WYSINWYG (What you see is NOT what you get)

The old adage of appearances being deceptive is true on many different levels.  More abstractly consider the problem of communicating the properties of an object or of a concept C from point A to point B.  There are many reasons why this communication is imperfect.  In fact, there could be a problem at every stage of the communication process.

First of all not all of the properties of C are transmitted to B. This could be deliberate, such as in the game poker or the game Mastermind, where after each trial, only the number of in-location matches and out-location matches are revealed, not their positions.  Or it could be limitations of the physical world, for instance, we can only see the outside of an object, (i.e. you can not judge a book by its cover).

Secondly, the transmission medium could distort the information.  For instance, we don't see as well at a foggy night.  In communication theory, noise in the transmission channel can make the information at the receiving end different than at the transmitting end.  Typically, information is lost in this process, i.e. it is not possible to recover all the information transmitted from point A from the information received at point B. Or this could be a fundamental consequence of quantum physics, where the act of observation influences the object being observed.

Thirdly, the sensor apparatus used to receive the information can be deficient.  Our eyes can only register lights in the visible spectrum, our ears can only hear in the range of approximately 20Hz-20kHz.   This means that something that is invisible (or inaudible) to one animal (or machine), may not be to another. For instance, our warm bodies emit infrared light, but we cannot see it in the dark.  However, night vision goggles allow us to see in the dark by using an infrared sensor to capture that radiation and convert it into visible signals that we can see.  I have always wondered about the invisibility cloak in the Harry Potter novels or the invisibility serum in H.G. Wells' The Invisible Man, whether they work across the entire electromagnetic spectrum.  If they only work in the visible light spectrum, then they could be easily defeated with modern technology by using either an infrared light source (a warm object does by itself emit infrared radiation) or ultraviolet light source (a black light) and using an infrared or ultraviolet sensor to "see" the invisible person.  In addition, there are other sensor modalities, e.g. sonar which uses (ultra)sound waves or radar which uses electromagnetic radiation at the radio waves range, that need to addressed for such invisibility cloaks to be truly invisible.