Eye Bubble

What you need

  • Bubble solution - washing up liquid with some water in a ratio of about 1:1 will make reasonably strong bubbles.
  • A black bin bag.
  • Masking tape.
  • A straw.
  • A large white lampshade, conical if possible, and preferably without metal attachments. It will get damp so don't use a good one.
  • A table somewhere bright.

What you do

Cut a large square out of the black bin bag, larger than the base of the lampshade and tape it to the table (don't use a good table for this experiment).

Pour a little bubble mixture in the middle of the square and use your straw to blow a bubble. If your bubble keeps bursting try adjusting the amount of water in your bubble mixture and make sure your black bag is smooth and flat.

Place your lampshade over the bubble and look down. Watch as the pupil of your bubble eye slowly dilates...

What's going on?

Light is carried in waves, and just like water waves they can combine to make much bigger waves when the crests coincide, or will cancel each other out when a wave crest coincides (or is 'in step with') with a wave trough. This is called, respectively, constructive and destructive interference.

A bubble is a thin curved sheet of water and detergent and has two surfaces, its outside and its inside. Light reflects off the bubble in the same way that light reflects off a window: any light hitting the bubble will be split; some will reflect off the outside surface, some will reflect off the inside surface, and the rest will pass straight through.

Light reflects differently when passing from air to water than from water to air. From air to water - as in a reflection from the outer surface of the bubble - the light wave is completely reversed: a trough becomes a crest in the reflection, and vice versa. From water to air - as in a reflection from the inner surface of the bubble - the crests remain crests and the troughs remain troughs. If there was nothing else going on, these two reflected waves - one from the inner surface and one from the outer surface - would completely cancel each other out.

However, there is more to it than that. Any light reflecting off the inner surface of a bubble will have travelled slightly further than light reflecting off the outer surface of the bubble. This slight difference in path length puts the two reflected waves out of step. White light is made up of a spectrum of different colours all with different wavelengths, and if the waves are out of step by one of these wavelengths then that particular wave and colour will be cancelled out (destructive interference). This means that the light will not appear white, but will be coloured by those waves which have escaped the destructive interference.

All this means that the thickness of the bubble, which affects the path length of the two reflected waves, determines the colour we see. For thicker, newer bubbles the difference in path length is long, and long wavelengths are cancelled out - leaving the bubble looking blue-green. As the bubble gets thinner the green and yellow part of the spectrum is cancelled out and the bubble starts to look blue-red. Finally when the bubble is very thin the blue short wavelengths are cancelled and the bubble looks red-yellow. Beyond this the bubble becomes so thin it may burst and the path difference is tiny, all the light is completely cancelled - the bubble appears to not reflect any light at all.

The lampshade provides an even white light surrounding the bubble and the interference of this white light explains why we see the coloured 'iris' of our eye bubble. The pupil is formed and grows as the bubble gets older. Water slowly flows from the top of the bubble towards the table and the bubble thins at the top. This thinning reduces the amount of reflection so the 'pupil' looks black.