I’m researching for a project about greenhouse gasses and I can’t find an answer on why this is. Please explain!!
In: Physics
UV is relatively high energy compared to IR and is able to move electrons to a higher energy state whereas IR instead is only able to slightly vibrate the bonds of molecules and this movement is what causes the heat.
They are both forms of energy so are both “heat”. We can only feel IR energy as heat because our bodies has sensors on our skin that can recognize it as heat. You can be burned by UV without really feeling it (like a sunburn).
With regards to greenhouse gases both are forms of heat but IR is the one trapped by green house gases and bounced back to the earth, its also the main type of heat generated from the earth and sent out into space (the earth is a giant ball of molten iron with a thin crust on the outside after all).
It’s the other way round: hot things emit IR radiation and not UV. The reason is thar IR radiation is low energy enough that our earthly hot things can emit it; for a hot thing to emit UV, it must be astronomically hot, like a star or an electric welding torch.
Well they both do really. 5 Watts of UV absorbed and 5Watts of infrared absorbed are the same amount of heat.
IR tends to be more associated with heat because warm things emit more blackbody radiation in the infrared range. Making something hotter can shift the peak blackbody frequency higher into the visible light range, and even into ultraviolet and beyond, but at those temperatures things tend to be on fire.
The greenhouse effect works because glass (or greenhouse gasses in the case of the earth) absorbs infrared, keeping heat that would normally be lost to the outside via blackbody radiation from escaping. The actual source of heat in the first place is full spectrum light from the sun which includes uv, and visible light.
Heat is not a thing by itself. Heat is the transfer of energy in a thermodynamic system. An object can emit or absorb IR and UV radiation. So IR and UV can transfer heat but are not heat b themself.
If an object is hit by electromagnetic radiation and absorbed it you will heat it up, that is regardless if it is UV, Visible light, IR, or other frequency ranges like microwave, radios waves, X-ray, or Gamma ray.
One way to transfer heat it through radiation. All objects emit thermal radiation that is electromagnetic radiation, the frequency range, and the amount of radiation at each frequency depends on the temperature.
Infrared light slower energy per photon than visible light and ultra-violent has higher. Higher energy photons require higher object temperature
Stuff that is at the normal surface temperature of the earth only emits IR light.
To emit visible light they need to be 525 °C, 977 °F, 798K that humans can see as a glow . So if you heat up metals, rocks like lava, or something else that is when they start to emit visible light.
At some point, they start to emit UV light too and the sun at 5,778 K (5,505 °C, 9,941 °F) do emit a lot of UV. The temperature required is not common on earth but it reached when you use welding equipment especially eclectic arch welding.
Humans often call IR radiation for heat but that is only because feeling how it heat up our skin is the only way we can detect it without instruments. Visible light heat you up the same way but we can see it.
The result is if an object is hot enough it does emit UV light like the sun but at a lower temperature, no visible or UV light will be emitted.
Greenhouse gases are the one that is good at absorbing the light frequencies that earth emmit but transparent the main frequencies the sun emits.
IR radiation doesn’t emit heat. Rather, it warms up things that it hits.
The full electromagnet spectrum is as follows, from longest wavelength to shortest.
Radio, Micro, infrared, visible, uv, x-ray, gamma.
Radio waves are very long. Ones used for communication typically have wavelengths measured in feet. That’s why the long antenna, so that the full wave can be captured.
Shorter microwaves and longer infrared waves are absorbed by water very well. This is why microwaves are used to heat foods, and why infrared light can heat up your skin and convey to you that you are near something hot.
Uv waves are so small that they don’t effect things on the scale of objects, but on the scale of atoms. They produce light radiation that is described as “ionizing,” a fancy way of saying that they can break down molecules. The most important molecule to us humans is DNA, which UV can destroy, causing cell death and subsequent sunburns. However, this breakdown of molecules doesn’t really cause the object they’re a part of to heat up on a large scale, and, thus, heat isn’t felt by people.
Side-note:
There is a physical phenomena known as blackbody radiation. In simplest terms, as objects heat up, they emit light. The hotter the objects are, the more powerful the light that they emit is. You can Google pictures showing the spectra emitted by different temperatures, but, it’s how old incandescent lightbulbs work, why things glow when they get hot, and why flames exist. Usually, though, things that humans heat up produce most of their radiation in the form of IR. That’s why you can feel the heat of a strong fire from feet away. You aren’t being heated through the air, but through IR light. There are also various indoor heaters that blast a room with IR.
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