October 24, 2013
When you take a course in something you find out new words and phrases about that topic. When I was taking geometric optics courses in school I learned how mirrors work, how light rays reflect and the resulting image they create. Your standard flat mirror creates a virtual image of you or of what you see in the mirror, virtual image being the new phrase. Thinking about that bathroom mirror, when you look at it, your reflection looks like it forms behind the mirror somewhere….the virtual image. It’s funny to watch an animal see themselves in a mirror because they think it’s another animal farther away from them.
But I started wondering how can light come from us (reflect off of us) and be perfectly put back together to create that virtual image. This is where I pull out my simplified Feynman explanation (links to 1 video of his explanation below) and try to explain it, assuming I understand this quantum effect. First, the light (photons) from us hits the mirror….everywhere. When I say everywhere, I mean everywhere. The optics classes taught me to use just one line or ray to make the virtual image but that doesn’t explain the details.
Maybe this will help with all the rays going everywhere. Some of them that are at a certain angle of reflection end up canceling out. Destructive interference…which effectively means no light occurs. Let’s say the time it took for that ray that wasn’t really at the right angle to create the virtual image, it met up with another ray which was just so slightly at a different angle and thus the time was just so slightly different, they crashed into each other and poof! Nothing left (basically). But the majority of the reflecting rays being at the right (correct) angle, create the virtual image because they survived and didn’t cancel each other out. I hope this makes sense. Doesn’t change how a mirror works but may help understand what happens to all those rays that really are everywhere.
There’s more! Briefly, those photons don’t just bounce off a surface like a ball. They have to interact with the atom’s electrons. And metals are used in mirrors (like aluminum) because they have a lot of electrons (and that’s why there’s metal in our wires that conduct electricity). We still call it reflecting though. The photon gets released at the same wavelength that it came in. If it didn’t, our virtual image colors might be all messed up!
One more thing about mirrors. There are a few cities that have erected huge mirrors (heliostats) to bring sunlight into their towns. Northern cities lose a lot of sunlight in winter. Check out these stories of these cities use of these mirrors.