Why do surfaces get darker when they are wet?

[itchyfrog]

A couple of pictures to get started:

I'm not a physicist, but a professional artist, so an overly technical explanation may not have any meaning (especially if it's maths!). However I am very interested in how light interacts with matter and I am puzzled as to what is going on here.

Firstly, the dry stones have mostly diffuse reflection and a small amount of specular reflection which is blurred by the roughness of the stones (you can see a faint sky reflection on them).

The wet stones obviously have a much greater specular component and you can see things being reflected quite clearly. However the diffuse component appears to be much darker - this is especially clear in the second photo where the small remaining wet patches are much darker than the surrounding areas.

I believed that the reason the stones get darker is because the layer of water is reflecting a greater proportion of the light directly (specular reflection) so there would simply be less light for the diffuse material to reflect, hence it appears darker.

However this may be too simplistic, for instance maybe the layer of water is changing the refractive index of the surface thereby somehow contributing to this effect.

Any answers or help with this problem would be greatly appreciated. I did try a search but came up with nothing.

 

[itchyfrog]

Just to add to the discussion:

another artist has suggested that the difference is due solely to the change in the specular characteristics of the stones. The dry stones appear brighter because you are looking at the combined diffuse reflection and the scattered specular reflection, whereas in the wet stones the specular reflection is concentrated by the smoothness of the water but the underlying diffuse reflection is unchanged.

This sounds good in theory, but I'm not sure if it can really account for the big difference in brightness: the dry stones would have to have a pretty high specular component to account for all that extra light causing a scattered sheen on the surface.

Another problem I have with this idea is that it implies that the stones would also get darker when they are polished, since the specular reflection would become less scattered.

Does this sound plausible?

 

[Andy Resnick]

http://www.victoria.ac.nz/scps/staff/johnlekner/publications/darkerwhenwet.pdf

There are two effects in this model- an increase in total internal reflection, and the refractive index change cause by the presence of a thin film.

 

[fawk3s]

water, like any other material, swollows energy. more energy swollowed, less energy (light) reflected.

oh yes.. I am stupid. :D

 

[itchyfrog]

http://www.victoria.ac.nz/scps/staff/johnlekner/publications/darkerwhenwet.pdf

There are two effects in this model- an increase in total internal reflection, and the refractive index change cause by the presence of a thin film.

thanks - that was very informative :D

 

[ehild]

The water cowering the rough surface of the stone makes a smooth layer on it, and the light reflected from the front surface of this water layer is dominated by specular reflection. Watching the surface at the proper angle at sunshine ( equal to the angle of incidence of the light) the surface appears very bright, otherwise it looks dark.
Still there is diffuse reflection and scattering at the stone surface, but the irregularities of the stone are surrounded by water now. Reflection, both specular and diffuse, and scattering too, depend on the refractive indexes of the reflecting surface and of its surrounding, and the higher the difference between them the higher is the reflection. Your stones are dark by themselves, I mean that they would appear dark grey, almost black, when polished. I think the material is some kind of silica, and the refractive index is about 1.45-1,6. The refractive index of water is 1.33, that of air is 1. In case of dry stones, the difference between the refractive indexes is high, and the difference is much lower when the pores of the stones are filled with water.



ehild