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We can all appreciate pretty shiny things, but what makes them shiny?

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Sources:

https://van.physics.illinois.edu/qa/listing.php?id=1973
http://www.uni-ulm.de/fkp/lehre/Physics%20Lab%20Anleitungen/optical%20properties%20of%20metals-2
https://link.springer.com/content/pdf/10.1007/BF03215148.pdf
http://scienceline.ucsb.edu/getkey.php?key=3903 https://www.photonics.com/EDU/Handbook.aspx?AID=25501
http://nvlpubs.nist.gov/nistpubs/jres/4/jresv4n2p189_A2b.pdf
Nothing grabs your eye quite like SHINY.

And when it comes to shine, nothing beats a good metallic sheen. From mirrors to jewelry, metals are prized for their ability to glitter like, well, gold.

Their shininess is basically a built-in property, and there’s a good reason why: It’s a quirk of the weird way their electrons behave. In a metal atom, the outermost electrons are only weakly attached to the nucleus. When a bunch of metal atoms come together in a chunk, these outer electrons wiggle free, creating what scientists usually think of as a negatively-charged “sea of electrons” flowing around a network of positively-charged ions.

This moving sea is a big part of what makes metals … metal. For example, you can hammer them into different shapes without cracking them because the electrons can move to compensate for the atomic nuclei shifting around. And all those free electrons are great at conducting electricity.

They’re also why metals are shiny. What we call “shine” is actually reflected light, but it doesn’t just bounce off the surface like a tennis ball. Instead, when a wave of light hits the sea of electrons, the electrons absorb the light wave’s energy, which makes them wobble a little.

That movement generates a second wave of light — one that radiates back up from the surface. That’s what we see as a shine. The free electrons in metals are especially good at this vibrating-to-match-visible-light thing.

But different metals shine in different ways. That’s because depending on the metal, the sea of electrons will have a different mass and density, which affects how freely the electrons can move — and therefore, whether they can vibrate to match the incoming wave. Most metals are silvery because they reflect most visible light, but not all of it, which makes them a little darker than pure white.

A few metals, like gold and copper, aren’t able to reflect the bluest wavelengths of light, so they shine with a reddish or yellowish hue. And there are no guarantees that a metal will reflect higher energy waves, like UV and X-rays, at all. Waves that aren’t reflected penetrate into the metal, where they can either be absorbed or pass right through, depending on both the type of metal and how thick it is.

Silver, for example, is one of the best reflectors of visible light, which is why it was one of the first metals used for mirrors. But it’s terrible at reflecting UV rays. If I locked you in a thin silver box and put it in direct sunlight, it would be completely dark in there, but you could still get sunburned.

You know, hypothetically. Metals like tin and aluminum are much better at reflecting ultraviolet light, which is why they’re used in roofing. Lead, on the other hand, can reflect even the high-energy X-rays that penetrate most metals, making it an ideal way to protect the parts of your body that your doctor doesn’t want to X-ray.

That’s also why Superman can’t see through it. So, metals can shine in lots of different ways, but it all comes from the same thing: a bunch of electrons moving around. Now, if I could just find a silver box ...

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