We had all three grandkids this weekend, which is always a recipe for mayhem. In the backyard, Job had managed to get Rose pretty wet so she was mad and went in. Then he threw a wooden rubber band gun which hit Bri in the butt. He had to, he was out of ammo! But she was kinda mad at that. So I had to read Job the riot act, in the course of which I may have inverted him over the water trough. In this position, he observed that the water had frozen in a thin clear sheet. This distracted the both of us, and we proceeded to try to break the ice by punching it. Which was a mistake because that ice was thicker than it looked, and felt like punching a brick wall. A quick search found a small rock, and a few quick jabs poked a hole in the ice. Some cracks formed, and Job said there were rainbows in the ice!
To make sure there was no oily stuff in the water, we took off our gloves and started picking out chunks of ice. Close up, we could see that the rainbows appear within the ice, along cracks that went through its thickness. Job discovered optical interference! Presumably the iridescent colors result from light bouncing around in the cracks. Interference makes the colors as light waves reinforce or cancel out each other in the resulting melee. Wow, that water was cold! Job and I beat a hasty retreat to get our hands under some warm water.
Inside we made our own rainbows (“Newton’s rings”) with two glass slides. Like the ice, the two pieces of glass offer closely spaced reflective surfaces for light to bang around between. In the picture, I’m putting some strain on the slides to give a little curvature and force them close enough together to get good interference. It’s lit with a compact fluorescent bulb, so the colors are a bit weird, since fluorescents don’t give us a continuous spectrum.