seeing coffee

Looking at coffee

coffee at Watch House
Observing the colours in our coffee can reveal much more than just the chemistry of the cup.

How do you see your coffee in the morning? Through blurry eyes, a red-ish/brown liquid that you may admit to noticing more for its aroma and taste than for how you look at it? But what is it about that lovely red colour of a fresh filter coffee viewed through sunlight? And what about the way that the glass jar curves towards you and then bends away, how do we perceive distance?

The colour question has historically been more problematic. For Aristotle, the rainbow was composed of a mix of three colours, which fitted with Pythagorean numerology*. Newton thought there were seven, which fitted with the harmonies in music theory. Goethe (who also developed a colour theory) liked to quote “If you show a red rag to a bull it becomes angry, but a philosopher begins to rage as soon as you merely speak of colour”**.

Today, in schools we are taught that there are three primary colours for light: red, green and blue. This is because all colours of light can be observed by careful weightings of these three colours and when the three are combined we see white light. But what does it mean that light has primary colours? Is not light just a vibration, why is it that we see colour at all? It comes down to our physiology and how we sense the world.

It was Thomas Young (who also showed the wave-like properties of light) who first proposed that these three ‘fundamental’ colours were associated with three types of ‘resonator’ in our eyes. The idea was significantly developed by Hermann Helmholtz during the 1850s. Each type of receptor responded to light at all frequencies but responded most sensitively in a smaller range. Generally humans have three types of frequency sensitive (and so colour sensitive) receptors, though those with colour blindness have fewer and there are even some of us with four. Most of us though, have three types of receptors sensitive in the red, blue and green regions of the spectrum. Hence we perceive the light as white if these three types of receptor are stimulated equally, that is, if we combine blue, red and green light. The red colour seen as you brew a fresh pour over of coffee in front of a window through which sun is streaming at dawn, is red because of these activated red-sensitive cone receptors in your eye.

Sun-dog, Sun dog
A ‘rainbow’ of colour as seen in a ‘sun dog’ observed in central London. But what is colour really?

But Helmholtz went further than this. Have you ever been staring at a bright object and then turned away towards a dark wall and had the experience of seeing the same bright object ‘projected’ on the wall but in a different colour? Both Goethe and Helmholtz observed themselves as they ‘saw’ these phantom images and watched the images as they changed colour before eventually fading. While Goethe incorporated his observations into his general colour theory, Helmholtz linked the phenomenon to these same cone receptors in the eye. He realised that if your red-sensitive colour receptors had become saturated by watching a bright red object (such as a red-hot piece of iron for example), they would not respond so quickly when you looked away at a blank bit of wall. So if you, for example, ordinarily perceived the wall as white, because the red-colour receptors had been taken out for a while, your blue and green receptors would dominate while the red would not respond and so cause you to observe a greener phantom image. Would we ever see a green phantom coffee?

Unlike the question of the colour, the question of depth perception has some thoroughly more modern elements. For while many had thought about how we realise that space has depth, the binocular effect of our two eyes was not realised until relatively recently. In fact, that two, 2D images taken from slightly different angles and viewed separately through each eye appeared as if they were a 3D image, was only discovered in 1838. Prior to that, it had been thought that perhaps we knew about depth because of our learned familiarity with the size of objects, much as Fr Ted explained the distant cows to Dougal (which is one of the clips here).

Shadows reveal a lot. From the position of the light source to information we interpret as informing us about how different objects relate to each other. And again, why is it that shadows appear blue on snow?

Apparently between 1855-59, 29% of scientific papers concerning the eyes were about this problem of stereoscopy or binocular vision. Helmholtz’s contribution to the debate was to show how much of our realisation of depth was a learned but unconscious process and also how much relied on the involuntary movement of our eyes to ‘calibrate’ the surroundings after fixing on something. That movement of your eye that is impossible to control but you can watch in others as they concentrate is there for us to check that what we think we are seeing is what we are seeing.

Just how much is revealed to us, about the coffee and ourselves, by our gazing at it? When Feynman discussed colour vision in volume I of his Lectures on Physics, he wrote “We make no apologies for making these excursions into other fields, because the separation of fields… is merely a human convenience, and an unnatural thing. Nature is not interested in our separations, and many of the interesting phenomena bridge the gaps between fields.”*** Our world is intricately connected, we only have to gaze at our coffee to have an intuition as to how much this is true.

This post is one of a series about the contributions of Hermann von Helmholtz to how we understand the world around us. The introduction is here and it will be followed by thinking about what we hear in our coffee, the heat of our coffee and, of course, what happens when we stir it.

*”The Rainbow Bridge: rainbows in art, myth and science” R.L. Lee Jr, A.B. Fraser, Penn State University Press (2001)

**”Helmholtz: from enlightenment to neuroscience” M. Meulders, MIT press, (2010)

***”The Feynman lectures on physics volume I”, Feynman, Leighton and Sands.