1. Apr 1, 2006

PPonte

Why does my schoolbook say that infrared radiation is the electromagentic radiation with most thermal effect?

I thought that the more energetic a certain electromagnetic wave is, greater is her thermal effect.
It seems to me that infrared radiation is the electromagentic radiation with most thermal effect, because for the temperature that the objects on Earth are, it is the radiation whose percentage of absortion is greater, because the emition of this radiation is also the highest.

2. Apr 1, 2006

andrevdh

The interaction of light and matter is a quantum process - meaning that it occurs in packets, it is not absorbed (or emitted) in continous quantities. If you look at the spectra (emissions) emitted by atoms you will see spectral lines, indicating that the atoms emit only some enegies of light. Now the reverse is also true - the atoms only tend to absorb only the packets of light that they normally emit. The electromagnetic spectrum is divided into different regions each with different ways in which the rays are generated and detected like at one extreme we find radio waves generated by oscillating charges and at the other end we find gamma rays - extremely high energy electromagnetic waves which are created by oher processes. Matter alos responds differently when absorbing the different types of radiation - ultraviolet gives you a tan while microwaves makes water molecules oscillate to and fro. Infrared emissions are the emissions from bodies that are warm - any body that is warm emits such radiations. When these emissions are emitted the heat energy escapes in packets from the body. When these packets are absorbed by other bodies they experience an increase in their temperature due to the absorption of the infrared radiation energy.

Last edited: Apr 1, 2006
3. Apr 1, 2006

nrqed

Its because different physical processes will lead to different types of electromagnetic waves. For example, atomic transitions in most atoms will lead to emission (or absorption) of UV, IR and visible wavelengths.

Many nuclear transitions lead to emission of gamma rays.

Now, atoms which are moving around due to thermal agitation emit mostly in the infrared (at ''normal'' temperatures...If you go to sevreal thousand Kelvins or beyond, thermal agitation will lead to visible wavelength emission or even UV and so on). This radiation is due to their thermal agitation and not to atomic transitions.

4. Apr 1, 2006

nrqed

Just to add something: I have talked about emission but similar reasoning applies to absorption.

If the molecules in your hand are bombared with infrared radiation, it will tend to move them around, creating heat. If they are bombared with gamma rays (lets say), the gamma rays will be tend to break the bonds and other nasty effects.

5. Apr 1, 2006

PPonte

Thank you. This was the precise point.
But look, although the gamma rays break bonds, will not they also heat my hand?

And if a beam of visible light is inciding in my hand? It will not break bonds. But it is not also true that it will heat my hand and not just more than an infrared beam would do, because an considering percentage of the photons will not be absorbed?

6. Apr 1, 2006

nrqed

To get really to the core of the answer would invoke the phenomenon of resonance. Are you familiar with this?

The key point is that, sure, all wavelengths will create a little bit of thermal agitation. But a *large* effect is only produced by a certain range of wavelengths, in the IR part. Ìf you push and pull at certain frequency on a child on a swing, you will make it move back and forth. But certain frequencies will cause larger oscillation. It is not true that the faster you push and pull the higher the child will swing.

Patrick

7. Apr 1, 2006

PPonte

Perfect. Thank you very much!