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Gallery 2

# Bubble Projector

Iridescence is created through interference between different wavelengths of light.

## How it works

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.

## Things to try or ask around the exhibit

• Look at the image projected on the wall. Do the colours on the wall match the colours on the bubble film?
• Does the bubble film look thicker at the top or at the bottom of the ring?
• Look at the bubble film from different angles and check whether the thickness of the film and the angle of the light changes the colours you can see.
• Try to work out what colour is being cancelled for each colour you see. Look at the Coloured Shadows exhibit for some clues!

## Background

Soap bubbles reflect rainbow-like iridescent colours off their surfaces. Iridescent colours appear to shimmer and change when you look at them from different angles because the colours are created by interference between different wavelengths of light.

In the exhibit, you may see bands of colour progressing down the film because the film gets thicker towards the bottom of the ring as water drains out. (Note that in this exhibit, the projected image on the nearby wall is upside-down, so the image of the film is thinner at the bottom and thicker at the top). The bubble film appears iridescent because as you move around, you are peering through different thicknesses of the film as seen from your point of view. This changes which colours are cancelled by the film.

A normal bubble is made from very thin films of soap and water. When white light reflects off the outside surface and inside surface of a bubble wall, 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!

“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

## Finding the science in your world

Iridescent colours can be seen in oily patches sitting on roads, or within peacock feathers and butterfly wings which contain tiny structures that cause light to interfere and produce the shimmering colours.