h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I'll admit this question never occurred to me until I read it (Jake Stonebender asks it in one of Spider Robinson's Callahan books), but it is an obvious one to ask--

Water is a clear, colourless liquid. So why does your towel darken when you splash some on it?

(And, if the only answer you can come up with is 'Because the towel is wet,' like my husband, that's not the point of the question. Why should being wet darken anything?)

My answer? 'I dunno.'

Funny I should happen along to this particular article and this particular comment - I was wondering if there'd been an article on water written already, and found this excellent one - but that's not what's funny, it's that you've just asked a question I've always wondered about myself.

That is, why do certain things (like clothing, one's hair, paper, etc.) that can absorb water, why do they appear darker when they absorb water? Hair is famous for this, as well as towels, jeans, most any clothing fabric in fact ... I wonder if perhaps it has something to do with the way light is affected by water, perhaps less of it being reflected and more of it being interfered with in some way by the water molecules saturating the fabric. Hmmmm. Maybe we should move this over to "ask H2G2."

A furhter corrolorary is why do certain materials become transparent with the application of water? (i.e.. T-Shirts?)....

This is way too complicated for me, but lets give it go:

(1) Porous objects (fabrics, paper and concrete) become darker when they are wet because the many tiny reflecting surfaces that cover their surface become filled in by the water and cease to reflect specular light back to the observer. This makes the object appear darker.

(2) Fabrics also have a relatively high refractive index (in relation to air or water). When wet their refractive index drops and their selective optical absorption of dyes (colour) predominates.

(3) Fabrics which become transparent when wet tend to have very low amounts of dye. Their opacity or whiteness is due to them scattering/reflecting light indiscriminately with respect to its wavelength. Once wet their refractive index drops and the amount of light scattered is reduced, thus allowing you to see through the fabric.

(4) Transparent when wet fabrics are also fundamental to the sexual development of boys and the continuing well being of men, without which, they would not even begin to understand the difference between eros and procreation.

I'm impressed...

Have a mufflewhump...

Indeed.....

what the heck is a mufflewump?!
just asking?
LAUREN
ARE THEY LIKE GOLDFISH?

*points to name*

If you want them to be

I'd forgotten about Mufflewhumps... How churlish of me...