The Demon in the mirror

The Misty demon!

My bathroom has a mirror facing the window so after a shower, when the mirror is misted over, I see something like this picture. My head is a grey silhouette and my face obscured by the mist. The weird thing is the eyes, which are replaced by rings. Each eye sees just the rings round its own image so if you shut one eye, the demon seems to wink back! In fact, if you take a photo, the camera sees rings where its lens should be, so I had to fake the picture. (I also increased the contrast to make the rings more obvious.)

Being a retired science teacher, I wanted to know what caused the rings and being retired, I had time to waste. Bath-times took on a whole new meaning, as I used torches and tape measure, camera and microscope to investigate. In the end I cracked it and wrote it up for Physics Education. The published paper is here, reproduced with permission from the Institute of Physics. In outline, the effect depends on each droplet acting as a lens to produce a tiny bright source of light. Light from one of these point-like sources is reflected at the mirror and then diffracted by the same droplet, producing a diffraction pattern which is symmetrical about the normal to the mirror. What you see is the sum of these over many drops. (Other droplets also cause diffraction, but in random directions.) The angular diameter of the rings depends on the thickness of the glass; drop size affects brightness and coherence.