In optics the refractive index or index of refraction n of an optical medium is a dimensionless number that describes how light, or any other radiation, propagates through that medium. It is defined as
where c is the speed of light in vacuum and v is the phase velocity of light in the medium.
For example, the refractive index of water is 1.33, meaning that light travels 1.33 times faster in a vacuum than it does in water.
Physics of invisibility.
If you want to make something invisible,you have to ensure that the index of refraction of the object and the medium where you are hiding it remain the same and also that the object is transparent and colorless.
Those are not mystical drops of water from heaven in the animation, but mere polymer balls. The reason why they turn invisible once dropped into water is because their index of refraction and that of water are the same and as you can see they are also colorless and transparent. Thus creating the illusion of invisibility.
What would happen if they were colored is a topic for another post.
Fun Fact.
If you would like to take a bite at something like this, then japan has the perfect dish for your palette.
EY FATTY BOOM BOOM 