Physics in Your Fish Tank: Swedish Fishing

Try this experiment if you want to spear some candy fish, if you can catch them.



What you Need

  • A clear container ( a glass, a jar, or a fish tank would work, but make sure there aren’t any fish in the tank if you choose this option)
  • Kabob spears (available at most supermarkets)
  • Swedish Fish
  • Water

What to Do

  1. Fill your container with water.
  2. Drop in some candy fish.
  3. While looking through the container from the side, take a kabob spear and place it in the water like in figure 1.
  4. Look at the top of the water line. Do you notice that the kabob spear doesn’t appear to be straight?
  5. Remove the kabob spear and observe. Is it still straight? Figure 2 shows what you might be observing.
  6. Put the spear back in the water and try to spear the fish, while looking at the water line and the spear in the jar.
  7. Once you’ve speared the fish, remove it from the water, eat* and enjoy.

    *Caution: Fish may be wet and slimy

Figure 1


Figure 2

What’s Going On?

When you look at the spear from the side of the container, you should see the spear appear to be in two separate parts, breaking where the water level starts. If it doesn't appear to be broken it should at least appear to be bent. When you pull the spear out, you can see that the spear didn't bend. This is an optical illusion created by refraction, or the bending of light.
When light travels from one density to another it bends. In the case of the spear in the water, light went from the lesser dense air to the more dense water. When light moves between these two densities it bends, which make objects, appear to bend when the object is passing between both areas of density. How much the light bends is known as the index of refraction.

Try This!

  • Try doing the experiment with different glass containers. Does the shape or thickness of the container change the difficulty?
  • Try covering up the middle of the container, so you can only see the top and the bottom, but you can’t see the top of the water. Try the experiment again.  How does this affect the difficulty of spearing the fish?
  • Observe where the top of the water meets the spear from different angles. How does the angle you are viewing the spear from affect the appearance of the spear?

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