Water can do some really cool things to light. We saw in the disappearing coin trick that light refraction could make a coin vanish in front of your eyes. You may also notice that when you are at the pool, objects in the water are not really where you think they are when you look at them from above the water.
This turning arrow trick is another excellent example to show your kids how light refraction works. Just like magic, you are going to make the arrow point the other way.
How Light Refraction Reverses the Arrow
- Glass or jar
- Draw an arrow on a piece of paper with the marker. It can point left or right.
- Pour water into a glass of water.
- Slowly lower the paper with the arrow behind the glass.
- Look through the glass and see the arrow pointing the other way!
My kids couldn’t believe their eyes. How in the world did the arrow magically change directions?
I told my kids to draw other objects so that we can reverse their directions. So this short, simple activity turned into an hour of drawing and seeing how the drawings look behind the glass of water.
Science Behind the Reversing Arrow Illusion
When light passes from a fast medium (air) to a slow medium (water), refraction, or bending of light, occurs. Since air molecules are far apart, light can travel through the air quickly and with very little refraction. However, water molecules are close together and slow down the light and cause the light to bend.
In this experiment, the light travels from the arrow through the air, through the glass, into the water, through the glass and air again, before reaching our eyes. The light paths cross at a focal point, and the image appears to be reversed horizontally.
In simpler terms, the light coming from the tip of the arrow refracts on its way to your eyes and instead of being on the right side, it is now on the left side.
If you move closer to the glass, you will notice that the arrow appears in the same direction as it is drawn on paper. This is because now your eyes are now closer to the image than the focal point.
- Try placing different objects or drawings behind the glass of water and see if all of them reverse directions.
- Use a much larger glass. Does the size of the glass affect how far you need to be to see the arrow in reverse?
- Use a clear rectangular container instead of a rounded glass or jar. Does the arrow still change direction?
- Use baby oil or another type of clear liquid instead of water. How does the medium inside the glass matter?
Light refraction is the basis of many magic tricks. I hope you and your kids enjoy this activity!
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