BS: Why is foam always white?

Subject: BS: Why is foam always white?
From: Sooz
Date: 23 Jul 04 - 09:22 AM

Whatever colour the agent that causes the bubbles, be it bubble bath, shampoo, washing up liquid etc the foam is always white. It even works for beer. Why is this?
Been puzzling Mike and I for a long time.

Subject: RE: BS: Why is foam always white?
From: Rapparee
Date: 23 Jul 04 - 09:29 AM

What other color would you like it to be? You can change it, you know.

Guinness foam is sort of beige.

Subject: RE: BS: Why is foam always white?
From: Pied Piper
Date: 23 Jul 04 - 09:30 AM

Foam is white (or the colour of what light is falling on it) because the tiny bubbles are acting as mirrors and reflecting light not transmitting it through the coloured liquid.
You can see your face reflected in used engine oil or black paint.
PP

Subject: RE: BS: Why is foam always white?
From: GUEST
Date: 23 Jul 04 - 09:32 AM

except that it's not (foam) - it is usually pastel(pale beige, pale blue, pale pink etc) - but I have even seen it dark grey or black.

Subject: RE: BS: Why is foam always white?
From: DMcG
Date: 23 Jul 04 - 09:35 AM

The pat answer is that white light is mixture of all the colours and the size and number of the bubbles gives a large number of fine reflecting surfaces which gives an averaging effect making things look white. However, thinking about it some more, the question leads on to deeper questions. If you took a spectral analysis of light from a foam, how complete would the spectrum be? Is the analysis for heads of Guiness consistant across all pints and is it the same or different to washing-up liquid foam? Dunno ...

Subject: RE: BS: Why is foam always white?
From: Rapparee
Date: 23 Jul 04 - 09:36 AM

Since I'll drink Guinness foam and not that of dishwashing liquid, it'd better be different!

Subject: RE: BS: Why is foam always white?
From: mooman
Date: 23 Jul 04 - 09:47 AM

Insulating foam for buildings is a yellowy-orange!

moo

Subject: RE: BS: Why is foam always white?
From: Once Famous
Date: 23 Jul 04 - 10:00 AM

Why is sperm always white?

Subject: RE: BS: Why is foam always white?
From: CarolC
Date: 23 Jul 04 - 10:37 AM

Foam that collects in eddies on much of the Potomac River is a really nasty yellow.

Subject: RE: BS: Why is foam always white?
From: muppett
Date: 23 Jul 04 - 10:51 AM

A slight digression from the subject,with the colour of my skin, if I joined the flag crackers of craven, could I whiten my skin?

Subject: RE: BS: Why is foam always white?
From: Bee-dubya-ell
Date: 23 Jul 04 - 11:35 AM

My desk foam is white, but the cordless foam is black. All foams used to be black umtil someone imvented the Princess foam which came in colors. Now foams come in lots of colors but they omly last a couple of years amd it's cheaper to buy a mew ome tham to fix a brokem ome.

Subject: RE: BS: Why is foam always white?
From: Blackcatter
Date: 23 Jul 04 - 11:39 AM

Like Guest said about foam is pastel, typically.

So this thread is pointless since it is based in an incorrect observation.

Subject: RE: BS: Why is foam always white?
From: Dave Bryant
Date: 23 Jul 04 - 11:48 AM

Have you never seen coloured Crazy-Foam ?

Subject: RE: BS: Why is foam always white?
From: Sooz
Date: 23 Jul 04 - 12:40 PM

In the interests of scientific research I have just spent 45 minutes in the bath with my favourite lavender bubble bath. The foam was definitely white although individual bubbles did reflect charming little rainbows as the light caught them. I feel much refreshed.

Subject: RE: BS: Why is foam always white?
From: Micca
Date: 23 Jul 04 - 01:43 PM

Martin, Sperm is white and urine is yellow so that you know whether you are going or coming!!

Subject: RE: BS: Why is foam always white?
From: Joe_F
Date: 23 Jul 04 - 06:38 PM

Foam isn't always white, but for foam to be colored, the liquid on which it is formed has to be extremely strongly colored.

The reason that (say) beer is yellow is that as light passes thru it, certainly particles suspended in the water absorb some of the light in other parts of the spectrum. When you look thru a glass of beer, you are seeing the effect of a substantial pathlength in the beer (several inches), over which a ray of light gets plenty of chances to be absorbed.

When a ray of light goes into foam, it repeatedly encounters boundaries between liquid and air. At each of those boundaries, it has a chance (a few percent) to be reflected; but it gets hundreds of such chances. Thus, the ray is almost certain to be reflected out of the foam eventually, and when that happens, it has spent most of its time in gas (inside the bubbles), and very little of its time in beer (in the walls between the bubbles). Thus, the beer gets very little chance to absorb light from the ray before it is reflected out to the eye. It hasn't gone thru much beer, so you get it almost all back, which is what "white" means.

Subject: RE: BS: Why is foam always white?
From: GUEST,Noddy
Date: 23 Jul 04 - 07:00 PM

Sperm isn't white, it's sort of creamy-colored, kind of off-white like ivory. The fact that Martin Gibson doesn't know this indicates he has lived a much more sheltered life than I had thought. No wonder he has so much unfocused hostility. If I was a member of Mudcat, Marty, I would PM you and explain how you can get some experience in this area of life, through a simple means of self-manipulation that is quite easy to master once one understands the basic concept. I figure once you know how to do it you'll probably spend only about 1 tenth the time on Mudcat that you do now.

Subject: RE: BS: Why is foam always white?
From: GUEST,Noddy
Date: 23 Jul 04 - 09:20 PM

If you didn't get it Marty, that's me flirting with you.

Subject: RE: BS: Why is foam always white?
From: Joe_F
Date: 24 Jul 04 - 06:54 PM

For "certainly" read "certain". And for "air" read "gas" (carbon dioxide in the case of beer).