Science · 6 pages · 12 min
Why a straw looks bent in water
The straw is straight, and light from the wet half bends at the surface.
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From the opening
A straw looks bent in water because light from the underwater part changes direction at the surface. Your eye traces those rays backward in a straight line and puts the straw where it is not.
The straw is not bent, and the water is not a dye or a wrench that twists the plastic. Lift it out and the straight shape was there the whole time.
The mechanism is more particular than the slogan. Light travels more slowly in water than in air, so at a sloping boundary the wavefront slews and the ray bends. Looking from air into water, rays from the submerged straw bend as they leave, and the eye, assuming a straight path, misplaces the submerged half.
Follow it one step further. The bend is small when you look straight down and strong when you look at a shallow angle. That is why the straw seems to kink right at the surface, and why the underwater half also looks closer to the surface than it really is.
The case worth keeping in mind is specific. A pencil in a clear glass of water shows the same break to anyone standing off to the side. Move until you look nearly straight down into the glass and the kink eases, which is the viewing angle changing, not the pencil.
That case has a boundary. The simple kink assumes a flat water surface and a view from the air side. Ripples scramble the direction from moment to moment, and looking from underwater out into air brings a critical angle beyond which the surface behaves as a mirror.
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Inside
- Light slowing down in water
- Why the kink sits at the surface
- Why pools look too shallow
- What a straight-down view does
- Where the idea stops applying
- A distinction worth keeping
Sources
- Refraction at an air-water boundary
- Snell's law relating angles to the speed of light