A pencil is placed with a diagonal orientation within a 2/3-filled glass of water. At the surface of the water, the pencil appears to be misaligned or broken; the portion of the pencil above the water is shifted relative to the image viewed under the water. The bending of the path of light as it passes from the water to air causes the observed distortion of the image of the pencil.
Next, I poured water into a glass and placed an arrow on a piece of paper about 4 to 5 inches (10-12 cm) behind the glass of water.
How does this work?
well, your not crazy and you not seeing things. In fact, you have just demonstrated a physics concept called refraction, the bending of light.
When the arrow is placed a particular distance behind the glass, it looks like it reversed itself. When light passes from one material to another, it can bend or refract. In the experiment that I just completed, light traveled from the air, through the glass, through the water, through the back of the glass, and then back through the air, before hitting the arrow. Anytime that light passes from one medium, or material, into another, it refracts.
Just because light bends when it travels through different materials, doesn’t explain why the arrow reverses itself. To explain this, you must think about the glass of water as if it is a magnifying glass. When light goes through a magnifying glass the light bends toward the center. Where the light all comes together is called the focal point, but beyond the focal point the image appears to reverse because the light rays that were bent pass each other and the light that was on the right side is now on the left and the left on the right, which makes the arrow appear to be reversed. See the diagram in the video which shows it better.
You can see when I pass the smiley face with a wink behind the glass the wink reversed, but when I passed the words Kid’s Fun Science right behind the glass it did not. The difference was the smiley face was farther back behind the focal point and the words were right against the glass. Try this at home and see how far back you have to place the paper and where the focal point is. Let me know in the comments below.