For anyone who's curious, its because the glass ball has the same refraction index as water meaning they refract light in the same way. Light changes when it moves from one refraction index to another so if they're the same, then the object becomes invisible.
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megasean3000
Nov 17, 2014 at 06:30AM EST
PoolCleaningRobot
Nov 17, 2014 at 09:20PM EST