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Message # 13228.6.1

Subject: Re: Could be the refractive index of water and the reflectivity of white fibres

Date: Thu 18/11/04 05:07:08 GMT

Name: AnthonyX

Email: anthonyx@jowc.net

Website:

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I'd agree... more or less.

"White" fibres aren't necessarily opaque. The substance may be transparent, but due to the refractive index relative to air, there is refraction and both internal and external reflection. When spun into fibres and woven into cloth, you end up with all sorts of surfaces which end up scattering the light (not to mention all the refraction), and since no particular color is being absorbed, the overall effect is more-or-less opaque white. Assuming the material in question (such as nylon) has a refractive index close to that of water and accepts water, then when soaked, all those original surfaces no longer play a role in refracting or reflecting light; in effect, fabric and water become a single optical medium. Due to surface tension, the boundary between the water and air will tend to "smooth over" all the surface irregularities. The result is a single, more or less uniform and transparent layer of fabric and water. Fabrics which only go translucent rather than transparent may do so because...

o the refractive index of the material is not very close to that of water

o the material itself is partially or completely opaque

o the material doesn't "wet" in water - some substances (like teflon) have a tendency to repel water, which would prevent water from filling all the gaps and forming a "composite" medium of only fabric and water. Any air pockets, especially tiny ones, create surfaces for reflection and refraction, scattering light and creating a degree of opacity.

How's that?

In reply to Message (13228.6)

Could be the refractive index of water and the reflectivity of white fibres

By John - Thu 18/11/04 02:36:03 GMT

Website:

I think it’s to do with the refractive index of water and the reflectivity of white fibres

Sadly, physics textbooks do not provide an explanation of why wet white fabric is translucent, and I doubt anyone has published a scientific paper called “The optical properties of the wet T-Shirt compared to those of its dry counterpart”.

Having said that, someone has done some research into this subject because someone once said in a sunburn protection news story on TV, that “wet clothes provide no UV protection”; not strictly accurate, because dyed and heavier clothing does provide some sort of UV protection, which I read on a total coverage swimwear website.

Going back to the mechanism of what is happening; this is mainly guesswork, but what I think is happening is that when you see “transparent” white wet clothing, you are not looking straight through it, but instead the image you see is from light that has passed into the water on the clothes, becoming trapped to a degree by the water's refractive index, bounced off fibre surfaces onto other nearby fibre surfaces until it has escapes out of the other side when it is travelling nearly perpendicular to the water surface. It then reflects off the wet body or undergarment surface below before making the return journey by the same manner. You are therefore not looking at light that has travelled in a straight path, but light that has bounced about a lot. On dry clothing, the light hits dry fibres that do not reflect, but just scatter a portion back giving the white surface you see. Dark clothing does not go as translucent because the dyes in the fibres absorb much of the light.

In reply to Message (13228)

why does white clothing become transparent when it is wet ?

By nemo - Thu 18/11/04 01:06:58 GMT

Website:

why does white clothing become transparent when it is wet ?

does someone know ?

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