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Author: Subject: What is stronger? Sunlight or artificial light?
LearnedAmateur
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[*] posted on 9-4-2018 at 06:11


Except none of the planets are black-bodies, which are objects that absorb radiation perfectly hence their name. Planets directly reflect a lot of the light received by the Sun, especially ones with lots of white like Earth’s clouds and icy parts.



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Vomaturge
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[*] posted on 9-4-2018 at 13:45


If you had a perfectly black object in a vacuum exposed to sun, it would get hotter and hotter until it emitted as much energy as it was absorbing. The sun is about 5800 Kelvins (~5500C). At that temperature, each square centimeter of the sun emits about 6.4 kW of light, infrared, and UV. If you exposed a "blackbody" in a vacuum to that much heat and light per square centimeter, it would heat up until it reached the temperature of the sun. To get that much heat and light, it would have to be very near the sun, maybe even in it (yes, I know it would also not be in a vacuum, and would have convection or conduction heating too if it was in the sun) But, if you were further away, like near earth, the insulated blackbody would only get maybe .14 watt per square centimeter of heat from sunlight, because the sun is so far away and the view factor (https://en.m.wikipedia.org/wiki/View_factor) is very low. The object would heat up until it was radiating .14 watts per square centimeter, which is the same as 1.4 kW per square meter. To radiate that much heat, it would have to heat up to about 396 Kelvin, or 123C. But remember, sun only hits one side of an object at at time. If the "blackbody" absorbed heat on one side, but emitted it on both sides(e.g.. if it was a good thermal conductor, or changed orientation regularly), it would only have 700W per square meter to radiate, which would require a temperature of 60C. If it was a sphere, it would have four times as much area as was presented directly to the sun. To radiate the average of 350 W/M^2 it's temperature would be 280 K, or 7Celsius, with some spots hotter and some cooler. Hey! It's about 5C outside right now. I suppose the Earth is almost a blackbody, and the part I'm in is at an almost average position at the moment.

Okay, back to RawWork's original question: which source of light/heat rays would work best for heating stuff? Obviously, you can get a lot stronger of a source of thermal radiation from a heat lamp, burning charcoal, etc if it's up close than if it's far away. If they are close enough, they might be more intense then the sunlight reaching Earth. However, if you try to focus a light source using a lens, the size of the focused spot depends on how large and close the source is. If you have a magnifying glass or magnifying mirror, you can give this a try right now. Try focusing the light from the sun into a bright, hot, point. Then, try with a frosted or fluorescent light from a distance of 2 or 3 meters. You will get a much bigger (and probably lightbulb shaped) spot of light. Now, if you bring the magnifying glass very close to the bulb (like 20 cm), you will have a hard time focusing it, and if you do get it to focus, you will get an extremely large spot, but it won't be any brighter. With a filament bulb, you might be able to get a smaller brighter spot, but it will never be as bright as the spot from focusing the sun. Even if the light source is put so close to the lens that it is stronger than the sun, that will just create a bigger spot. It won't be brighter. Another thing to keep in mind is that most of the energy from a lower temperature source like burning charcoal will emit more infrared energy then visible light. A glass lens will absorb this energy, and get hot, but it won't transmit it. If you use a lens to focus the heat from burning charcoal, you will end up with a rather large spot of warm light, that likely won't even be hot enough to boil water. If the source is a barbecue sized charcoal fire a meter from the lens, the spot might even be cooler than the radiant heat without the lens. You can focus radiant heat from a concentrated but distant source, but you can't focus an up-close diffused source very well, even if both are just as hot and bright without a lens.

Lasers are kind of in a class by themselves. They can be focused into a very fine point. They also can be emitted in a fine beam, which can travel huge distances while staying bright. They would seem like an ideal way to heat things. Unfortunately, they are not very efficient, and the ones which are powerful enough to be useful are big, expensive, and use a lot of power. A normal laser pointer will hardly heat anything, and the "high-powered" lasers that are just a watt or two are no better then a magnifying glass and sun.

The best way to heat an experiment would be either to use a lens/mirror and the sun, or else have a heat source very close to the target. If you put your reaction container in a bigger insulated furnace (with heating filaments, an electric arc, fire, etc) then the container will get a lot of radiant heat even without any way of focusing it, because it's so close to the source. Also, there will be hot air and hot surfaces to further heat whatever you are trying to heat up by touching it. Finally, some of the heat that doesn't get absorbed by the "target," or that gets radiated back from it, can just be absorbed by the walls of the furnace, and conducted or radiated back.

Edit: [rquote] Why people have to wear face shields when working with torches? [rquote]
for one thing, there is very little atmosphere to filter UV light between your eyes and the torch (I'm assuming you're talking about welding or cutting metal) for another, you would also need some kind of face mask if you had a job involving looking straight at the sun. Intense light sources are focused to a tiny point by your eye lenses, and could get hot enough to burn the very sensitive tissues in the back of your eye. If your job involves welding, you have to look closely at what you're doing. To do that safely, a mask is in order. Also protects your face from drips of metal or oxides.

[rquote]you will imagine it as kW for one hour, as it's easy to convert.[rquote]
It looks like you've been studying energy and power in the last week or so. Keep up the good work; these principles will be very useful for understanding all kinds of scientific concepts:)

[Edited on 9-4-2018 by Vomaturge]
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AJKOER
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[*] posted on 10-4-2018 at 12:21


With respect to chemical activity induced by light, as opposed to heating by direct light, there is an interesting construct of so called 'diffused' sunlight (as opposed to direct sunlight) like what can occur on a cloudy day. See, for example, "TiO2-NiO p-n nanocomposite with enhanced sonophotocatalytic activity under diffused sunlight", abstract at https://www.ncbi.nlm.nih.gov/pubmed/26968646 and also "Diffuse Light for Better Plants" at https://www.growertalks.com/Article/?articleid=20729 .

[Edited on 10-4-2018 by AJKOER]
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