In the world I'm building it is controlled by two powers. Light/Dark. No need to get into any details about Dark magic. My question involves skills attributed to bending Light. As in manipulating light rays. In my world Light Magic is used in many different ways, but when it comes to bending light rays, my character has learned how to make himself invisible by bending the light around him, rendering himself invisible. If bending light rays to disappear yourself were possible, would you be able to discover the invisible person through thermal detection?
Strictly, no, but....
The "thermal" detection you are talking about is detection of [infrared] radiation. This is electromagnetic radiation just like visible radiation, but at a different wavelength. So-called "night vision" goggles work by detecting the infrared and rendering it in false color as visible light. If your character's power bends all wavelengths of light, it bends the infrared, too.
BUT THE FIRST -- if the spell only bends visible light (perhaps it involves "eyes", which can only detect a narrow range of frequencies), then your character can be detected by heat, if the Bad Guys think of it.
BUT THE SECOND -- Even if the spell deflects all wavelengths, your character's enemies could tell your character's body heat from a rock by direct contact. Although this seems unlikey, it allows for the possibility of body heat diffusing by the air molecules that hit his or her body, or absorb infrared radiation inside the effect of the spell. So the Bad Guys might detect your character's general vicinity by spotting a diffuse area of heated air molecules.
But those decisions are up to you.
but when it comes to bending light rays, my character has learned how to make himself invisible by bending the light around him, rendering himself invisible. If bending light rays to disappear yourself were possible, would you be able to discover the invisible person through thermal detection?
Your character is performing a very special (and difficult) light bending, forcing every incoming photon on exactly one half orbit before proceeding with the same direction and on the same axis. Depending on the incoming area covered, this is more or less effective. If the area covered is a full sphere, you have perfect invisibility (barring some sophisticated phase detection methods; possibly, looking at your character through a slate of Iceland spar could reveal a sort of glassy bubble).
This kind of light bending is effective only against incoming light - it cannot act on outgoing photons, that is, body heat.
- case one: they pass unimpeded, and a thermal imager (or a snake, or a mosquito) can "see" the body heat.
- case two: they're reflected backwards (a "half orbit" for outgoing light is straight from whence it came), and your character starts feeling really, really warm, and is forced to open his Tarnkappe and let the steam out every now and then.
Whoever can bend light that way would have little trouble in bending it in a much easier way. So your character does the bubble bending with an almost full bubble, and bends all outgoing light - heat included - into a tiny pinprick exactly under his feet. Nothing goes out, no thermal imager can now detect him.
He already needs to do the "incomplete bubble" trick, otherwise he'd be blind as a bat and unable to see whatever is outside his bubble. He should still be able to hear (and use ultrasound) and use any non-electromagnetic ESP he has, as well as any radiation that doesn't get bubble-bent: radio, microwave etc.
Note: the guy will still disrupt sound echoes: a very sensitive person can feel when something is in a room they know that shouldn't be there. Also, sonar will still detect him. More technological methods such as capacitance detectors and phase-shift remote sensing, as well as the more advanced radars and lidars, will not be fooled - they won't maybe succeed in telling what is there, but they'll detect that something is there.
It depends what you mean by light.
If you mean visible light, then yes, you would be able use thermal imaging to detect the presence of the person making themself invisible.
If you mean electromagnetic radiation then no, the person would be able to use the same light bending technique to bend the infrared light (heat radiation) that they are emitting. They would be invisible to both the naked eye and infrared/heat sensors.
Depends on how you want these powers to work. Infrared radiation is primarily responsible for heating the earth, and it is a very low frequency form of light waves. Thermal imaging cameras work by looking at the difference of infrared thermal radiation being emitted from objects.
If your Light Magic users could bend this kind of light, then they could theoretically hide their heat signature.
Infrared is light, just as the visible light.
Glowing things radiate visible light only because the frequency of the radiated photons increase with the temperature. Also your body glows, but it radiates only infrared light.
If the invisible, infrared light falls into the "dark magic" topic in your world, then only the dark magicians can create invisibility. If it falls into the "light magic" topic, then the light magicians can create invisibility.
If light-bending cannot affect thermal infrared, then obviously body heat won't be affected.
If light-bending is affected but you cannot bend light emitted from inside the zone back toward itself, then thermal infrared will still escape, and you can be detected. Interestingly, if you hold a (visible) light, it can also be seen, possibly being partially bent and giving a deformed image. An eerie, liquid floating light with no visible source could be used for great effect at parties!
If you can completely bend light back, then your body heat cannot escape and reveal your presence. But then you have one big problem: your body heat cannot escape. Which means you will be very hot very fast. You probably want to pack ice or cryogenic liquid gas, so it can absorb your body heat and allow you to stay hidden longer before suffering a stroke.
Also note that you cannot insulate yourself from your own body heat: being the heat generator yourself, you can only dump heat to something colder, so air around you will never be as hot as yourself. Unless you are using a heat pump (like a refrigerator), but those are heavy, power-hungry, and the power-source itself will produce more waste heat than will be extracted, so the total heat will rise even faster.
Give up on thermal detection, but you should be able to detect a person from using a calibrated measuring laser. The laser will take longer to go around the body than it would to go straight, so the apparent distance to the other side of the person will be longer than it should be.
Or you can choose any of... - the movement of air from their breath. - the carbon-dioxide presence from their breath. - weight sensors / floorboard creaks, etc. - incongruities in the light-bending technology. - the fact that they keep bumping into things because they cannot see! - the fact that they may also freeze to death from lack of (or surplus) heat.
If you define "light" to be "electromagnetic radiation" then the light bender would be able to bend the infrared radiation associated with heat. However, they would still need to deal with conduction. If they leaned up against a pole, the pole would warm up due to heat conducted from their body. This would be detectable, unless the light bender took the time to hide that as well. Eventually the light bender would get awfully busy hiding the warm trail of objects that they touched.