Did the Chopstick Snap? Did It Move Backward? The Strange Images in a Glass of Water
Ken Kuwako, Science Trainer here. Every day is an experiment!
Have you ever been surprised by a chopstick in a glass of water that looks snapped in two, or even seems to have a backwards twin? This time, I used a glass and a pair of disposable chopsticks to explore some strange tricks of light and the “images” they create.
Put it in the middle of the glass, and it looks perfectly straight
First, I dropped a chopstick into a glass and watched what kind of image it made. When I held the chopstick upright near the center of the water and looked at it from the side, it looked perfectly straight.
That’s because the light from the chopstick at the center leaves the glass almost head-on, so it barely bends. Light bends when it passes at an angle through the boundary between two different materials, like water and air. This is called “refraction.” Light that goes straight in doesn’t bend at all, so the chopstick looks just the way you’d expect.
Move it to the edge, and it looks broken
Now let’s move the chopstick toward the edge of the glass. Suddenly, it looks like it’s been cut right at the water’s surface! 
Near the edge, the light from the chopstick hits the glass at a slant and bends a lot. As a result, the part underwater and the part above the water appear to be in different places, so the chopstick looks bent or broken. The same refraction is why your legs look short in the bath, and why the bottom of a pool looks shallower than it really is.
Hold the chopstick sideways and slide it behind the glass…
Next, I held the chopstick horizontally and slowly passed it behind the glass. Partway through, a mysterious image appeared, moving in the opposite direction from the real chopstick! It then merged with the real chopstick, and as I kept going, it looked like the chopstick was coming out from the edge of the glass. 
Something’s popping out from the right!
They merge!
A glass of water is actually a “lens”
So why does this happen? Seen from the side, a round glass of water works just like a magnifying glass, which is a “convex lens” that gathers light. The light from the chopstick behind the glass gets bent as it passes through the water, comes together at one point, and then spreads out again.
With a convex lens, an object far from the lens appears flipped upside down and left to right. With a glass of water, the image is flipped left to right. That’s why the chopstick’s image seemed to move in the opposite direction inside the glass. The glass itself is thin, so you can think of the water inside as doing most of the work.
As you move the chopstick, the real one and its image in the glass move in opposite directions. After the two meet, the chopstick seems to emerge from the edge of the glass. That’s because the path of the light keeps switching as the chopstick changes position.
“Water lenses” are all around us
There are plenty of other examples of water acting like a lens. If you look closely at a water droplet on a leaf, the veins of the leaf can look bigger. And if you look at a view through a clear plastic bottle filled with water, it can look warped or even upside down.
All you need is a clear glass, some water, and a chopstick (or a pencil). Fill the glass with water, hold the chopstick behind it, and slowly move it around to see what kind of image appears. What happens if you change the width of the glass, the amount of water, or the distance to the chopstick? It’s also fun to make guesses with friends and family about why things look the way they do!
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Science is full of wonder and fun, and it’s closer than you think! This blog collects fun science experiments you can do at home, along with tips to help them go well. Try searching around! ・The content of “Kagaku no Netacho” (Science Idea Notebook) is now a book! Details here. ・Learn more about the author, Ken Kuwako, here. ・For requests (writing, lectures, science classes, TV supervision, appearances, and more), click here. ・Get article updates on X!
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