The other day I was talking to a primary school child about condensation, what it was, where to see it etc. So I asked,
“Do you drink coffee?”
“Do you drink tea?”
(I started to worry about the future generations). Nonetheless, I pulled out my cup of steaming coffee and pointed to the water droplets around the edge of the mug (which are very common if you haven’t warmed your cup before pouring your hot coffee into it) and noticed a sudden expression of recognition cross the child’s face.
“Like when you breathe on a mirror?”
Yes, exactly so (and probably a much better example for a kid anyway, the problem of being an adult with a one track mind!). As the child had realised, the science in your coffee cup is connected to phenomena that occur elsewhere in the world. In the case of condensation, it occurs when the temperature of the surface onto which condensation happens is below what is called the “dew point”. Determined by the relative humidity in the environment, the dew point is the temperature below which water vapour in the air will condense into liquid water.
Of course the dew point gets its name from the dew that can form after a chilly night. Which brings us to another property of those water droplets that form around the rim of your coffee mug. Although it is not easy to see on the mug, each droplet is acting as a lens, focussing the light that falls onto it. As the surface of the mug is fairly flat, rather than form spherical droplets, the drops that form on the side of the mug are squashed hemispheres. This is not the case when dew forms on grass. Tiny hairs on the surface of the grass protrude from the leaf meaning that the water droplets form into spheres (which is, incidentally very similar to the reason that a duck is so waterproof). When the sun comes up, each sphere of water focusses the sunlight onto the grass behind it which reflects it back, right in the direction it came from.
This means that if you stand with your back to the sun and look at your shadow on dew covered grass, you will very probably see a region of bright light surrounding your head, your heiligenschein. German for “Holy light”, heiligenschein is the effect of all of those spherical dew lenses reflecting the sunlight back towards you. You can only see the effect around your ‘anti-solar’ point (a position defined as being 180º from the Sun from the viewpoint of the observer, see here for what this means visually). This means that while you will see heilgenschein around your head, or around the shadow of the camera that you use to photograph it, you will never see the halo around someone else’s head even while they themselves can clearly see it.
I’m sure there’s some sort of metaphor there, perhaps one to contemplate next time you’re drinking a hot, steaming coffee.