mirror

Mirrors and virtual images

Mirrors and virtual images

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.

http://www.mirror.co.uk/news/world-news/giant-mirrors-rjukan-norway-reflect-2506643

http://web.archive.org/web/20110622095120/http://www.timesonline.co.uk/tol/news/world/article634117.ece

Rjukan, Oslo mirror

Rjukan, Oslo mirror

 

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Fryscraper and curved effects

Fryscraper and curved effects

As you may know, a mirrored curve in a surface (concave) can focus light and magnify as well. Makeup type mirrors come in handy when you need to see yourself, biglike. There are formulas that can calculate the amount of magnification which is basically comparing the size of the object (your real face for example) to the size of the image (the magnified face). How does 10x look to you? Helpful when you can’t put on those reading glasses.

I have a Stirling Engine (supposed to be my son’s) that has a solar collector. It’s one of those bowl-shaped mirrors that concentrates sunlight which then makes the engine work if I get it aligned with the sun just right. But what if you are an architect and you design a tall curved building with mirrored windows? The result is some intense sunlight that has burned part of a Jaguar car parked in front of the building along with burning other things. When the sun is at the right location in front of that building the heat that is focused can reach the boiling temperature of water. Just having light reflect off a flat mirrored window into your face is bad enough but focusing the light to be hot enough to melt or burn plastic or boil water?

This makes me wonder about skyscrapers in general. What’s the purpose of building the highest? I know. We’re no different than those guys who wanted those huge pyramids but no one wants a pyramid anymore. It’s all about how much you can put in one place and call it useful. This London “fryscaper” as it’s been called is also known as the walkie talkie building because it also has that shape. If we really wanted to make the best use of the structure I think the architect might have been onto something but just didn’t do it the right way. Concentrated solar power (aka, CSP) could have been a secondary goal to generate heat for the building or run some generator for electricity. What really is a green building if it can’t use some of that sun instead of heating up the bricks on it or reflecting it on it’s neighbors?

This also made me wonder (I do that a lot) how much of the sun can we collect in a mirror and focus that light somewhere to be useful instead of frying the sidewalks and people. Apparently there’s a lot of real energy producing “plants” already doing this. We just need the architects and concentrated solar power engineers to meet up so they can put this all in one building.

http://en.wikipedia.org/wiki/Concentrating_solar_power

 

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