Pull it out and it's perfectly straight. The bend isn't in the chopstick — it's in the path the light takes, and in your own eye. Scroll down and take "seeing" apart.
"Seeing a bent chopstick" = light × refraction × a virtual image.
The tip doesn't glow on its own — it scatters the light that falls on it in every direction, and one thin bundle happens to fly into your eye. You never see the chopstick itself, only this light.
Light moves through water at only about three quarters of its speed in air. The instant it crosses the surface, its speed changes — and so does its direction. That's refraction. Passing from water into air, light bends away from the normal (the imaginary line perpendicular to the surface, which all angles are measured against).
Your eye only knows the direction the light arrives from — not that it turned along the way. The brain assumes light travels in straight lines and extends that direction backwards. Where the traced lines cross is the virtual image: no light actually starts there — it's a position your brain invents, and it sits shallower than the real tip.
Every underwater point gets lifted a little — shallow points less, deep points more. Connect all the virtual images and the chopstick kinks upward right at the waterline. It isn't the chopstick that bends — it's your judgment of where each point is.
That's the law of refraction — Snell's law. Water has refractive index n ≈ 1.33, air n ≈ 1.00: going from water into air, sin θ₂ ＝ 1.33 × sin θ₁, so the refraction angle is always larger than the incidence angle. One handy consequence: seen from straight above, an underwater object appears at roughly 3/4 of its true depth.
The light a fish reflects bends at the surface too: the fish you see is a virtual image, and the real one is deeper, below it. To hit it, aim under the spot where it appears. Fishermen have known this by feel for ages — now you can take it apart and explain why.