The changing arrows illusion
Optical illusions fascinate both children and adults alike. One simple yet captivating optical illusion can be created using basic materials – a transparent cup, water, and a paper with arrows. This visual trick demonstrates how light bends and focuses as it passes through different mediums. With just a bit of refraction, we can easily fool our eyes and brains into seeing something different than reality. The changing arrows illusion teaches basic physics principles like light refraction in an engaging hands-on way.
This experiment is fun for classrooms or science fans to try at home and explore how optical illusions work. The surprising arrow reversal is sure to puzzle and intrigue audiences of any age.
Materials:
- A white page with printed or drawn arrows.
- A transparent cup with straight sides
- Water
What to do?
- Place the cup in front of the arrows.
- Fill the cup with water until it covers the first arrow. It will seem as if the arrow flipped and is pointing the opposite direction.
- Move the cup, the arrow will go back to its original direction.
- Place the cup again in front of the arrows and fill with water until both arrows flip.
How does this happen?
Light travels in straight lines, however when light passes from one material to another it changes direction. This change depends on the type of material the light is passing through and the angle at which the light hits the material.
In this case, on the path of the light from the arrows to our eye there is air, glass, water, again glass and again air. When we add water to the cup, we turn the cup into a sort of convex lens. A convex lens bends the light rays inwards, meaning it concentrates them towards one point (a magnifying glass is an example of a convex lens).
The point where the light rays meet is called the focal point. When the light rays pass through the focal point they intersect and the ray that came from the right is now on the left and the ray that came from the left is now on the right. Therefore, the arrows flip.
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