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Re: [Phys-l] optics




The water makes the rock reflect like a mirror as water is somewhat
reflective itself.

~ 2% at water and air interfaces. Limestone is > 90%

Why is it that when you
wet a rock, it becomes darker and more reflective?

May become less reflective -- It becomes specular instead of matte.

If one wishes to know how the rock will appear polished, wet it. The smooth surface of the water "polishes" the rock. The same effect makes roads invisible at night in rainy weather.

bc suggests wetting different minerals, e.g. limestone compared to a dark mineral to decide about the second point.


On 2009, Apr 27, , at 07:29, Anthony Lapinski wrote:

Here's a seemingly simple question that I've wondered about even since I
saw it done in a geology class over 20 years ago. Why is it that when you
wet a rock, it becomes darker and more reflective?

I'm thinking it has to do with both reflection and refraction.

The water makes the rock reflect like a mirror as water is somewhat
reflective itself. And the water-rock interface has a smaller refractive
index difference (compared to air-rock) and is absorbed more (reflected
less) by the rock. I'm not exactly sure.

Does this all make sense?

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