A few questions about the electromagnetic spectrum

[itsthemac]

Can any wavelength of the electromagnetic spectrum cause objects to heat up? If you shined a bright enough flashlight at someone through a vacuum, would they start to feel warmer? What about if you directed radio waves at someone (If they were intense enough)? Could that cause them to heat up?

And would certain areas of the spectrum cause objects to get hotter faster than other areas? It seems to me like it would make sense that the higher-frequency waves would carry more energy, and thus cause something to heat up more than lower-frequency waves. When you stand in sunlight and feel heat from the sun, is that mainly because of the visible light from the sun hitting your skin and causing it to heat up? Do UV rays account for more of the heat that you feel than the visible light rays?

Also, I was trying to find answers to this online before I posted here, and I came across a yahoo answers question, where someone asked "How does the sun's heat get to Earth if heat needs a medium?"

The response was "Radiant heat needs no medium, it is nothing more than long wavelength light... infrared light as a matter of fact. This is the heat you get on your skin when standing out in the noon day Sun."

Is this true that it's just infrared light that's accountable for you feeling heat from the sun? Why would this be the case? Wouldn't the higher-energy wavelengths of light produced by the sun like visible light and uv rays be the main culprits?

thanks

 

[Phyisab****]

Well you could assume that the absorption spectra of the human body is (maybe poorly) approximated by that of water. http://en.wikipedia.org/wiki/File:Water_absorption_spectrum.png . Infrared radiation is commonly associated with heat because of the thermal radiation of objects near room temperature http://en.wikipedia.org/wiki/Thermal_radiation. Judging from the above absorption spectra of water though, I do not think it is accurate to say that the heat you feel from the sun is only from infrared radiation.

The amount of heat transferred will depend on the energy of the radiation as well as the efficiency of absorption.

 

[itsthemac]

Well you could assume that the absorption spectra of the human body is (maybe poorly) approximated by that of water. http://en.wikipedia.org/wiki/File:Water_absorption_spectrum.png . Infrared radiation is commonly associated with heat because of the thermal radiation of objects near room temperature http://en.wikipedia.org/wiki/Thermal_radiation. Judging from the above absorption spectra of water though, I do not think it is accurate to say that the heat you feel from the sun is only from infrared radiation.

The amount of heat transferred will depend on the energy of the radiation as well as the efficiency of absorption.

Interesting, thanks. I was surprised to see that, judging by that absorption spectrum of water, the human body absorbs almost all of the light around visible light very well, but barely absorbs visible light itself. If there is any truth to the comment about infrared light being the main cause of our feeling heat from the sun, this would definitely help explain why that is.

 

[Phyisab****]

You would also need to look at the solar spectrum which makes it through the atmosphere. I was kind of assuming though that the answer you found was not fully correct. I think they were making the mistake of thinking only infrared radiation carries heat. That sounds really stupid if you think about it but it is a common misconception I've heard that whenever em radiation is associated with heat, you must be talking about IR.

 

[itsthemac]

You would also need to look at the solar spectrum which makes it through the atmosphere. I was kind of assuming though that the answer you found was not fully correct. I think they were making the mistake of thinking only infrared radiation carries heat. That sounds really stupid if you think about it but it is a common misconception I've heard that whenever em radiation is associated with heat, you must be talking about IR.

It's frustrating when something's a common misconception and you're trying to learn about it for the first time and you keep finding answers from people with that common misconception, like I did with that yahoo answers post.

So just to set the record straight for my own curiosity, if there were a hypothetical object that absorbed the entire range of the electromagnetic spectrum, and had the same absorption at all parts of the spectrum, would it get hotter faster as higher frequency em radiation was emitted at it, as compared to when lower frequency em radiation was emitted at it? This is what my intuition tells me, but I'm still in the self-doubting early stages of learning about this topic, so I'm desperate for confirmation.

 

[Phyisab****]

It would depend what the spectrum of the radiation you are sending at the object looks like and the intensity at different frequencies. If you absorb the same amount of photons at two different frequencies, E = hv so yes higher energy photons carry more energy. Usually though you won't find the spectrum defined as a relative number of photons that varies with frequency. If the spectrum gives intensity as a function of frequency, then you are directly given the power carried at different frequencies. In that case, the same amount of power at different frequencies is still the same amount of power.

 

[Bob S]

Read about microwave heating of water (e.g., tissue) at 2.45 GHz (e.g., magnetron tubes in microwave ovens) at

http://en.wikipedia.org/wiki/Microwave

My brother was in charge of a Nike missile site near New York City, and they used to occasionally focus their microwave radar antennas (dishes) at birds in the trees about half a mile away. The birds quickly flew away.

The military has a microwave "active denial" , "directed energy" or heat-ray microwave system:

http://en.wikipedia.org/wiki/Active_Denial_System

Unlike IR or visible light, the microwave energy penetrates clothing.

Bob S

 

[fluidistic]

I've read the thread, good questions and good answers. I'm no expert at all being myself a student of Modern Physics so feel free to correct me and teach me some.
I would like to give the following information: http://en.wikipedia.org/wiki/File:Solar_Spectrum.png. In words, the Sun radiates the most in the visible spectra. Photons in the visible spectra carry more energy than infrarred, micro waves and radio waves. So they should heat more a black body. However as you pointed out, the human body isn't a black body and it might behaves more or less like water, though I'm really unsure of this afirmation. I'm pretty sure than black humans are very different from white skinned humans. The black skinned humans would absorb light in the visible which would in turn heat them much more than white skinned men.
Apart this, yes water absorbs photons with almost any wavelength except in the visible spectra. I don't really like wikipedia's picture provided in this thread for the detailed visible spectra. Take a look at http://www.btinternet.com/~martin.chaplin/vibrat.html (scroll down the page, last picture). You can even see that water absorbs more photons with wavelengths corresponding to the red color than any other in the visible spectra and the least absorbed are photons with wavelengths corresponding to blue. Hence I might even guess that's why water appears a little blue, but I might be wrong.

Almost last thing: I'm not sure that photons carrying an enormous amount of energy do heat up more than visible-infrarred light's photons. If you have the Compton effect taking place or maybe even the photoelectric effect, I'm not sure real bodies would heat up as much as with visible light. It probably depends on the bodies.

What about if you directed radio waves at someone (If they were intense enough)? Could that cause them to heat up?

Yes. As those waves have very, very few energy, it would require lots of them (great intensity) to heat up someone. Since we have a lot of water in our body and that water absorb radio waves, the answer to your question is yes. Unless I'm missing something like a diffraction of those waves with our body... I'd appreciate a doubt-clearing on this, if someone can provide it.Last thing: notice that different bodies absorb differently the photons. For instance water molecules absorb microwaves while when you shred light on a metal, mostly its free electrons "absorb" the waves and not the molecules themselves. I'm not sure I should use the word absorb or collide, but the electrons on the metal's surface interact with the light, not really the molecules/atoms themselves which differ from the case of water.Edit: to Bob s: very interesting.
However I think that IR penetrates clothing, or some of it. Take a look at the wikipedia's picture of http://en.wikipedia.org/wiki/Black_body#Radiation_emitted_by_a_human_body.

 

[Bob S]

Edit: to Bob S: [The microwave stuff is] very interesting.
However I think that IR penetrates clothing, or some of it. Take a look at the wikipedia's picture of http://en.wikipedia.org/wiki/Black_body#Radiation_emitted_by_a_human_body.

The military has found that the most efficient way of converting electricity to directed energy (heat ray) weapons is by using microwaves (similar to the ones in your microwave oven). There is also the airborne laser weapon (which I am not considering here).

Bob S