# Heat and Infrared radiated from an object

Summary:
Infrared without heat?
Dumb question.

Is there a direct heat energy relationship between a heated object and infrared radiation? 1 to 1 relationship? Or can IR go up without there being much heat?

jim mcnamara
Mentor
Your question has a couple of oops statements I think.
For heat let's use degrees K (or Celsius). Ok?
The first problem is that IR is just a part of a spectrum: gamma rays, x-rays, ultraviolet, visible, microwave, IR, radio. So microwaves make "heat" when they hit water molecules, for example. Visible light feels warm on your skin. So it can "heat" your skin, as another example. IR can do the same thing.

gamma rays can disrupt chemical bonds.

So, help me out here. What do you want to know? This is a huge topic.
Sorry for the marginal answer, best I could do.

anorlunda
Staff Emeritus
Also pleas tell us if you are interested in heat or temperature

hmmm27
Gold Member
There's nothing special about the infrared spectrum, excepting that - at the temperatures we're used to, on Earth - it makes up the bulk of thermal radiative emission/absorption.

Drakkith
Staff Emeritus
Is there a direct heat energy relationship between a heated object and infrared radiation? 1 to 1 relationship? Or can IR go up without there being much heat?
Assuming you are asking about the IR emitted by a hot object, there is absolutely a relationship. Most objects closely obey Planck's Law, which lets us calculate how much power is emitted by an object in various electromagnetic frequencies, including IR. In general, the hotter something is, the more radiation it emits and the higher the average frequency of this radiation is. So an ice cube emits less radiation than my big fat cat who's occupying my lap currently, and the radiation the ice cube emits is of a lower frequency on average.

This is why heating up a piece of metal turns it first red, then orange, then white and explains why it becomes increasingly brighter as it heats up. At first the temperature is too low to emit hardly any visible light, so the metal is just its normal color, though it might me scalding hot and you can feel the IR radiation it emits if you hold a hand close to it. Once the metal heats up a bit more it becomes a dull red and you can see it glowing, even in the dark. Hotter still and it becomes orange, then yellow, then white. As it gets yellow and then near white the amount of radiation it emits in the visible frequencies is so much that it could light up a room. As well it should, as this is exactly how an incandescent light bulb functions.

Hotter still, such as when arc welding, and you start to get appreciable amounts of UV radiation. This is why they say never watch someone welding or you'll hurt your eyes.

Note that as you increase the temperature the amount of radiation produced at ALL frequencies increases. So metal that's white hot is emitting more IR radiation than when it was red hot.

Also, keep in mind that the color vs temperature scales you see online usually work pretty well for your eye, but not always for cameras, as the following video demonstrates:

BvU, jim mcnamara and russ_watters
Your question has a couple of oops statements I think.
For heat let's use degrees K (or Celsius). Ok?
The first problem is that IR is just a part of a spectrum: gamma rays, x-rays, ultraviolet, visible, microwave, IR, radio. So microwaves make "heat" when they hit water molecules, for example. Visible light feels warm on your skin. So it can "heat" your skin, as another example. IR can do the same thing.

gamma rays can disrupt chemical bonds.

So, help me out here. What do you want to know? This is a huge topic.
Sorry for the marginal answer, best I could do.
I am interested in IR radiation therapy that some medical researchers experiment with, but was wondering if high levels of heat are needed for higher levels of IR radiation? Perhaps depends on the material? I know fireflies produce light between red and green and produce no heat at all. Are you sure visible light produces heat?

Drakkith
Staff Emeritus
I am interested in IR radiation therapy that some medical researchers experiment with, but was wondering if high levels of heat are needed for higher levels of IR radiation?
Yes. All that IR radiation generated is absorbed by the person and felt as heat. There's no way to get around this if the goal is to pump IR radiation into someone.

I know fireflies produce light between red and green and produce no heat at all. Are you sure visible light produces heat?
Fireflies don't produce light via incandescence, which is what hot objects like incandescent light bulbs, hot metal bars, and stars do. Incandescence is the emission of electromagnetic radiation (including visible light) from a hot body as a result of its high temperature. Instead fireflies use bioluminescence, where a series of chemical reactions directly release light within a narrow band of frequencies. For fireflies there are several peaks in the spectrum between about 550 and 600 nm (source).

And just to be extra clear, there is a difference between something hot producing light vs light producing heat. A laser doesn't use heat to produce its light (by that I mean that the laser isn't incandescing), yet even lasers that use a tenth of the power of a common LED light bulb can burn through wood. Visible light produces heat when absorbed, as does all EM radiation including IR.

I am interested in IR radiation therapy that some medical researchers experiment with
What exactly are you trying to do? I hope you're not planning on using it on yourself or anyone else. Under no circumstances should you ever perform any 'therapy' or any other medical procedure on yourself or others unless instructed to by a doctor. Even something seemingly mundane like IR therapy might lead to skin problems (or potentially something else) over time. I can't remember what it's called, but I do remember hearing something about the heat from laptops messing up people's skin from prolonged use. So now we have people with skin problems on their upper legs and groin from simply using a laptop every day.

BvU
No, no self treatments. Just curious if one could get IR therapy without burning oneself.

bioluminescence can never emit light in IR spectrum?

Drakkith
Staff Emeritus
bioluminescence can never emit light in IR spectrum?
Given the right molecules it might. It's just that IR radiation isn't a good way to 'see' due to its lower energy vs visible light. Visible light has enough energy to cause certain molecules to deform inside your eye, which is the first step in a cascade of reactions that enable you to see. IR, with its lower energy per photon, can't do this.

hmmm27
Gold Member
bioluminescence can never emit light in IR spectrum?
There's a deepsea nightmare fish that uses 708nm light (almost IR) to hunt prey

Everything at that depth has lost the red visual receptors (since red from the sun doesn't reach that far down, its useless).

So the fish has a flashlight : internally it starts off as blue-green luminescence, that absorbed by a protein which emits red, that through a brown filter that takes out almost everything, leaving only almost-IR. Which illuminates prey that can't see it.

But, the fish doesn't have any red receptors either.

Reflected light from the prey runs through a chlorophyll mechanism that upshifts it into the green which the fish does have receptors for.

As a final insult to sanity, the source of the chlorophyll is from the fish's prey's diet.

There's a deepsea nightmare fish that uses 708nm light (almost IR) to hunt prey

Everything at that depth has lost the red visual receptors (since red from the sun doesn't reach that far down, its useless).

So the fish has a flashlight : internally it starts off as blue-green luminescence, that absorbed by a protein which emits red, that through a brown filter that takes out almost everything, leaving only almost-IR. Which illuminates prey that can't see it.

But, the fish doesn't have any red receptors either.

Reflected light from the prey runs through a chlorophyll mechanism that upshifts it into the green which the fish does have receptors for.

As a final insult to sanity, the source of the chlorophyll is from the fish's prey's diet.
That's pretty interesting.