Blow a bubble and the first thing you see is colour: magenta and green bands sliding and swirling over the surface. Yet soap solution has no dye in it. Pour some in a glass and it looks clear. The colour is made by the thickness of the film, and a bubble is a thickness map you can read with your eyes.
A soap film is a sandwich: a thin layer of water held between two sheets of soap molecules. When light hits it, a little bounces off the front surface and a little more goes through and bounces off the back surface. Both reflections come back to your eye together, but the one from the back has travelled further, across the film and back again.
Light is a wave, so the two reflections can line up crest to crest and add, or crest to trough and cancel. There is one extra twist: the reflection off the front, where light goes from air into water, comes back flipped upside down. The one off the back does not. Whether a colour survives depends on how many of its wavelengths fit into that extra trip.
White light is a mix of all colours, and each has a different wavelength, so a given thickness cancels some colours and boosts others. Take away part of the spectrum and what is left looks coloured. A film about 200 nanometres thick knocks out the green and looks purple; a little thinner and it is the blue that goes, leaving orange and gold. Drag the probe and watch which parts of the spectrum go dark.
A fresh film is thick at the bottom and thinner at the top, because gravity is always pulling the water down. As it drains, every thickness band slides downward, so the colours march down the bubble in order. Evaporation, air currents and small differences in surface tension stir the film at the same time, which is why the bands curl into marbled swirls instead of neat stripes. Press Blow on it to see.
Once the film is thicker than about a micrometre, the cancelled colours sit so close together across the spectrum that the leftovers mix back towards white. That is why the bottom of a draining film looks pale and washed out, while the vivid colours live in the thin part above.
Keep watching the top of the film and something odd happens. First it turns a bright silvery white, then patches of it go black, as if there were a hole. There is no hole. The film has become so thin, a few tens of nanometres, that the back reflection barely lags the front one at all. Because the front reflection is flipped, the two now nearly cancel for every colour at once, and very little light comes back.
The black spot is one of the thinnest things you can see with the naked eye.
Isaac Newton noticed it more than 300 years ago. In his Opticks of 1704 he describes watching rings of colour sink down a bubble until a small round black spot grew in the centre at the top. Today these are known as common black films, roughly 30 to 50 nanometres thick. Some soap mixtures thin even further, to a Newton black film of about 5 nanometres: little more than two layers of soap molecules with almost no water between them. A film that thin is close to its limit, and the bubble usually does not last much longer.
Now press Yellow lamp. Under light of a single colour, like the orange-yellow of a sodium street lamp at 589 nanometres, the rainbow turns into bright and dark stripes. Every step from one dark stripe to the next means the film got thicker by half a wavelength inside the water: about 220 nanometres. Count the stripes from the black top downward and you have measured the film without touching it.
The same trick is at work in a sheen of oil on a wet road, and it is used on purpose in the anti-reflective coatings on glasses and camera lenses, where a layer about a quarter of a wavelength thick makes the two reflections cancel. A bubble just does it for free, with a film that is constantly redrawing its own map.
Sources: Newton, Opticks (1704), Book II, Part I, Obs. 17; thickness ranges for silver, common black and Newton black films from soapbubble.dk and the physics literature on soap films. The simulation uses the standard two-surface thin-film formula (refractive index 1.33) with an approximate human colour response.