Have you ever gazed into your coffee as you take a mouthful only to get disturbed to see a distorted view of your face looming back at you from the coffee? Has it struck you that while you often see such reflections, you rarely see shadows? Try it first with water and then coffee. Can you, perhaps, see a shadow on the coffee where you cannot see shadows on the water? Why would this be?
For a shadow to be visible on a surface, the surface must scatter enough light so that the contrast between shadow (where there is no light to scatter) and non-shadow (where the surface is illuminated) can be seen. Although a shadow (or at least the relative lack of light) is always going to be present behind any obstacle, it is whether or not it can be seen on the surface of the water/coffee that is at issue here. Pure water is of course quite transparent. Without anything in the water to scatter the light (such as mud for example), the light passes straight through the water to the other side. Overall, not enough light is scattered back from the surface of the water to generate the contrast required for seeing shadows. Seeing shadows on pure water is going to be hard.
By contrast, coffee contains suspended particles, in fact they are part of the very essence of the drink. These particles offer a surface to scatter the light back towards the observer and so highlight the shadows formed by the object between the coffee and the light. It strikes me that different brew methods will result in different amounts of sediment and suspended particles in the coffee and therefore a greater or lesser tendency of the coffee to reveal shadows. Perhaps if anyone does notice that it is harder to form shadows on coffee prepared by a Chemex than a French Press (for example) they could let me know using the comments section below.
Shadows have been used by philosophers to illustrate by allegory how we perceive the world around us. In the tale of Plato’s cave a group of prisoners are held in a cave such that they can only ever see the shadows playing on the cave’s wall. The shadows are formed by a fire behind the prisoners that the prisoners cannot see. As they can only see the shadows, they start to think that it is the shadows themselves that are ‘real’. It is a tale questioning the reality of what we currently see and also our inability to adjust to the differences between looking directly at the Sun or discerning shadows in the dark. In the story of the cave, it is the fire, or the Sun that causes the shadows that deceive the prisoners. No consideration was given to the role played by the wall on which the shadows dance. Yet we can see from our coffee that to understand the world of shadows we do not merely need a light source. To understand shadows, we need a surface from which to reflect the shadows. Perhaps we need to spend some time contemplating our coffee, the shadows and what they can tell us about the world and how we see it.
For details about this and other phenomena involving light and its interaction with the world around us, see: “Color and Light in Nature”, David K. Lynch and William Livingston