How can you see raindrops if water is transparent?
For the same reason you can see air bubbles in water. These materials have different indexes of refraction, which means they bend light by different amounts, simplistically speaking. Another effect of two materials with differing refractive indexes in contact is that the boundary between them tends to reflect light. Both of these effects combine to make a transparent object in a transparent medium visible.
Note that diamond, which has a refractive index nearly identical to water is almost completely invisible when immersed in that liquid.
Ok thanks. I guess that makes sense.
what exactly makes something transparent?
Diamond's index of refraction is not even close to that of water.
Glass is a little closer (n_glass=1.5) but still quite different.
My high school physics teacher did a good trick. I think it was glass in glycerin (or something). Very similar indices of refraction. You couldn't see any part of a test tube that he had immersed in a beaker full of the stuff. He teased us that it was "molten glass" (i.e. that he had actually melted the test tube).
Reminds me of the "fire water" demo my chemistry teacher did. He put some sort of light flammable liquid into a beaker of water and lit it, producing a nice flame on the surface. When we were leaving the class a friend of mine decided to do the teacher a favor and put out the fire, he poured water into the beaker until it overflowed. I'm not sure what he was thinking but seeing a river of fire flow out of a beaker and spread across the tabletop was a nice ending touch to the demo.
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