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The light that you and I see with our eyes is just a small range of frequencies on a much larger spectrum called the Electromagnetic Spectrum.

Different frequencies of light interact with matter in many different ways depending on the atomic structure of the material, and it's electromagnetic properties.

For instance Glass is clear to us because it permits light in the visible frequency range to pass straight through it, but move a little higher in the spectrum to Ultraviolet and glass is no longer "see through".

X-Rays are another example of this. In the x-ray frequency our bodies are clear, like glass almost, while our bones are not clear because they have a different structure than our skin and organs.

When light interacts with matter a couple different things can happen:
You can have reflection: where the light bounces off completely.
You can have refraction where the light enters the material and passes through it. Or you can have a combination of the two, where the beam splits into two.
You can also have absorption, where the energy from the light particle (or Photon) is absorbed and converted to another form of energy. This is what happens in the photo-electric effect, which is what Einstein published his first paper on. In the photoelectric effect the energy from a photon is tranfered to an electron in the form of kinetic energy. Basically the photon comes in and smack the electron sending it flying off. These jumping electrons can then be harnessed to produce electricity.

This is the underlying principle behind a photovoltaic cell, this is how solar panels convert sunlight into electricity.

We have millions of electromagnetic waves passing through our bodies at any given time. Most pass right through us with no interaction, but some of the higher energy waves can and do cause interference to our bodies sometimes harmful.

Now that I've given you the basics on electomagnetic waves and their interaction with matter we are ready to talk about how to make things invisible...