Put a straight straw into a glass of water at an angle and it can appear broken or bent exactly where it crosses the surface.
Take it back out and the straw is perfectly straight.
The water did not bend the straw.
It bent the path of the light reaching your eyes.
Light changes speed in different materials
Light travels differently through air, water, glass, and other transparent materials.
When a light ray crosses from one material into another at an angle, its speed changes and its direction usually changes too.
That change in direction is called refraction.
Light coming from the underwater portion of the straw travels through water and then crosses into air before reaching your eyes.
At that boundary, the ray bends.
Your brain assumes light came in a straight line
Your visual system is very good at using incoming light to estimate where objects are.
Most of the time, treating light as though it traveled in a straight line is a useful shortcut.
With refraction, that shortcut creates an error.
Your brain extends the refracted light rays backward and places the underwater image where those straight-line extensions seem to originate.
That apparent position is shifted from the straw’s actual position.
The part above water is not shifted in the same way, so the two portions no longer appear to line up.
Instant broken straw.
Why does the effect depend on viewing angle?
If light crosses the boundary straight on, there is much less directional change.
At steeper viewing angles, refraction becomes more obvious and the apparent displacement can be larger.
That is why moving your head around a glass can make the underwater part of the straw seem to shift.
The straw has not started wandering.
You are changing the geometry of the light paths.
Why does a swimming pool look shallower than it is?
The same effect can make objects underwater appear closer to the surface.
Light coming from the bottom of a pool refracts as it leaves the water. Your brain traces those rays backward and places the bottom at an apparent position higher than its true location.
That is why judging water depth by sight alone can be misleading.
Refraction is not merely a classroom trick involving pencils and beakers. It affects what you see every time you look through the boundary between air and water.
Why do eyeglasses use refraction?
Lenses are shaped to bend light in controlled ways.
Eyeglasses redirect incoming rays so the eye can focus them more accurately on the retina. Cameras, microscopes, telescopes, binoculars, and many other optical devices rely on the same basic principle.
The bent-straw illusion is refraction being inconvenient.
A lens is refraction being hired professionally.
Does light always bend toward the same direction?
No. The direction depends on the materials and the angle at which the ray crosses their boundary.
When light moves between materials with different refractive properties, the geometry is described by a relationship called Snell’s law.
You do not need the equation to understand the glass on the table.
The important part is simple.
The straw is straight.
Light from its underwater portion changes direction on the way to your eyes.
Your brain assumes the light did not change direction and puts the image in the wrong place.
The optical system works beautifully most of the time.
This is one of the occasions when it gets confidently fooled by a glass of water.
A related explanation is Why do mirrors reverse images?, which looks at another familiar effect from the same general system.
And that’s the short explanation.
Related explanations: Why do mirrors reverse images? and Why does your recorded voice sound weird?