system
Given the following post, respond as if you're commenting on Reddit:
human
ELIF: Water is transparent but a stream of water has shadow.

I was waiting for the bus and I realized that the fountain upwards stream had shadow, but since water is transparent how is this possible?
gpt
Mostly, this depends on the shape of the water.

When light moves from air to water or back, it bends (refracts) based on the angle it hits the water or air. Because a stream of water is round, the light bounces away from the stream when passing through it, leaving a shadow where the light didn't end up.

This is why the bottom of a pool is not in shadow: the light hits the top of the pool mostly straight, and so continues to the bottom. If it is refracted, it is all bends the same direction, so you still get an even amount of light at the bottom.

Similarly, if you have ever seen a waterbug, or other lightweight object floating on the surface of a pool, you will notice they create round shadows where they touch the water. This is because they make "dents" in the water which bend the light away and leave a dark spot.

The opposite effect happens with light from air to glass: a magnifying glass concentrates the light by bending it inward.

TL;DR: light bends when it hits the water. If the shape of the water is round or curved, all the light bends away from the water, leaving a shadow.