Optical ripples

This interesting pattern showed up on my bedroom wall this morning — a spot of sunlight shining through the window. I’ve processed this photo to enhance the fairly subtle vertical “ripples” in the brightness of the light.

The spot of light in the photo is about 4 inches (10 cm) wide, but the light that made the spot passed through a 32 inch (80 cm) wide window. This happened because the early morning sun was low in the sky, right on the horizon, and it happened to be at just the right compass bearing to shine almost (but not quite!) edge-on to the window.

So what causes these ripples?

My first thought was that we had some vertical streaking of dirt on our windows (if you saw our windows, you’d think this was quite likely <smile>!). But as far as I could perceive, the dirt on the window was quite randomly placed. My next thought was that there were vertically-oriented thickness variations in the pane of glass — and this I was able to verify simply by looking at reflections in the glass, while viewing at an almost edge-on angle from one side. When I did the same thing from below, there were no visible variations. Bingo! Slight thickness variations in the pane of glass, with a vertical orientation, are causing this phenomenon.

I’ve never noticed these thickness variations before, most likely because they’re very small, and we don’t normally look through windows edge-on. I googled a bit to see if I could find a reference to these ripples, but I didn’t find anything directly relevant. In one description about how modern window glass is made (with “Pilkington’s method"), I did learn that one side of a pane of glass is “fire polished” as it moves down a conveyor — I speculate that the fire-polishing machine is responsible for the ripples, and that they’re linear because the glass is moving in only one direction past the fire-polishing machine…