I don't remember a lot from the optics section in Physics but isn't it basically that you certain lights only reveal other things at that same frequency or wavelength? Or something like that.

Well, to human beings they're not in our visible spectrum. It's like a color wheel, where violet fades into red just fine. If ultraviolet and infrared were added, they'd fade into each other strangely well, and red and purple would be pretty far off.

Purple doesn't exist It is your brain warping violet and red light together to make what we see as purple.  There is actualy no wavelength for purple light, its all in your head

Infrared is only "Heat" becasue our environment is about 300 Kelvin and at this temperature we release the most amount of blackbody radiation at this wavelength.  You are emitting as much radiation as you are absorbing because everything around you is about the same temperature.  If you heat something up hot enough, say to 2000K, it starts to emit much of its radiation in the form of visible light which is why a glowing iron rod appears red after being heated.  And if you go into space, you have a temperature of about 2.7K which is due to the cosmic microwave background radiation.

So the actual wavelength is only dependent on temperature.  Its only infrared at room temperature.