1Two things to notice first
A bubble wall has two sides — and light is a wave
You only need two ideas. Watch each one:
Light bounces off both sides
A bubble's wall is a tiny-thin sheet of water with a front and a back. Some light reflects from each side, so two copies return. The front reflection also turns a wave's peaks into dips; the back reflection does not.
Two waves can add or cancel
Light travels as a wave. When two copies of the same wave line up crest-on-crest they add into something bright. When they fall out of step, they cancel and you see nothing.
2So what can the two bounces do?
They line up, or they cancel
The front bounce and the back bounce are the same light — but the back one travels a little farther through the wall. That tiny extra trip decides everything:
That color shines
For one color, the two bounces come back crest-on-crest. They add up — and that color glows bright.
That color vanishes
For another color, the two bounces come back crest-on-dip. They cancel — and that color disappears.
3Your turn — shine a light on the wall
Watch one beam become two bounces
Here is a slice of the bubble wall, zoomed way in. Drag the slider to swing where you shine the light. Watch one beam hit the wall and split into a front bounce and a back bounce — and watch the back one take a longer trip through the wall.
4Now let the wall get thinner
A bubble's wall keeps draining and thinning 💧
A colorful patch can keep getting thinner. Imagine the top film becomes so thin that the back reflection's extra trip is almost zero. What will the two reflected waves do then?
Guess before you drain it
Keep thinning an already colorful bubble. When the top film is almost gone, will its reflected colors grow brighter or will that patch turn dark against our dark background?