Why Does a Chopstick Look Bent in Water?

The chopstick does not actually bend at the waterline; the light changes direction. Light reflected from the submerged part speeds up as it enters the air. Unless it crosses the surface perpendicularly, it bends away from the normal to the interface. The eye usually assumes that arriving light has travelled in a straight line, so it extends the refracted rays backwards and places the underwater part at a shallower, slightly displaced position.

Light from the portion in air does not cross the water–air boundary, so that part is not shifted in the same way. The two apparent positions fail to line up near the surface, creating the impression of a break. The same effect makes a pool bottom viewed from above appear shallower than it really is.

This is a virtual image formed by refraction, not water magnifying the chopstick and not mirror-like reflection. The displacement is usually more noticeable from an oblique viewing angle. When the line of sight is close to perpendicular to the surface, the bending is smaller and the apparent break weakens. Refractive index describes how light propagates differently in different media; the chopstick itself never changes shape.

Source: https://openstax.org/books/university-physics-volume-3/pages/2-3-images-formed-by-refraction