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Invisible ink seems to work only on paper. Can you use it on real objects by coloring them thus making them invisible. Is there another way to hide something in plain sight and reveal it only if you have a special light?
Question Date: 2017-06-24
Answer 1:

What an interesting question!
​There are many ways to make invisible ink. The most common is heating up lemon juice on paper, which causes a chemical reaction called oxidation and effectively burns the lemon juice into the paper. This wouldn't necessarily work with any other surface, but there are many other ways that you could make something appear invisible.

You have hit upon a broad and fascinating field of study about how materials interact with light.

Light, or more generally the electromagnetic spectrum, consists of much more than what we humans can see.

click here to see a picture of the electromagnetic spectrum

We (classically) characterize light as a wave​ (things get more interesting with quantum mechanics, but that's a story for another day). As a wave, light can have wavelengths that span all the way from a football field (e.g., radio waves, the same you listen to on the radio) to the size of an atom (e.g., x-rays, the same you use to see your bones). Most of these wavelengths are invisible to us humans. In fact the visible spectrum is only a small sliver of the overall spectrum.

So we need to change our definition of invisible from what we as humans cannot see to an object that is transparent to a part of the electromagnetic spectrum. Whether a material is transparent to some wavelength of light is dependent on what we call its electronic properties. We can use the physics of what makes glass look transparent to us in the visible spectrum to "engineer" materials that look invisible to many types of light.

One example you can try for yourself at home is using a black light. A black light is a type of light that emits mostly in the UV spectrum. This is the part of the electromagnetic spectrum that sits next to the visible range. This means you'll see some things you won't see with just light in the visible spectrum. But be careful and make sure to have adult supervision.

Another example is something that is under active research- how to make large objects appear invisible to a particular part of the light spectrum. This is being explored with metamaterials, or materials not ordinarily found in nature. Metamaterials are those that have been specially engineering to bend light in such a way that looks like the material appears to vanish. This is known as metamaterial cloaking. You can manipulate what kind of material you use or use complex geometries to do this. Read more about it here.

​Hope this helps!

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