light scattering

Seeing the light at Cable Co, Kensal Rise

coffee in Kensal Rise, Cable Co

Cable Co, coffee in Kensal Rise

It was fairly late on a February afternoon that we came upon Cable Co on Chamberlayne Road, (opposite Kensal Rise station). With a fairly ‘industrial’ type look, there are plenty of tables at the edge (and in the window) of the café at which to enjoy your coffee. There are also plenty of coffees on offer. Although I had an Americano, I noticed (too late) that pour-overs were available. Coffee is roasted by Climpson and Sons. As it was late in the day, the remaining cakes in the display case all looked to be nutty (or at least likely to be nutty) and so, sadly, I had to wait until I got home for my slice of cake. It was good coffee though, even without the cake, but in a bit of novelty the coffee came ‘deconstructed’, so I got to add the amount of water that I preferred, a nice touch.

Golden light from the setting sun streamed in through the windows (which is a navigation clue & tells you which side of the road this café is on). The effect of the Sun was to bathe the café in light and to silhouette our fellow coffee imbibers making the café take on a film-like atmosphere. The light had another effect though. The steam rising from both the jug of water and my espresso became far more visible than it would normally have been. I watched as the steam clouds formed vortices and turbulent patterns, one fluid (steam) moving through another (air). It was very difficult to catch this in a photograph, a fact that I took in support of my idea that it is impossible to catch the beautiful, beauty is necessarily transient (but my companion in these reviews took as evidence in favour of their idea that I really ought to use a “proper”, manual, camera and not my iPhone).

Steam, scattering, colour

Steam rising from hot water, seen at Cable Co, Kensal Rise

Still, those turbulent rising patterns of steam were visible and that implies that light was being scattered from the droplets of water in the steam. The size of the droplets influences the colour that we perceive when we view the steam clouds. If the clouds appear white, it is because the droplets that are scattering the sunlight have a diameter roughly equal to (or greater than) the wavelength of visible light. The wavelength of light varies between about 400 nm (violet) to 700 nm (red) which means that these water droplets have to be at least 700 nm across. To put this in perspective, the smallest particles of coffee in an espresso grind are about 10 μm diameter which is 14 x bigger than the droplets in the steam cloud.

Of course, how water droplets scatter light above a steaming coffee has implications for our understanding of why the clouds in the sky appear white (and why the sky is blue). Someone who did a lot of early work in understanding the way that light scattered off water droplets in air was John Tyndall (1820-1893). Tyndall was an experimentalist as well as a famous communicator of science. He regularly gave lectures at the Royal Institution that included demonstrations of the experiments that he himself was working on¹. One of these involved scattering light from water droplets (and therefore demonstrating why he thought the sky was blue).

Interior of Cable co

Light streaming into the cafe.

The idea is that sunlight scatters from water droplets differently depending on the diameter of the droplet. When the water droplets are approximately the diameter of the wavelength of red light, 700 nm, there is very little wavelength dependence to the light scattering. Practically this means that the droplets will appear white. If on the other hand, the droplets are much smaller than the wavelength of light, the light scattering starts to be wavelength dependent. So as the droplet gets smaller, blue light (short wavelength) gets scattered a lot by the droplets, while red light (long wavelength) is not scattered so much. This means that if you are looking at a cloud of steam formed by these small droplets at an angle between the sunlight and yourself (say, 90º), the cloud will appear to have a blue tinge. If on the other hand you look straight through the cloud at the sunlight coming in, it will have a red-hue because the blue light will have been scattered out of the cloud leaving only the red colours to come through.

The experiment can be easily demonstrated at home by using very dilute milk in water (see video here or further explanation here). If you put a few drops of milk in a glass of water and then look at the colour of the milky-water as a function of angle, you should see it change from red to blue as you move the glass relative to the light source. The connection with the blue sky seems clear, small particles (in-fact, they can be as small as molecules) scatter blue light preferentially and so, apart from at sunrise and sunset, the sky will appear blue. As Tyndall wrote:

“This experiment is representative, and it illustrates a general principle…. that particles of infinitesimal size, without any colour of their own, and irrespective of the optical properties exhibited by the substances in a massive state, are competent to produce the colour of the sky.”²

Cable Co is at 4 Bridge House, Chamberlayne Road, NW10 3NR

¹A Vision of Modern Science, John Tyndall and the role of the scientist in Victorian culture, U. DeYoung, Palgrage MacMillan, 2011

²Quoted in John Tyndall, Essays on a Natural Philosopher, Ed. WH. Brock, ND. McMillan, RC. Mollan, Royal Dublin Society, 1981


An opportunity to become a cafe-scientist

coffee, Timberyard, wooden tray

A great place to sit and do some citizen science: Timberyard, Seven Dials has plenty of seats outside.

There are many things to be gained from putting down your smart phone when you enter a café. Firstly, there is the opportunity to fully experience the coffee. The sounds as it is made, the smell, the taste, even the feel of the coffee. Then there is the opportunity for people watching; their behaviour as they order their coffees or have their meetings or try to alleviate boredom while playing with their smartphones. Of course, there is also the opportunity to look at the history of the café and its surroundings, to think about a café-physics review or just slow down and notice things. There’s always something interesting going on.

If you are lucky enough though to be in Athens, Barcelona, Belgrade, Berlin, Copenhagen, London, Manchester, Milan or Rome there is now even more reason to put down that phone while you savour your coffee. By doing so, you could be helping scientists with a few questions that they have about atmospheric pollutants. If you are not in one of those cities, you miss out this time, but you may want to keep reading because if enough people get involved now, perhaps next time the iSPEX-EU project may come near you.

contrail, sunset

What sort of aerosols and pollutants are floating in the atmosphere above your head at this moment?

The question is, what are the atmospheric pollutants that are in the air near where you are now? Perhaps you are in a café on a main road and the answer seems obvious, it is those cars and buses that keep passing by. But there are in fact many forms of atmospheric aerosols or particles and they range in size from a few nanometers to tens of microns (which, in terms of coffee grind is from much smaller than the smallest Turkish coffee to approximately the size of a small particle in an espresso grind). Is it really so clear that where you are, in the centre of that big city, is that polluted? If on the other hand you are on the coast in Barcelona, just how salty is that salty sea air? The iSPEX-EU project allows you to measure it and find out.

These particles of dust, salt and soot etc. can have  an effect on human and animal health, so clearly we want to know more about their distribution and their prevalence. But there are also, more subtle reasons why we may want to know about them. They may have an effect on global warming and they are certainly needed in order for clouds to form, (though as yet we still do not fully understand this process). We need more data about what aerosols are around and where they are to start to know what questions to ask (let alone answer) about health, the climate and cloud formation. Yes, we have satellite measurements and pollution data at specific locations, but what people are missing is that local information. What are you actually breathing? When you look up at the blue sky, what pollutants (or other type of aerosol) are you looking through? Can we get enough data to know how the air quality varies between the cafés of Hackney and those of Hammersmith?

Skylark Wandsworth

Another ideal cafe for iSPEX-EU measurements, great coffee and a lovely outdoor seating area at Skylark cafe, Wandsworth Common

To get this data the scientists involved in iSPEX-EU need people, many people. People who are willing to spend 5 minutes turning their iPhone (sadly it is an iPhone-only project) into a pollution detector. The more people that they can get measuring, the more data that they will be able to obtain. All you need is an app from the App-store and a (free) device that fits over your iPhone camera which you can pick up from somewhere local to you. Then, you just take a seat outside the café on a lovely blue sky day between now and the 15th October, aim your phone at the sky and take a series of photographs which are shared back with the scientists coordinating the project. If you are curious to know how your air quality compares with that in another participating city, you can check the live map to see how the measurements are going across Europe.

The device works by looking at the colour spectrum as well as the polarisation of the light reaching the camera as a function of angle. This information gives tell-tale clues as to the size of the aerosols as well as their prevalence. There is a lot more information on the website of the iSPEX-EU project and so I would recommend that if you do want to know more, you click their link here. In the meantime, why not sign up with iSPEX-EU, take a seat outside in that café and enjoy a great coffee knowing that, as you do so, you are contributing to our understanding of atmospheric science.

If you do decide to participate, please let me know of any great locations that you find, both for the coffee and the measurements, or share your pollution measurements with me in the comments section. I look forward to seeing some great data on the live map.

To get involved with the iSPEX project, you can follow the link here.


Dappled with Dew

Part of my morning routine can involve a walk through a local park. Each day reveals how the seasons are affecting the plants, bird life etc. This morning on walking through the park, I was treated to the spectacle of a thick layer of dew, shimmering and spectacular, glinting in the sunlight.

dew, surface tension, everyday physics, slow morvement

The dew this morning

Taking out my phone, I tried to take a picture of the scene for later and yet, what came out in the image was not the brilliant scene before me but instead some blurry grass. The ‘immediacy’ of the sight struck home. As with so many of the gifts that nature provides, attempting to take a photograph of it somehow just doesn’t quite capture the beauty of the moment. There are some great photographs of sunsets or sunrises, but part of the attraction of the image is not the photograph itself but our memory of those brilliant sunsets that we have experienced. The photograph is suggestive of the beauty that the photographer saw but somehow, the fullness of that beauty has not translated into the photograph.

As we stop to enjoy the moment, rather than photograph it and rush off to our morning appointment, we can start to notice what it is about it that captivates us. From my viewpoint, the majority of the dew this morning formed a silver blanket on the grass. It was this that caught my eye initially. Yet as I observed the dew, individual droplets came into focus and, because of the angle at which I was viewing them, they appeared as blue, as a slightly different blue and then other different colours. The physics of the rainbow was being revealed before me, one metre away on the grass. If I moved, the clues to these mysteries would disappear.

It was a reminder to slow down and notice things, who knows what we’ll see.  Perhaps you will disagree and say that it is just my poor photography skills that are the problem.  Please disagree in the comments section below!  Alternatively, if you agree and want to share a moment of beauty and everyday physics, please also share that in the comments section below.  I’ll finish this post however with an excerpt from the thoughts of someone who obviously did stop, slow down and observe his world.  The excerpt is from “Inversnaid” by Gerard Manley Hopkins:

Dew, surface tension, everyday physicsDegged with dew, dappled with dew,
Are the groins of the braes that the brook treads through,
Wiry heathpacks, flitches of fern,
And the beadbonny ash that sits over the burn.

What would the world be, once bereft,
Of wet and of wildness? Let them be left,
O let them be left, wildness and wet;
Long live the weeds and the wilderness yet.