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.
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