Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: hamza on 15/09/2007 21:02:02
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I am confused about this fact that infra red causes heat. according to what i know , infrared is long wavelength portion of light and therefore it should have low energy ( long wavelength is actually low energy. right??)So, if energy is low than why does it causes heat (which is energy)?..
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Yes, long wavelength is equivalent to low energy, by the equation Ε=hc/λ where the wavelength is λ. The infrared radiation is a result of the atomic oscillations in a substance due to temperature. The reason that the radiation is so low in energy is that most things are at a relatively low temperature. However, when you heat materials up the radiation they emit goes up in energy; iron is a perfect example. It changes from infrared to red to orange to white to blue and even ultraviolet and beyond. The increased frequency (decreased wavelength) is due to the higher frequency atomic oscillations at higher temperatures.
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Any object, at any temperature, will radiate a continuous spectrum of electromagnetic energy. Depending on the temperature of the object, its spectral peak may be at infra red (warm), red (red hot - burning) or yellow ( white hot - like a light bulb filament) etc. Even the coldest regions of space still radiate microwave energy. This relates to Mr Andrew's comments about vibrations within the material. Higher temperature implies more vibration. What we used to call the 'heat' in an object is now referred to as its ' internal energy'. The temperature is really a measure of the average energy of each molecule.
Rather than saying that infra red "causes heat" it is more meaningful to say that hot objects radiate with a peak of infra red energy.
The material is not really relevant. What IS relevant is the 'colour' of the surface, which can alter the amount of energy radiated or absorbed at various wavelengths and the actual temperature.
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Any object, at any temperature, will radiate a continuous spectrum of electromagnetic energy. Depending on the temperature of the object, its spectral peak may be at infra red (warm), red (red hot - burning) or yellow ( white hot - like a light bulb filament) etc. Even the coldest regions of space still radiate microwave energy. This relates to Mr Andrew's comments about vibrations within the material. Higher temperature implies more vibration. What we used to call the 'heat' in an object is now referred to as its ' internal energy'. The temperature is really a measure of the average energy of each molecule.
Rather than saying that infra red "causes heat" it is more meaningful to say that hot objects radiate with a peak of infra red energy.
The material is not really relevant. What IS relevant is the 'colour' of the surface, which can alter the amount of energy radiated or absorbed at various wavelengths and the actual temperature.
Everything ok, however IR radiation DOES heat an object hit by it. Example: when you approach a fire, you feel hot.
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One of the things to think about is the difference between feeling heat, and things getting hot.
The IR radiation from a fire will make you feel warm, but UV radiation is often not felt significantly at all until it has caused significant damage to your skin.
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Quantum theory was actually born of the study of black-body radiation. Max Plank developed a law to describe the distribution of radiation over the different wavelengths at different temperatures and to do so he had to assume that atoms had discrete energy levels...E=nhv.
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If you download the Wolfram Mathmatica free demo program it includes a blackbody radiation simulator.
http://www.wolfram.com/products/player/
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Quantum theory was actually born of the study of black-body radiation. Max Plank developed a law to describe the distribution of radiation over the different wavelengths at different temperatures and to do so he had to assume that atoms had discrete energy levels...E=nhv.
What I coloured in blue it's not exact, even if it's quite so; he assumed the energy of atomic vibrations, happened in discrete quantities, with E = hν. It was only Bohr who first proposed the atomic model with discrete energy levels.
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The fact that the individual quanta of the radiation are lower in energy than light is not relavent. it is the quantity of quanta that determines how much heat energy is produced for example we talk about light and infra red radiation a being emitted by bodies at several hunderds or several thousand degrees K. bodies that are very cold (say only a few degrees above absolute zero) emit microwave radiation as their natural radiation. however you know that wehen you put a glass of water into a microwave cooker and zap it with a lot of photons of microwave radiation it can get very hot indeed.
The question then arises why would the radiation naturally emmited only by something very cold be available in big enough quantities to make thigs hot. The answer is there are two principal ways of creating electromagnetic radiation one of them is called the thermal process ie heating something to the right temperature and letting it radiate energy. A tungsten filament light bulb is just such a source of electromagnetic radiation. The other sources are called non thermal because the quanta are generated by processes that generate specific quanta like the magnetron valve that greates the microwave rediation in the microwave oven the LED that creates light directly from electricity in a semiconductor junction or the florescent tube or lazer that creates light from a discharge in a gas.
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It all comes down to the type of interaction between electromagnetic (EM) radiation and the atoms or molecules that it interacts with.
"Heat" is caused by atomic and molecular motion. There are three types of motion: translational (moving from one place to another), rotational (rotating around a point) and vibrational (moving back and forth rapidly).
The reason we don't feel heat from visible, ultraviolet (UV), etc. is because it's TOO high energy. These types of EM radiation affect the electrons, stimulating them to higher energy levels. The electrons can fall back, but usually produce EM radiation of the same type (e.g., visible -> visible). Many times, the light gets converted to light of lower energy (visible -> infrared (IR)).
EM radiation of lower energy (e.g., IR and microwave) on the other hand stimulate molecular and atomic motions, precisely what we feel as heat. IR stimulates vibrations, while microwaves stimulate rotations. Every molecule requires a specific frequency of radiation to be stimulated, so if you just shine visible light on an white object, you don't feel much heat. With colored objects, a certain wavelength of light is absorbed, then released as IR radiation.
Dick
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The fact that the individual quanta of the radiation are lower in energy than light is not relavent. it is the quantity of quanta that determines how much heat energy is produced for example we talk about light and infra red radiation a being emitted by bodies at several hunderds or several thousand degrees K. bodies that are very cold (say only a few degrees above absolute zero) emit microwave radiation as their natural radiation. however you know that wehen you put a glass of water into a microwave cooker and zap it with a lot of photons of microwave radiation it can get very hot indeed.
This is the important point.
If you put a fully-absorbing object under a 500watt source of radiation then that object would heat up just as fast regardless of whether it was microwave energy, infra-red, visible light, ultraviolet, or X-rays.
The shorter wavelength/"higher energy" radiations have more energy per photon, but if you have sources of equivalent power you just get correspondingly fewer photons for the "higher energy" radiations.
In practice there are differences in penetration-depth, so in the case of a human radio waves would mostly pass through without heating you, the microwaves might heat you to a depth of some centimetres, the infra-red might penetrate a few millimetres and the visible light slightly less still. The X-rays may penetrate deeper again depending on their photon energy.
I can assure you that if you put your finger directly in front of a powerful video- or cinema projector lens then it feels very hot even though there's very little infra-red in the beam. A cinema projector typically has upwards of a 1000 watt xenon bulb!