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A science observation : A dry surface " say Plaster" can appear quite light when dry. ( say light pink or white.) On the addition of wetness , for example water, the hard surface of plaster will turn instantly quite dark.

 

 

+ Question. What does water do at the surface that turns the phenomenon from reflection to absorption ?

 

 

 

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It reduces scattering

http://en.wikipedia.org/wiki/Scattering

We had a discussion on this before so you might want to search for it. One effect (to expand on John Cuthber's response) is that you change diffuse reflection to specular reflection and reduce the amount of light scattered in your direction. Wet surfaces can be quite light and bright if you look at the proper angle and satisfy Snell's law.

We had a discussion on this before so you might want to search for it. One effect (to expand on John Cuthber's response) is that you change diffuse reflection to specular reflection and reduce the amount of light scattered in your direction. Wet surfaces can be quite light and bright if you look at the proper angle and satisfy Snell's law.

 

 

 

Is there or where is there a search engine within Science forum ?

Is there or where is there a search engine within Science forum ?

 

Upper right of the page, just below your username. Clicking on the "gear" gives advanced search (though if you use it, it annoyingly defaults to searching help items, so be sure to click on "forums")

Upper right of the page, just below your username. Clicking on the "gear" gives advanced search (though if you use it, it annoyingly defaults to searching help items, so be sure to click on "forums")

 

I have searched . But so far in vain.

http://www.scienceforums.net/topic/41987-wet-spots/

 

Though my faulty memory was recalling a longer discussion

http://www.sciencefo...1987-wet-spots/

 

Though my faulty memory was recalling a longer discussion

 

(1) Guess this must be the relevant point from the Wikipedia link :-

 

"Surfaces described as white owe their appearance to multiple scattering of light by internal or surface inhomogeneities in the object, for example by the boundaries of transparent microscopic crystals that make up a stone or by the microscopic fibers in a sheet of paper. More generally, the gloss (or lustre or sheen) of the surface is determined by scattering. Highly scattering surfaces are described as being dull or having a matte finish, while the absence of surface scattering leads to a glossy appearance, as with polished metal or stone." Wikiipedia Scattering

 

I can get my head round this to some extent, although I am not quite sure what is happening on a photon by photon level.

 

Do the photons get absorbed into a transmission mode and thus the surface appears dark.?

 

(2) The other link speaks about " diffused " light caused by water having roughly the same refractive index as fibre (1.3 , one the other 1.33 ). Am I to understand that "diffusion" is a transmission mode ? Thus the light does not get scattered when wet but is transmitted through the substance, but does get scattered when dry ( thus sort of reflected external light), to some extent?

I am a little confused with " diffused".

Edited October 21, 201114 yr by Mike Smith Cosmos

(1) Guess this must be the relevant point from the Wikipedia link :-

 

"Surfaces described as white owe their appearance to multiple scattering of light by internal or surface inhomogeneities in the object, for example by the boundaries of transparent microscopic crystals that make up a stone or by the microscopic fibers in a sheet of paper. More generally, the gloss (or lustre or sheen) of the surface is determined by scattering. Highly scattering surfaces are described as being dull or having a matte finish, while the absence of surface scattering leads to a glossy appearance, as with polished metal or stone." Wikiipedia Scattering

 

I can get my head round this to some extent, although I am not quite sure what is happening on a photon by photon level.

 

Do the photons get absorbed into a transmission mode and thus the surface appears dark.?

 

(2) The other link speaks about " diffused " light caused by water having roughly the same refractive index as fibre (1.3 , one the other 1.33 ). Am I to understand that "diffusion" is a transmission mode ? Thus the light does not get scattered when wet but is transmitted through the substance, but does get scattered when dry ( thus sort of reflected external light), to some extent?

I am a little confused with " diffused".

(sorry; I had a longer response written out but my browser crashed twice while typing it out and looking something up, and I don't want to do it a third time)

 

Smooth surface = forward scattering rough surface = scattering in all directions. If you don't see the light, it will appear dark.

 

If the water is of a smaller index than the material, it will reflect less light and transmit more to the material, and any light scattered from the material has another chance to scatter off of the water/air interface before getting back into the air.