The light was shining through the cut glass window of our door this morning...
...casting rainbows on the kitchen walls and floor.
We didn't have much time for an in depth science lesson, as D(age 13) was getting ready to head out the door for camp. There was just enough time to fill our one clear glass mug (all our glasses are colored) with water, to show the children what would happen if we placed the mug in the way of the light coming in through the door.
The mug wasn't making a new rainbow (you can see the colors reversed on the right side of the picture above). The colors were coming from the light passing through the angled glass in the door. So, when we let plain white light pass through the cup, it stayed white.
Instead, the light, colors and all, was being refracted (or bent) through the cup at such an angle as to make the reflection on the other side appear to be flipped from the original, much like when you look at an image (like for instance the graduation invitation below) through a glass of water.
Hold it close to the cup and it will look pretty normal.
Move it away, and the picture will start to blur.
Move it a little bit more, and the picture will become clear again, but appear to be reversed.
Putting our cup down into a rainbow on the floor (with a piece of white paper underneath), we could see exactly how the light was bending in such a way as to come out of the cup, crossing, and then reversing.
The next time you have a rainbow reflecting on your floor, you might want to give it a try too - it's a surprisingly simple, but effective demonstration, with no expensive light box, lasers, or lengthy lectures required.