Why is a puddle reflection wobbly?

After the rain, a puddle can hold a picture of the sky, a tree, or even your face. Sometimes that picture wiggles, though the tree stands still. Why is a puddle reflection wobbly?

The answer starts with light, the brightness that lets you see things. In the daytime, the sun gives us light. Some sunlight hits a tree and bounces away in lots of directions.

Some of that light travels from the tree to the puddle. The water bounces some of it toward your eyes. You see a reflection, a picture made when light bounces off something.

When the water stays still and smooth, the puddle acts like a mirror. Light from the tree keeps bouncing the same way. That makes the reflection look still.

But a breeze can push against the top of the water. That push makes ripples, small waves that travel across the puddle. A falling raindrop can make ripples too, by nudging the water where it lands.

Here comes a little breeze, sending ripples across our puddle. Can you guess what will happen to the tree's reflection? The tree stays still, but its watery picture starts to wiggle.

Each ripple has little slopes, like tiny hills in the water. A slope bounces light in a different direction than a flat patch does. Those slopes change which bits of the tree's picture reach your eyes.

As the ripples move, their slopes keep changing. Light bounces in changing directions, so parts of the reflection seem to stretch, squeeze, and wobble. The water moves the picture, not the tree!

When the breeze stops pushing, the ripples slowly fade. The water grows smoother, so light from the tree bounces the same way again. The tree's reflection settles down too.

Light, reflection, ripples. A puddle reflection looks wobbly because moving ripples bounce light in changing directions toward your eyes.

Now you know the little secret behind a puddle's gentle wiggle.