If you pour water from a container, the flowing water stream seems to cast a shadow. I am not sure you can call it a shadow, but it definitely is not letting all light through it. How is this possible and what uses can it have?
Optics – Why Does Water Cast a Shadow Even Though Transparent?
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Best Answer
A large amount of water (i.e. if the path of light through it is long) will simply start absorbing light, as it's not completely transparent. For smaller amounts, as when pouring it from one container to another, this is mostly negligible. However, there is also surface reflection. A small amount of the incident light will be reflected off by the surface.
The much larger contribution, however, will come from refractive effects. If you look closely, there are not only areas which are darker than the uniformly lit surroundings, but some will also be brighter. The stream of water forms shapes that act similar to a lens and will divert light off its original path. The patches where the incident light would have gone without the disturbance will then be darker, the places where it is directed to instead will be brighter.