on the angle at which it hits the tea surface and by the density of the tea. The fact that you have to be able to see the bottom of the cup to see this effect, makes tea ideal for viewing it. (If your coffee is transparent enough to view these dancing lines of light, you may well want to check that you are brewing it correctly).

I’m not an optics person but it strikes me that there are at least two easy ways for these light patterns to form. Firstly, small waves on the surface of the water/tea will cause the light hitting the waves to be refracted by different angles as they go through the water. The patterns that form on the bottom of the pool/cup will therefore move with the waves. It is easy to see how such waves could form in a swimming pool, it is not so easy to imagine them in a tea cup. A second way to form these patterns is if the light is refracted through regions of different density, such as slightly hotter and slightly cooler tea. Such regions will occur in a tea cup because the tea is being cooled at the surface by contact with cool air and so there will be a continuous convection process in the cup. Warm water is less dense than cold water* and so will refract light slightly less than cold water will. Consequently, as the slightly cooler and slightly warmer regions of tea bend the light by slightly different amounts you should see patterns forming on the bottom of the cup as different amounts of light get to the bottom at each point.

So we have two possible causes for the light patterns on the bottom of a tea cup. How could we distinguish between them? Perhaps it would be an idea to get two identical cups, one filled with cold water, one with hot water (or a clear tea such as peppermint). Which shows the dancing filaments? Both of them, neither of them? Another experiment could be to observe the filaments in a cup of hot tea and then wait for the tea to cool. Do the light patterns fade as the tea cools?

tea pot science

Not always coffee. Tea can be interesting too.

The link to forensic science comes from the fact that light passing through transparent substances of different density will be ‘bent’ by different amounts. Imagine a drug dealer has been caught with some illegal substance wrapped in cling film. Although it looks to us like any other piece of cling film, that piece of film has been made in a specific factory at a specific time. This means that the roll of cling film that this piece was taken from will share variations in thickness and density with the cling film wrap. A type of cling film ‘finger print’. The density variation in the cling film can be photographed with a technique called the Schlieren photograph which exploits the fact that the light is refracted by varying amounts as it passes through these varying densities. If the police can get hold of the cling film in the suspected drug dealers home, this too can be imaged. If the ‘finger prints’ (changes in density etc.) of the two samples of cling film match, the suspect may be in significant trouble. The motto of this: Ensure that you have a decoy roll of cling film to hand before wrapping anything or, what is probably much better, spend time contemplating your tea in a café instead.

What do you think causes these patterns? What do your experiments reveal? Comments always welcome, please leave them in the box below.


* Between 0-4ºC the density of water decreases with decreasing temperature. For the purposes of this blog article it is assumed that you are drinking normal tea at around 60ºC rather than ice tea.