# Soap film

##### Demonstration

Coloured bands of light are produced in thin films because of path differences and interference.

#### Apparatus and materials

Beacker, 400cm3

Beacker, 1000cm3

Copper wire frame

Soap solution, fresh

Glycerine

#### Health & Safety and Technical notes

For a below, bend a frame of copper wire, 16 or 18 SWG, as shown. The vertical circle at the top should have a diameter at least 5 cm, preferably 7 cm or more.

Put soap solution in the 400 cm3 beaker. Soap bubble liquid from toy shops does very well. Or make a mixture of detergent and water (preferably the detergent used for washing woollen fabrics).

A dilution of 1 in 100 is probably best. A dilution of detergent of 1 in 10 gives a rather streaky pattern, but the film is strong. 1 in 1000 gives a film with closely spaced fringes but the film is weak.

Only if you need to, add glycerine to make the film stronger — it will make the colours poorer.

#### Procedure

a Dip the frame in soap solution to make a film. Use this film as a mirror to reflect light from the sky to students. As the film drains, it thins, and interference bands appear.

If you discourage evaporation, the film will last a long time, even when thinned. Place a large beaker, wet inside, over the frame carrying the film as it stands on the bench-top.

b Blow a soap bubble and catch it on a small piece of carpet made of synthetic fibre. Place a big beaker over the resting bubble.

#### Teaching notes

1 Apart from providing an opportunity to show the beauty of soap films, this experiment is bound to provoke discussion as the film thins and eventually bursts.

The coloured bands demonstrate that different colours of light have different wavelengths.

2 Light is reflected from the two surfaces of the soap film, and the two waves then interfere. Their path difference is twice the thickness of the film. The thickness of the film is irregular, so the coloured patterns are irregular too. The coloured bands provide a reminder that different colours of light have different wavelengths. This is particularly important if students have only seen Young’s fringes produced by a single wavelength laser.

3 Just before the film breaks, the thinnest region becomes invisible, because of the phase change on reflection at the less optically dense, back surface.

Students generally expect the light-waves from two surfaces very close together to reinforce, not cancel each other. They will be surprised by the 'black spot'. A slight draught makes the experiment more convincing, and dispels any idea that the black region is a hole in the film. The most convincing test is to poke the black region - a good way of breaking any soap film. You will need to judge whether or not it is appropriate to explain the effect.

4 More advanced students should understand that light reflected from the top surface undergoes a phase change of pi (180 degrees). This means that a bright fringe is formed whenever the path difference between the two waves is twice the thickness of the film plus a half wavelength.

5 Remind students that street lights provide good light sources for viewing interference and diffraction phenomena. These include interference in thin films such as oil films on a wet road, and moisture on a car windscreen. Students can look at the lamps themselves viewed through net curtains or eyelashes.

This experiment was safety-checked in February 2006