Bubble Projector

Shine white light through a thin sheet of bubble film and watch the colours that are projected onto a wall. The colours you see will tell you just how thick the bubble film is in places. Use the fan to make the colours swirl around.

Explanation

Bubbles don’t just contain a bit of air, they contain a lot of science!

Soap bubbles often seem magical because of their fragility and the rainbow-like colours reflected off their surfaces. These colours are known as iridescent colours because they appear to shimmer and change when you look at them from different angles.

The colours are made by interference between waves of light. Each bubble’s ‘wall’ is made of a very thin film of soap and water. White light reflects off the two surfaces of the bubble film—the outside surface and the inside surface.

The light that travels through to the inside surface travels a little bit further than the light that reflects off the outer surface. The two reflected light waves recombine with each other when they meet at the bubble’s outer surface.

The means that the thickness of the bubble film determines whether the two light waves are in-phase with each other (crest-to-crest), antiphase (crest-to-trough) or just partially out of phase.

If the waves are in-phase with each other they will constructively interfere. You can think of this as the crests adding together to make bigger crests. This makes the light brighter. If they are antiphase with each other, they will destructively interfere and the light is cancelled out.

White light is made of all the colours of the rainbow and each colour has its own wavelength range. When the bubble film is the right thickness to destructively interfere with a particular colour (for example, red), that colour is cancelled, and the complementary colour appears in its place (in this case, cyan). So the bubble’s colours are actually giving information about how thick the bubble is at that point!

Extras for Experts

“Given a certain thickness of the bubble wall, a certain wavelength will be cancelled and its complementary colour will be seen. Long wavelengths (red) need a thicker bubble wall to get out of step than short wavelengths (violet). When red is cancelled, it leaves a blue-green reflection. As the bubble thins, yellow is cancelled out, leaving blue; then green is cancelled, leaving magenta; and finally blue is cancelled, leaving yellow. Eventually the bubble becomes so thin that cancellation occurs for all wavelengths and the bubble appears black against a black background.”

– The Exploratorium

You may see bands of colour progressing down the film because the film gets thicker towards the bottom as water drains out. (Note that in this exhibit, the projected image is upside-down, so the image of the film is thinner at the bottom and thicker at the top).

As the film gets thicker, it goes through various thicknesses where particular wavelengths are cancelled or added.

The colours may also spin and swirl as you blow on the bubble film and interfere with its thickness.

The bubble film is also iridescent because as you move around, you are influencing the thickness of the film as seen from your point of view. This changes which colours are cancelled by the film.

You can see iridescent colours on oily patches on the road.

You can also see iridescent colours on peacock feathers or some butterfly wings. Here the iridescence is caused by tiny structures on the feathers or the wings. These structures cause light to interfere and produce the shimmering colours.

Things to Try

Try to work out what colour is being cancelled for each colour you see. Look at the Coloured Shadows exhibit for some clues!
Make a permanent version of an iridescent thin film here:
http://scitoys.com/scitoys/scitoys/light/permanent_rainbows/permanent_rainbows.html

Questions to Ask

Can you think how the colours in bubbles are created? Where are some places you have seen similar colours?

Try an Activity

Permanent bubble colours
Here's a way to make a sheet of lasting, bubble-like colours.

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Coloured Shadows
Make a multitude of coloured shadows by blocking or letting through just three coloured lights.

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