Resonance

When I was a kid, I used to sit between two mirrors and revel at how I could project my image a few dozen times. Then I wondered... what would happen if I put light between two perfectly parallel mirrors and just have it bounce back and forth. If I removed a mirror, would there still be light inside? It took me like 15 years slap the label resonance on that thought but now I know. And that one time in when we stood on that grate at the park near the samurai school near Ishinomaki and could hear the echoes of our voices. That was awesome... resonance. I don't think it was a coincidence either.