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Noon Edition

Coffee Cup Illusion

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An illusion in a coffee cup, in this Moment of Science.

Things are not always as they seem, and this little demonstration will prove it.

All you need is a cup of black coffee and an overhead light. A single incandescent bulb works best. Position the coffee cup so that the light is relected in it. Look into the cup from a distance that allows the reflected light to just about fill the cup.

Now move your head quickly and smoothly toward the cup. The light appears to get smaller and farther away. The change is dramatic. The light seems to shrink to a quarter or a fifth of its original size, and the move away 10 times more than you moved.

This is obviously an illusion, since you know the light hasn't changed size or moved. However, as you move closer to the cup, the cup fills more of your field of vision than the light, so the cup seems larger, and the light smaller.

In trying to interpret the information being sent to it, your brain correlates smaller with farther away, and you perceive the light has move. Your brain cannot make an adjustment either; you can do this over and over again and the result will be the same. 

Again, fill a cup with black coffee, and place it so that a single light is reflected in it. Look into the cup from a distance that will allow the reflected light to fill the cup. Move your head quickly and smoothly toward the cup, and the light will appear to get smaller and move away.

Photo of coffee cup.

You can create an optical illusion using just a coffee cup and a light source. (Wikimedia Commons)

An illusion in a coffee cup, in this Moment of Science.

Things are not always as they seem, and this little demonstration will prove it.

All you need is a cup of black coffee and an overhead light. A single incandescent bulb works best. Position the coffee cup so that the light is relected in it. Look into the cup from a distance that allows the reflected light to just about fill the cup.

Now move your head quickly and smoothly toward the cup. The light appears to get smaller and farther away. The change is dramatic. The light seems to shrink to a quarter or a fifth of its original size, and the move away 10 times more than you moved.

This is obviously an illusion, since you know the light hasn't changed size or moved. However, as you move closer to the cup, the cup fills more of your field of vision than the light, so the cup seems larger, and the light smaller.

In trying to interpret the information being sent to it, your brain correlates smaller with farther away, and you perceive the light has move. Your brain cannot make an adjustment either; you can do this over and over again and the result will be the same. 

Again, fill a cup with black coffee, and place it so that a single light is reflected in it. Look into the cup from a distance that will allow the reflected light to fill the cup. Move your head quickly and smoothly toward the cup, and the light will appear to get smaller and move away.

Sources and Further Reading

  • Ingram, J. (1989). The Science of Everyday Life.
  • Senders, J. (1966). The Coffee Cup Illusion. American Journal of Psychology. 79, 143-45.
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