These clear pearls are made of polymers, which go by a variety of different names: jelly marbles, water balls, water crystals, and clear spheres. Polymers are individual molecules that are linked to form long chains of molecules. Any plastic object is a form of polymer.
The reason they’re invisible under water is because they have the same index of refraction as the water it’s submerged in. Light moving through the water doesn’t react to the boundary of the polymer. Therefore, it just continues on exactly as it does through water. However, when you remove all of the water, they become perfectly visible. This is because rays of light are scattered as they hit the objects in the bowl.
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