If water is clear why does it make materials appear dark?

  1. So water is completely transparent, it doesn't look darker in a glass so why when you tip it on cloth for example the cloth looks darker?
  2. jcsd
  3. mathman

    mathman 6,754
    Science Advisor
    Gold Member

    Water is not completely transparent.
  4. The way in which substances reflect light changes when they are wet. I do a lot of photography of wood samples, and it can be QUITE striking how different the surface looks between wet and dry.

    It is similar to when the surface is polished as opposed to rough. Rough, it has an effective surface area for light reflection that is much greater than what it has when polished (or wet). The greater surface area reflects more light and thus appears lighter.
  5. Water fills in the rough spots in a surface to effectively make it behave as if it was smoother. When light strikes a smooth surface it reflects in a specific direction. I'm sure you've seen lights reflected in water right? If you don't happen to be in the direction that the light is reflected then the surface looks dark. A rough, dry surface will scatter light in random directions so you are much more likely to be in the path of some of the light that is reflected, therefore the surface looks lighter.
  6. sophiecentaur

    sophiecentaur 14,730
    Science Advisor
    Gold Member

    The terminology they use with reflections is diffuse and specular (mirror-like). When you take a fine, irregular surface, the reflection is totally diffuse and the light from any small spot on the surface is reflected from all directions (from the lamps or the sun and the rest of the scene). So it will look very pale or white (modified by the pigment). Adding water, oil or a light varnish will fill in all the tiny holes and produce partly specular reflection (i.e a mixture of specular and diffuse) from the small liquid surfaces in the holes / gaps. This means that the image you will see is a poor quality reflection of some part of the surrounding scene this will be darker, in general (by excluding reflections from the lamps, to some extent), unless the viewing angle gives you a rough image of the lamp / sun - in which case it will look bright. Very thick varnish can produce very crisp images by reflection at the actual surface plus a semi specular reflection from the rough substrate beneath.
    When you buy photographic prints, you can notice that the best quality prints are on high gloss paper but these suffer from annoying reflections of room lights if the angle is just right (/wrong) and matte paper may be better for general display. The 'depth' effect you get with a polished surface will be because the image you see (even the poor quality ones for moderate polishing) appear behind the surface (as with a mirror).
  7. D H

    Staff: Mentor

    Wetting does not change diffuse reflection into specular reflection. It changes diffuse reflection into absorption. The primary action of wetting is to decrease the albedo.

    Consider a sandy beach. Silicon dioxide crystals, the main ingredient in sand, is transparent. The reason sand looks white is because incoming light bounces around a lot when it hits the surface. Each crystal is close to transparent. Light enters a crystal and changes direction because and silicon dioxide has an index of fraction of about 1.5. The light changes direction again when it exits the crystal. The process repeats when the light enters and exits another crystal. Those diffractions are pretty much random because the sand is an aggregate of crystals with little correlation in orientation from one crystal to the next.

    Some of the light passes to deep into the sand, causing it to be absorbed. However, a good percentage of the incoming light eventually escapes the surface it entered. Because of those multiple random diffractions, the light exits at a more or less random angle. That's diffuse reflection.

    When you add a wetting agent to the sand, the wetting agent fills the pores in the sand. This attenuates the diffraction angle as light enters and exits the crystals. More of the light penetrates deeper into the sand and is eventually absorbed. Those crystals aren't perfectly transparent. They do absorb some fraction of the incoming light. Allowing the light to penetrate deeper into the sand significant increases the chance the light will be absorbed.
  8. Baluncore

    Baluncore 3,305
    Science Advisor

    We appear to live in a universe of incompatible interfaces. We can only sense surfaces where the electromagnetic or acoustic impedance is mismatched.

    The wet surface of a substance will appear darker than the dry surface when the speed of light in water lies between the speed of light in air and the speed of light in the substance. The wet surface is effectively an impedance matching layer that aids in the absorption of energy. More energy is reflected by a single impedance change step than by two half steps.
  9. sophiecentaur

    sophiecentaur 14,730
    Science Advisor
    Gold Member

    There has to be some 'geometry' involved in this effect, though. Otherwise the roughness of varnish, glass or a wet surface surface would not be as relevant as it is, clearly (or frostily?).
    I guess that, like "how Aeroplanes Fly" question, the problem is a multi-faceted one and we should avoid sticking to just one explanation or we'll have a thread that's ten miles long and doesn't get anywhere. Impedance matching and absorption must be relevant. It would be interesting to know whether the most effective polishes have intermediate values of refractive index and whether the thickness affects the process.
  10. D H

    Staff: Mentor

    Do index of refraction and thickness affect the process? Sure. Wet onion skin paper with water and it doesn't get darker so much as it gets translucent. Use oil instead of water and the affect is much more pronounced. Index of refraction is the primarily culprit. Water has an index of refraction of 1.33. For oil its between 1.4 to 1.6, depending on the oil. For paper, it's about 1.5. Use an oil with the same index of refraction as the paper and voila! the thin paper becomes becomes nearly transparent.

    Geometry also plays a role. The substance has to be wettable. It has to be porous, or at least have a rough surface. Dry albedo also plays a role. Pouring water on an extremely low albedo black sand beach or an extremely high albedo white sand beach also isn't going to have much of an effect. The black sand beach already has a very low albedo. Wetting it can't reduce albedo by much because the albedo is already very low. With a very, very white sandy beach, the effect is also minimal. The re's not much absorption. Wetting the sand just makes the light penetrate a bit deeper before reemerging. On the other hand, pour water on a typical sandy beach with a dry albedo between 0.2 and 0.4 and the effect is quite pronounced.
  11. This is not the explanation !!!
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