Naked Science Forum
General Science => General Science => Topic started by: cuso4 on 13/05/2003 13:23:49
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As we know, UV light ,even visible light have higher frequency than infra red.But why do we use infra red (as we always mentioned the heat with it.e.g. The night monitor using infra-red radiation)to cook which is the best way(excluding microwave)instead of UV light and visible light.Why things can't be cooked in sunshine for our normal life.
Angel
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Probably it's that the higher frequency wave have more energy in them however it's also more penetrative and therefore that food or whatever do not have time to absorb the energy the waves are emitting.
X-rays and Gamma rays are very penetrating and will go right through the food without stopping. Microwave and Infra-red have lower frequency wavelength are long so not as penetrative. Light and UV mmm... I suppose it can but not maybe as efficient as Microwave or infra-red where more energy can be absorb.
Tom
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Light is electromagnetic radiation. Radio 4 and Neighbours (unless you have cable !) are conveyed to your living room in the same way that light reaches the earth from the sun - as electromagnetic waves with a frequency and a wavelength.
The light that we can see (visible light) comprises just a small range of wavelengths in the entire electromagnetic spectrum. But the sun also pumps out a range of wavelengths that we can't see. To come back to our earlier analogy, the tuner on your radio can only allow you to pick up a small range of radio frequencies. But taxis, police cars and aeroplanes use a different range of frequencies out of the range that your radio can detect. So it is with your eye, and this is the reason that you burn at the beach, even on a cloudy day because ultraviolet light, which your eyes cannot detect, will readily penetrate clouds. Thankfully most of it is mopped up by the ozone layer, otherwise life couldn't exist on the planet surface owing to the destructive effects of UV waves.
To consider now the question of cooking, infra-red radiation is the commonest means by which we heat things up. When you put a chicken in a hot oven, or you stand too close to a bonfire on Guy Fawkes night and you can feel yourself getting burnt, you are seeing the effects of infra-red radiation.
Hot bodies radiate energy in the infra-red spectrum. As they get hotter they also start to emit radiation within the visible spectrum which is why a hot piece of metal begins to glow red and eventually 'white' hot.
The microwave in your average kitchen uses very high frequency light waves (usually about 2.5 GHz or 2500 MHz). The wavelength of these waves is about 12 cm (given by c=fl where c is the speed of light and f is the frequency).
A useful property of radio waves at this frequency is that they are very well absorbed by the chief components of food, including water, fats and sugars, but are poorly absorbed by plastic and glass. This has the advantage of maximising the heating effects within the food. As the molecules which make up the food mop up energy from the radio (micro) waves they begin to move around more and hence become hotter.
Because radio waves penetrate well into food (although it can be patchy) you are not reliant on heat seeping slowly into the interior of something from outside, as occurs with a conventional oven. For this reason microwaves are much faster. To prevent local 'hotspots' and 'cold-spots' which are a consequence of the fact that any wave has a high energy (maximum displacement) and a low-energy (minimum displacement) segment, microwaves are equipped with a turntable to ensure thorough heating throughout the dish.
For top marks can anyone explain, given that microwaves a just a form of light, why you can see what is going on inside the microwave if it is equipped with a guard on the window to stop the microwaves (light waves) escaping ?
TNS
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Wow, good post there ! Must have taken you ages to type :p
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Also I don't know too much, but for some reason, infrared makes atoms want to bounce about and dance!!!!
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For the first time, I studied the door of my microwave and found out that on the inside of the door, there is a grey-ish material with small circular patterns all over it.
You know in some hot countries, people put a film(that's how I describe it anyway), one side is silvery colour and the other is black with the same circular pattern, on their car window to stop sunlight entering the car. On the outside the car windows act like mirrors because they reflect the light, but people can see from the inside. I think the microwave door must works the same way.
Angel
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Exactly, except micowaves are the waves that bounce off, not light waves ^_^
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Nope - remember microwaves ARE light waves - so why do the microwaves bounce off, but not the visible light waves ?
TNS
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My guess is that because microwaves are larger (as TNS said, about 12cm), so cannot get through the circular patterns and is reflected by the grey-ish material whereas the visible light waves are small enough to get through.
By the way, the formula E=hf (where h is Planck's constant) alos come into all this, higher the frequency the more energy the photon has.
Angel
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Yep, you're on the right lines.
The wavelength of the microwaves is about 12cm. To a 12 cm wavelength microwave the 2-3mm diameter holes in the metal on the door of the microwave oven resemble a solid piece of metal. But light from the visible part of the spectrum has a much smaller wavelength and so can get through. Hence you can SEE what is going on inside the microwave, but the dangerous microwaves are largely prevented from escaping !
Clever eh ?
TNS
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Quite[:)]
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do I detect a hint of sarcasm [;)]
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TNS, any idea which metal the microwave door is made of? Steel? Aluminium? And what about the grey-ish material?
Angel
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Pat, what would ever make you think that anyone uses sarcasm here[:D]?
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quote:
why do we use infra red to cook
Things tend to absorb light at the same frequencies as they emit it - so just about everything will emit infra red if you heat it up, so it will also absorb it, making it a good range of frequencies if you are heating somethign up. Also for the aforementioned reason it is easy to make (in fact about 90% of the energy produced by a normal blub is infra red).
Why do they not use a visible bulb?
A couple of reasons, visible bulbs have to get a lot hotter than an infra red one, so the filament is more likely to evaporate away and die, and if you were goign to put enough visible energy into an oven to cook with it, then you looked in, your eyes would focus the light onto the back of your retina - cooking your retina...!
quote:
Why things can't be cooked in sunshine for our normal life.
The earth is luckly far enough from the sun that it doesn't happen normally, because you loose heat fast enough for the sun not to get you hot enough to cook. If you moved to mercury, you would be able to do your barbequeing just by putting the meat outside as the sun is 5 times stronger.
You can simulate being closer to the sun by building a parabolic reflector (or a big lens), which will concentrate the sun into a smaller area.