People think of light as the visible stuff we see by, this is how we all knew light once until we discovered there were other kinds of light outside the range of what we see. Don't panic, I won't go into quantum physics (but it IS interesting!)
Most people now know of UV and infrared as light because they've been told about it, many even know the number given to 'the speed of light' but lack an understanding of other types of light such as microwave, x-ray, radio waves and so forth, and they also don't really know what the speed of light actually means.
The speed, the number they hear is a definition not a fact - 'light in a vacuum travels at 299792458 metres per second' (we used to be taught it was 2.8x10^8ms-1, accuracy has improved) - it's light travelling in a vacuum that travels that speed, and we use than in calculations characterized by c .. in water though it travels at 2.25x10^8ms-1. Light can travel much slower than what people think.. the 'speed of light' number we here is the supposed maximum but right now all the light around you is actually going slower than that speed.
Also in physics "light" can be the part of the electromagnetic spectrum we see, subjective visible light (400-700nm), or it can be the entire electromagnetic spectrum. Both are correct depending on context.
I say subjective visible light because it's only human perception that makes this 'visible' - some people see more of the spectrum, some less - so what you see as 'light' can be different to the next person.
but back to what light actually is, it's basically energy that travels in straight lines and I want to talk about it because some of the misconceptions are being used to support some extraordinary claims about the world around us.
Light as energy shoots off from it's source and travels along until something absorbs it, when it's absorbed it imparts the energy it carries to the thing that absorbed it, causing the thing to have more energy than it did, we call this 'heat'.
Take for example visible light, glass is transparent to visible light so light travels through it.. it travels through air, glass, the lenses of our eyes and it is absorbed by the chemicals in our retina and we use the chemical reactions that occur to see. However visible light does not travel through bricks, wood, sheets of metal or other such things we call opaque and instead is absorbed by these things
We can use this characteristic of the interaction of light and matter in some interesting ways as all things absorb or pass different lights through in different ways.. some things selectively absorbing specific wavelengths while passing others (they're transparent to these wavelengths) and some absorb only a small part of specific wavelengths, grabbing some of the energy while letting some of it through. In physics, photography and organic chemistry we use these traits just as we do in our eye to determine the nature of things. A gas chromatograph 'sees' stuff using wavelengths of light that our eye cannot, and we can compare what they see to stuff we know to determine chemical compositions..
There's another interesting thing light can do interacting with matter, some things can absorb the light of one wavelength and re-emit it as another, we call this fluorescence - shining a UV light onto certain dyes or minerals will cause them to absorb the UV and re-emit it as a visible wavelength that we can see.
The thing about all this is, the things that absorb light heat up when they do so, we're most familiar with this when we're talking about IR or Infra-Red light.. we know this as 'heat', we know this because we're taught this - but knowing this fact can lead to some major misunderstandings which really need to be resolved.
ALL light has the potential to be 'heat' in that it can cause the thing that absorbs it to heat up, it's just a matter of intensity. We know IR as heat because we've been familiar with heat for a long time and IR was the first light we found outside the visible spectrum. Knowing it as heat has led us to conclude (wrongly) that it and it alone is heat..
We know if we stick our hand against a hot iron or stand in fire we'll get burned. the reason the iron burns us is it's hot - it's absorbed a huge amount of energy from somewhere and delivered it to us in a huge blast that our body couldn't dissipate fast enough. The energy passed to us through 'conduction' .. but we also know standing too close to a fire or holding our hand too close to an iron can also cause burns, this is not by conduction but instead from radiation.
It literally causes radiation burns. Let that sink in. it is a radiation burn, the part of the electromagnetic spectrum, the radiation that burned you was infrared. The reason it burned you was it was an intense .. massively bright (invisible) light - a massive amount of energy that your skin absorbed, raising it's temperature faster than you could dissipate it and you get burned.
Other sources of radiation can burn you too.. ultraviolet, microwaves, x-rays, gamma rays - all these will burn you if they're intense enough. This doesn't mean you'll get radiation burns from being exposed to these at low level, just as low level infrared doesn't burn us but in fact keeps us alive by stopping us from freezing... and some of these types of radiation have benefits.
UV is generally painted as a bad guy for causing skinburn, but the fact is we need UV to manufacture the incorrectly named Vitamin D (it's actually a hormone not a vitamin) when the melanin in our skin converts precursor chemicals to this hormone - critical for our wellbeing. An appropriate amount is good, a lage amount becomes heat and we suffer a radiation burn. It's not just nuclear energy that causes radiation burns, ANY energy source - radio waves can cause radiation burns if they're intense enough. In fact this was how the heating effect of microwaves was discovered and how we came to have microwave ovens.. radio experimenters noticed those particular radio waves caused things to heat up as they absorbed the wavelengths ..
Interestingly, the Chernobyl nuclear facility explosion has also taught us things we didn't know about ourselves. They've been sending probes into the reactor core - a place considered extremely dangerous to us and they've found plants and fungi inside. Amazingly these guys are living exposed to massive levels of hard radiation with no ill effects, once species of fungi never seen before was of particular interest in that it fed off the gamma radiation. As we all know plants use visible light radiation in photosynthesis to convert CO2 and water to sugars.. seems this fungi was using gamma radiation as food too, it was a radiotrophe. Not only that, when they looked they found the fungi was using melanin in the chemical pathway to make it's food. It's quite possible having learned this that human melanin also can derive energy from gamma radiation (no, don't go playing in reactors folks..) No one ever considered gamma radiation could actually be used by our bodies.
No one generally thinks of microwaves as 'heat' .. put a glass in a microwave it doesn't heat up, nor do most plastics - they're transparent to microwaves so they just shoot straight through those materials - but water isn't transparent to microwaves - if we could see microwaves, water would look pitch black because it absorbs microwaves really well. Similarly if humans only saw using UV light, we'd not be able to see through glass because glass absorbs UV extremely well. Think of it like this, red glass looks red to us, but if we could only see red light, red glass would look clear. Clear? ;)
Now if you've a passing interest in the sciences you'll have bumped into stuff about carbon dioxide (CO2) and how it absorbs and traps heat - NASA has an article here that's worth reading on the matter.. go look at it. They'll tell you CO2 absorbs this energy then re-emits it - as the same energy, IR. This is a very strange thing to say. It's true CO2 absorbs infrared, just as water and water vapour does. But it causes these things to heat up, and heated up gas basically results in an increased pressure as the molecules with this added energy start zipping about faster. When they do this, they bump into other molecules of other gasses and cause them to also move faster - the physics is the same as billiard balls - slam one into another and they both move off with a share of the energy. What that NASA article is suggesting would be like say only red billiard balls can move, and if a red one hits a different coloured one it won't cause them to move. OK I know this is NASA, but they're still wrong. Any physics student will know this, it contradicts everything we know about gasses to suggest otherwise.
They also misrepresent CO2 by saying they're some sort of rare special gas - as you've learned above many things absorb different amounts of different types or wavelengths of radiation, and in fact oxygen is pretty good at absorbing UV. When it does, it gains energy and heats up, just as the CO2 molecule does when it absorbs IR. Nitrogen absorbs different wavelengths again - this is how we're able to tell what gasses exist out there in space, by looking at what parts of the spectrum they absorb!
A few facts: CO2 forms 0.04% of our atmosphere. O2 and Nitrogen together form 99% of our (dry air) atmosphere with water varying from very little to 5% or more.
We know the sun's output, the radiation it emits varies, we know it's variance is not huge, but he HAVE learned the spectrum changes and radiation wavelengths change.. a bit more UV across a period, a bit more IR at other times, more gamma rays or less, it changes across both long and short intervals.. this is relatively new information to us and we've not yet worked out the implications - but consider - carbon dioxide is only 0.04% of our air and O2 + N is 99%.
What do you think? Do you think it makes sense that minor fluctuations in energy input that could change the total heat in the system would affect 99% of the atmosphere most, or would it affect the 0.04% made up of CO2?
Remember 'heat' is not infrared, but just the effect of energy absorption. Yeah, I thought so too.. so do a lot of physicists.
Another thing to remember about gasses is they're refrigerants - maybe not always phase change refrigerants (turning from liquid to gas like water does), but they do what a refrigerant does.. they carry heat away from the source. We all know hot air rises - this is refrigeration. gasses that can absorb energy and move it, do. A gas that absorbs infrared, like CO2 is a very good refrigerant.. it absorbs the heat and rises quickly (because it's hot-state pressure is higher) and when it gets to a cooler place it dumps the heat and descends. Using 'heat' to drive refrigeration is an old idea - I actually saw an Icyball fridge out in Guildford near where I live recently and was seriously considering adding it to my collection of heat driven fridges but the $300 put me off.. I know it's cheap given the thing is near a century old, but I couldn't justify it.
The point is if a gas can't absorb a wavelength it lets it straight on through, same as glass lets visible light straight through, and only when it hits something that can absorb it will it do so. If our atmosphere was unable to absorb IR it would all pass straight through until it hit the ground or water where it would be absorbed.
Also just as I mentioned intensity of light causes burns most people don't know visible light can also burn you if it's bright, intense enough. In old movies they used tons of makeup not just to look pretty but because they used carbon arc lamps to produce the light.. so much light that the crew wore sunglasses, and anyone standing in front of it would get a visible light radiation burn if there long enough or were unprotected by makeup (they also produced massive amounts of UV, but this was blocked by a sheet of glass - remember, glass absorbs UV.. making those sheets so hot they could melt)
Picture a pot belly stove gently warming a room, dull black shape in the corner making you feel pleasant in winter. If you could see infrared light you'd be unable to sit in that room, the amount of IR pouring out would dazzle and blind you, it'd be like sitting in a room with 100,000 watts of spotlights pointing at your face. because that's exactly what it is.. it's just the light is shifted to a part of the spectrum you cannot see.
I hope this explains light as radiation and gives a little better understanding of how it works, it might also help the reader understand why some of us have issues with some things being discussed in the media by various people. Plenty of physicists (but not all) understand the nature of light, energy and gasses and are a little bewildered by some recent public policies - you might see why now too.
12:13 p.m. - 2016-12-16
Recent entries:
Authoritarian Oz - 2018-07-27
light, radiation, GAS! - 2016-12-16
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