The straw does not bend in the glass: light bends on its way to your eyes. Light from the underwater section changes direction as it crosses from water into air. Your visual system generally treats arriving light as though it traveled along a straight path, so that submerged section appears displaced. The upper, dry section does not shift the same way, leaving an apparent break at the waterline.
What you actually see at the waterline
Look through the side of a clear glass containing water and a straight straw. At certain angles the section below the water surface seems to start farther to one side than the dry section. Turn the glass or move your head and the apparent break changes. Pull the straw out and it looks straight again. Nothing has happened to the straw’s shape.
Why light changes direction
Transparent substances let light travel through them, but its speed differs from one substance to another. The change in direction when light crosses a boundary at an angle is called refraction. Water has a refractive index of about 1.33, while air is close to 1.00. When light travels from water into air at an angle, it bends away from an imaginary line perpendicular to the surface. The eye sees the outgoing light but cannot directly see its earlier underwater path.
The curved walls of a drinking glass introduce additional optical distortion. A flat-sided transparent container can make the basic water-to-air effect easier to observe.
Try a two-minute experiment with kids
- Fill a transparent drinking glass halfway with clean water.
- Insert a straight, brightly colored straw or wooden pencil at an angle.
- Look from the side, then slowly raise and lower your viewing position.
- Compare the appearance at the waterline and just above it.
- Remove the object, dry it and check that it never truly bent.
For another test, place a coin at the bottom of an opaque mug, move your head until the near rim barely hides it, and slowly add water without moving your eyes. The coin may appear to rise into view. For young children, use a nonbreakable cup and supervise water play.
Why is the effect stronger from some angles?
The bending depends on the angle at which a ray meets the boundary. Looking nearly straight down changes the apparent depth of the underwater section but may produce little obvious sideways break. Looking from a slanted side angle often makes displacement easier to notice. Glass thickness, curvature and colored stripes on a straw all affect what stands out.
Is this the same science as a rainbow?
They share refraction, but a rainbow also depends on internal reflection and the way different colors bend by different amounts inside raindrops. A straight straw usually appears shifted rather than rainbow-colored. You can explore another optical effect in our guide to rainbow colors on a CD: those colors arise mainly from diffraction by microscopic tracks, not the glass-of-water effect.
For further study, see OpenStax’s explanation of refraction and Snell’s law.
Frequently asked questions
Does water actually bend the straw?
No. A flexible straw may bend if physically pushed, but the apparent kink in this demonstration is caused by the light path.
Why does a swimming pool look shallower than it is?
Refraction can shift the apparent position of submerged objects toward the surface. Always rely on measured depths rather than appearance.
Can I use a pencil instead?
Yes. A pencil, spoon handle or rigid transparent-safe rod works, although a patterned straw can make the visual shift especially easy to see.
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