The water looks transparent, but when the water hit the stone, it looks as if it turns white? Why?
More complex surface -> more diffuse reflection, refraction. Same with grinding transparent glass to white powder.
What exactly is "spray" in this context?
All those optical effects mentioned above. A bit like the same reason why a pair of corrective glasses works for one person and not for somebody else.
I don't perceive this metaphor.
"the spray" here just like the drop created when the water hits the stone. For example, the bottom of the waterfall is white while the top is transparent.
Ah okay. I think A.T.'s answer is right. The huge number of droplets reflects and scatters the light in all directions, resulting in a white color instead of a transparent "sheet".
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