Materials
- Two colored pencils (different colors work best)
- Clear glass
- Water
Instructions
- Place the two colored pencils upright side by side behind an empty clear glass. Make sure both pencils are clearly visible through the glass.
- Look through the empty glass and note the position of each pencil.
- Slowly pour water into the glass until it is about halfway full.
- Look through the glass again at the pencils.
- Observe how the part of the pencils seen through the water appears shifted, making the pencils look like they have switched positions in the water-filled section!
- Add more water and watch how the effect changes.
What’s going on?
- Light bends when it passes from air into glass and then into water. This bending is called refraction. The curved surface of the glass and the difference in refractive properties between air and water cause light rays from the pencils to change direction.
- When the glass is empty, light travels mostly straight, so the pencils appear in their true positions. When water fills part of the glass, light passing through the water bends differently than light passing through air. This causes the image of each pencil to shift sideways in the water-filled region. As a result, the pencils appear misaligned or switched only in the part of the glass that contains water, creating the illusion that they cross or change places.
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