Why Does a Straw Look Bent in Water?

The straw has not actually bent. We see its submerged part because light reflected from it travels through the water, crosses the surface and then enters the air. Light moves at different speeds in water and air, so a ray crossing the boundary at an angle changes direction. This is refraction. A ray travelling from water into air generally bends away from the normal before reaching the eye.

The visual system does not directly register that change of direction. It traces the arriving ray backwards as though it had continued along a straight line. Each point on the underwater section is therefore assigned to an apparent position that is shallower and slightly displaced sideways. The part above the water is seen mainly through air and is not shifted in the same way. The two apparent sections no longer meet at the surface, so the straw looks broken or bent.

This is not an arbitrary illusion but a predictable apparent position produced by the ray path. When you look nearly straight down, the rays cross close to the normal and bend less; from the side, the displacement is usually clearer. A transparent glass wall may add a smaller extra refraction, so the vessel's shape can also affect the view. Placing a straight straw in clear water and observing it from different angles is a safe way to see geometric optics change where an object appears to be.

https://openstax.org/books/physics/pages/16-2-refraction
https://openbooks.lib.msu.edu/collegephysics2/chapter/the-law-of-refraction-2/