# Does light give off heat?



## kicken_bright (Jan 28, 2009)

So I have heard about this Wicked Lasers Torch and similar flashlights that can light things on fire! My question is this, do light waves have energy to set things on fire? Or is it the heat waves being produced along side the light waves? If it is actually the heat waves doing the fire thing, then isn't it true that energy is being wasted in the form of heat? Shouldn't a really efficient light produce minimal heat? :thinking:


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## StandardBattery (Jan 28, 2009)

There are no 'heat' waves traveling along side the light waves (well not as part of the wave, lets pretend the waves are traveling through a vacume). It's more a matter of the heat produced by various materials as they absorb the electromagnetic radiation (the light waves). The heat that might be produced is dependent on the frequency of the radiation and the material.


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## lctorana (Jan 28, 2009)

But is that strictly true for the Infra-Red part of the emitted spectrum?

After all, that's part* of how bar radiators give off heat - radiant energy.

_(* as well as conduction and air convection)_


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## kicken_bright (Jan 28, 2009)

I was under the impression that light waves would not energize material to produce energy in the form of heat, light waves should bounce off matter so that it can be seen by our eyes. I thought infrared waves were more in the department of heating and energizing matter.


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## kicken_bright (Jan 28, 2009)

Should pure light waves ranging from about 450-750 nanometers cause matter to heat up?


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## LukeA (Jan 28, 2009)

lctorana said:


> But is that strictly true for the Infra-Red part of the emitted spectrum?
> 
> After all, that's part* of how bar radiators give off heat - radiant energy.
> 
> _(* as well as conduction and air convection)_



It's all electromagnetic radiation, it's all energy. It's why it's uncomfortable to hold a hand in front of my 10-Cree light. There's several watts of radiated energy hitting your hand in (almost completely) the form of visible light. 

It's why the efficiency ceiling for white light is 241lm/W. A lumen is a certain amount of energy being radiated.


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## Mdinana (Jan 28, 2009)

kicken_bright said:


> Should pure light waves ranging from about 450-750 nanometers cause matter to heat up?


 
First off, there is no "pure light" wave. The range you're speaking of is just the range of EM radiation that the human eye can detect. technically speaking, it's no different than any other part of the EM spectrum.

Second, yes it can heat matter up. Heating an item is just a matter of absorbing energy so that the molecules vibrate faster. The equation E=hc/lambda gives the energy of a photon related to wavelength. Infrared, being below the red in the visible light spectrum, has more energy (that's why it's used to heat things up) than visible light. But if you ever get the chance to stand in front of a searchlight, you'll find they get hot pretty quick. Or go walk past some decorative lights in a cool rain - you'll see that they steam the rain off. Lastly... go touch a lightbulb when it's on. yup, it's hot.


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## Timothybil (Jan 29, 2009)

There are two forces at work here: 1. Yes, there is infrared radiation being given off by the light source, moreso with incans than LEDs. Some of that infrared will get transmitted out of the front of the flashlight, just like the visible light is. One of the reasons LEDs are more efficient than incans is because they produce less infrared per input watt than incans do. 2. When em radiation strikes a surface, three things happen. Some is reflected, some is absorbed, and some passes through. If the surface is a mirror, most is reflected, some is absorbed, and essentially none passes through. [The reason mirrors used in very high power lasers are silvered on the front is to cut down on the absorbsion. If the silvering was on the back side the substrate (glass, plastic, etc.) would explode from the amount of energy it would absorb over a very short period of time] A surface like a piece of paper will obviously reflect less energy away, and will absorb more. Sometimes even enough to reach ignition temperature.


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## kicken_bright (Jan 29, 2009)

So far I like where this thread is going! I have already learned a lot! But I still have a question. In the case of the wicked laser torch, is it the visible light waves or the infrared waves that are igniting things. I understand it is probably a combination of both, but is it logical to assume that there are a lot of infrared waves being produced? Would a 4,000 lumen LED or HID light things on fire?


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## 65535 (Jan 29, 2009)

Within a certain range all EM radiation will cause matter to heat up. IR-UV light (which includes all visible) can definitely heat things up. 

Pure lumens and brightness won't determine burning things. If you've ever noticed the only material ever burned by ultra high output mag mods and other burners is within a few inches of the light. The intense amounts of energy culminated at that spot creates enough heat to cause combustion.

BVH's 20KW carbon arc lamp won't light stuff on fire, but is the largest lamp around. It's simply doesn't have the culmination for such heating. Lasers are highly culminated.

The other thing about incan mag mod burners is the 150 watts you put into it turn into pure EM energy. From IR-near UV the incan lamp produces much more energy than just the visible lumens. If it produces 4k lumens and lights stuff on fire, about 100 of those 150 watts is IR energy which is absorbed even more readily than visible light and burns the paper or material faster.


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## lyyyghtmaster (Jan 29, 2009)

Yes, all EM waves will cause heating in the right material that is able to absorb them. Microwave rays at 2.45 GHz (122,450,000 nm) heat up water plenty fine in the microwave!



Mdinana said:


> Infrared, being below the red in the visible light spectrum, has more energy (that's why it's used to heat things up) than visible light.



Not to nit pick too badly, but it's actually the other way around. A photon of 400 nm violet light will have twice the energy of an 800 nm IR photon. It's just that IR photons are readily absorbed by many things and are available in great abundance in an unfiltered beam of incandescent light.

If an overdriven incan is putting out 50 lumens of light per watt of input, then its efficiency in creating visible light would be just under 20%. The rest is going (mostly) to infrared and (much less) UV. 

If you have one, try driving a royal blue emitter at .35W or more and putting its dome up to your upper lip. You will feel heat, and there is negligible light in anything other than the blue-green / blue / indigo range coming out of that! It's kind of strange when you realize what's going on! I'm not sure how safe it would be to do that long term, though....


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## broadgage (Jan 29, 2009)

A high power incandescent light source can certainly ignite paper, wood etc, and this has been demonstarted both to prove a point, and in a number of accidental fires.
Incandescent sources, and some types of HID produce considerable infra-red, and it is mainly this that ignites materials, the contribution from the visable rays being limited.

In the case of a source that produces very little infra-red, such as LEDs, then the visible light, could ignite materials, but in practice it is unlikely except in extreme cases.
The visible light is not hot in itself, but when absorbed by a non-reflective surface, is turned into heat which increases the temperature of that surface. A perfectly reflective surface would not become heated, and an imperfectly reflecting surface would be heated to a much smaller extent.

Lasers can readily ignite materials because the energy is concentrated in such a small area.
Other (visible only, no IR) light sources cant normally concentrate enough energy in a small enough space to start a fire, though it is no doubt possible.


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## electric sheep (Jan 29, 2009)

A photon(packet of light quanta) will strike the electrons in the probability cloud that makes up the surface of matter and cause them to change energy levels, when the electrons change levels they absorb and emit photons of different wavelengths. Energy is delivered absorbed and or reflected but the energy levels change dependent on the energy of the photons determined by their wavelength.


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