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In our tales and stories about the Middle Ages all of the finest, most chivalrous knights ride around in, of course, shining armor. Now, in the real world, most knights probably worried more about how well their armor protected them from swords and arrows than how shiny it was, but storybook knights always had to look dashing for their damsels in distress. They’d ride to the rescue, light shining like a beacon off their perfectly polished breastplates. In today’s experiment, we’ll learn a bit about the shininess of armor, and any shiny metal, using some basic kitchen supplies.
Materials:
- Aluminum foil
- Something flat and heavy, like a book
- Something scratchy, like a Brillo pad or steel wool
- Something soft, like a cotton ball or a soft cloth
Process:
- First of all, tear off a small piece of aluminum foil. Make some observations. Notice how one side is so shiny that you can almost see your reflection, and the other side is duller, less shiny. It’s still shiny, but not nearly enough to see your face in it.
- Crumble your foil up into a ball – really mash it together. Now open it back up again. What do you notice on the shiny side? Can you still see your reflection? What about the dull side?
- Using your book, flatten and smooth out your foil as best you can. Can you iron out all the wrinkles? When it’s as flat as you can get it, can you see your reflection again on the shiny side? Why do you think that is?
- Tear off a fresh piece of foil. This time, leave it smooth and shiny. Instead of crumbling it, start rubbing the shiny side with your scratchy material. Rub firmly, but don’t push so hard that you tear the foil. What changes does this make to the shininess and reflectivity?
- Now try scuffing up the dull side? Does it change as much as the shiny side?
- Okay, one last piece of foil. This time, make a prediction. You already saw what a rough material will do to the shininess of the foil. What do you think a soft material will do?
- Rub your last piece with your soft material. What can you observe?
Summary
So what makes metal shine? The cool thing is that the answer is the same for all shiny metals. So even though knights never made armor out of aluminum (it couldn’t be produced efficiently until the late 19th century), we can use aluminum as a cheap, easy model for the steel they did use.
The simple answer is that metal is very good at reflecting light, and that’s what makes it look shiny. It’s true, but it doesn’t really tell us much. Why does metal reflect light? Well, it has to do with the electrons in the metal. Electrons are incredibly tiny particles that make up atoms. Specifically, they orbit around an atom’s center, or nucleus, and can move up and down into higher or lower orbits. When an electron absorbs some energy it moves up (we call that getting excited). Electrons can’t hold extra energy for very long, and when they release it they drop back into their ground state, and the energy moves off away from them.
Metal has lots of “free” electrons that move around the metal from atom to atom, and this makes it easy for these electrons to absorb energy. Light is really just a form of electromagnetic energy, and so when light hits metal, these free electrons get excited. Pretty soon, though, they fall back to ground state, and when that happens, they give off some of the light that excited them in the first place. It’s never quite as much as they absorbed because they needed to use some to get excited.
Now here’s a strange but true factoid about aluminum foil (and all other shiny metal): scratched or scuffed metal actually reflects almost the same amount of light as shiny metal. That might seem weird since your foil had a shiny side and a dull side, and scratching made it really not shiny, and even just crinkling it made it so it wouldn’t reflect your face anymore.
Here’s the thing, though: smooth metal reflects all the light back in the same direction, so when you look at it it’s really shiny. All the light is going right to your eye. When you crinkle, scratch or scuff metal the scratches scatted the reflected light, making it go in all different directions. Since it isn’t hitting your eye directly anymore, the metal no longer looks shiny. That’s why there's a shiny side and a dull side on foil: when the foil is being made, it gets rolled out two long sheets at a time, and the sides of those two sheets that are touching each other create tons of tiny, tiny scratch marks that dull the metal. Check for yourself with a magnifying glass. You can see the micro-scratches!
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