[paul.fr]

Two questions about soap bubbles:

#1. when looking at the colours in a soap bubble (the washing-up liquid type), they are not in the same order as those in a rainbow (red, orange, yellow, green, blue, indego, violet), they do not even contain all the colours. Why is this?

#2. Why is it that those colours move around inside the bubble?



I realise Neil may have already asked this question, in one of his many posts []

[neilep]

I realise Neil may have already asked this question, in one of his many posts []

 [] []

[ROBERT]

http://www.thenakedscientists.com/forum/index.php?topic=3159.0

[paul.fr]

Thanks for the link:

originally posted by soul surfer in fecember 2005

Because of its size and construction a bubble's behaviour is adequately described by classical physics.  As the water drains to the bottom of the bubble the colours demonstrate the wavelength of light by thin film diffraction and quite interestingly dissappear as the top part of the bubble becomes invisible just before it bursts. 

Bubbles are the result of surface tension in a liquid.  The atoms or molecules in a liquid clearly have more attractive forces associated with them than a gas bcause the liquid manages to hold itself together when you put it in a container.  Throughout the body of the liquid as the molecules jostle together these attractive forces are balanced and the liquid can move about but at the surface these forces are not balanced and so the molecules at the surface are held down more strongly.  This excess of force is called surface tension and it is strongly dependant on the structure of the surface. Nomally water bubbles do not last very long because the water drains down and the bubble pops but if you add detergents a big change takes place. 

Now you know that oil and water don't mix the just form blobs in each other and some substances like salt dissolve in water and some substances like wax dissolve in oil.  So if you make a molecule that likes water at one end and oil at the other it will help the oil and water to mix by sitting on the surface of the water and oil (this is a detergent)  Detergernts or emulsifying agents can allow bubbles to form and last much longer than the would do in just water by effectively helping to hold the surface together and solwing down the way the water drains to the bottom of the bubble to allow the top to break and the bubble pop. 

One of the important characteristics of this soap film is that it is always trying to contract (hence the name surface tension) and be as small as possible so, in the absence of any other influences, it will be spherical because a sphere is the shape that contains the largest amount of Volume possible inside the smallest area of surface.

One other important feature if a bubble is that there is always a slight pressure inside it.  This is caused by the curved film of water and detergent trying to contract.  The interesting fact about this is that the more curved the film ie the smaller the bubble the higher this pressure is so small bubbles are much stronger and more stiff than big ones.

[paul.fr]

i see your  [] []
and raise you  [] [8D]

[ROBERT]

Thanks for the link:

originally posted by soul surfer in December 2005

Because of its size and construction a bubble's behaviour is adequately described by classical physics.  As the water drains to the bottom of the bubble the colours demonstrate the wavelength of light by thin film diffraction and quite interestingly dissappear as the top part of the bubble becomes invisible just before it bursts. 

Originally posted by Me:

Hi Soul Surfer,
sorry for being pedantic but, the colours seen on soap film are produced by interference, NOT "diffraction".
You are correct about the "invisible" bit: the black "holes" in these soap films are where the film is thinnest.

[paul.fr]

Thanks for the link:

originally posted by soul surfer in December 2005

Because of its size and construction a bubble's behaviour is adequately described by classical physics.  As the water drains to the bottom of the bubble the colours demonstrate the wavelength of light by thin film diffraction and quite interestingly dissappear as the top part of the bubble becomes invisible just before it bursts. 

Originally posted by Me:

Hi Soul Surfer,
sorry for being pedantic but, the colours seen on soap film are produced by interference, NOT "diffraction".
You are correct about the "invisible" bit: the black "holes" in these soap films are where the film is thinnest.

Sorry for missing your bit Robert. Nice pictures by the way

[Karen W.]

CAN I GET THAT IN a shirt? The colors are wonderful!!

[paul.fr]

CAN I GET THAT IN a shirt? The colors are wonderful!!

try tie-dying, it way give similar results.

[Karen W.]

YEAH BUT THAT IS SOMETHING>> Tye Dyeing is nothing .. You know what I mean?

[lyner]

The reason for the unexpected colours is that, as has been written above, there is interference. This has the effect of removing bits of the visible spectrum, so you see what's left as a colour. It is a 'non-spectral' colour and is often more vivid to the eye than just pure spectral colour. Birds' feathers and butterflies' wings have their colours produced this way. It looks far brighter than any colour produced with a simple pigment as used in paint. The wavelength that is eliminated  depends on the angle of viewing and the thickness of the film,  so , rather than seeing Red, orange yellow etc side by side, you will see (white minus red), (white minus orange), (white minus yellow) etc. as you see light from different angles.
Your poor old brain has difficulty making sense of the colours  and it can only do its best.  This is why, on colour tv displays, you can get a good  (apparent) yellow by mixing green and red light when, in fact , there is no actual light there, with the wavelength associated with yellow.

Actually, to be super pedantic, 'Diffraction' is a general term which describes any wave-summing phenomenon. "Interference" is really just a sub-set of diffraction, involving discrete rather than distributed sources .   

[ROBERT]

CAN I GET THAT IN a shirt? The colors are wonderful!!

try tie-dying, it way give similar results.

Be the coolest teacher in your school with this tie-dye lab coat



http://www.stevespanglerscience.com/product/1712

Will also come in handy if your school is putting on a performance of "Joseph" []
http://www.reallyuseful.com/rug/shows/joseph/

[paul.fr]

Wow, it looks realt trendy...i'm sure this could start a new teenage fashion trend!