Why Do Hot Objects Glow? - Black Body Radiation

you've certainly seen infrared goggles before and maybe you've seen this scene in predator where Arnold covers himself in mud to mask his body heat and the light it gives off but why is Arnold's body normally glistening in the infrared what causes this phenomenon [Music] whenever we discuss some phenomenon we almost always wind up somewhere in the quantum world because if you ask why enough times eventually that's where you'll wind up so it should come as no surprise that there's a bit of a quantum element to describe blackbody radiation but luckily we won't need to stick around here for too long the first thing to tackle is to figure out what exactly is eat I'm sure you've heard that heat is the kinetic energy of molecules or atoms within matter and that's 80% of the story but if we want to be a little more precise we can say that he is the excess energy that a molecule or atom contains above its ground state this can be represented visually as changes in rotation location and vibration these are all manifestations of excess energy when we supply heat to something we are simply forcing the molecules to undergo one of these changes friction is the interaction of the electrons between two surfaces repelling each other and thus moving them about this movement creates momentum or changes in kinetic energy and subsequently Heat now we have to enter the quantum world the electron and the electromagnetic field have a very intimate relationship because the electromagnetic field is a manifestation of charges and the electron has a charge or its own magnetic field changes in one directly influence the other they're very much like a boat in water the boat can't go anywhere without creating waves or ripples and the more aggressive the boat moves the larger the waves similarly the boat can be influenced by waves as well therefore whenever an electron changes its own magnetic field be that by changing its momentum or wavefunction it sends ripples or photons through the electromagnetic field that is to say a change in energy of an electron will always produce a photon as we leave the quantum world we keep this little factoid in mind changes in electron energy produced light blackbody radiation is ultimately the phenomenon we want to discuss today and having learned that changes in electron energy levels produce light we could hypothesize that maybe really hot objects glow because the electrons inside are changing their energy a lot let's see how that would work if we visualize a simple model of an atom we can see an electron whizzing about the nucleus it exists in an orbital and that orbital describes a wave function or energy state of our electron as long as the electron is in that orbital and has the same amount of energy as one would imagine it can certainly increases energy by transferring into a new orbital but not easily an orbital describes a wave function and waves are dictated by their boundaries if we imagine a guitar string in a box and we wanted to change its wave function we'd find unless the waves nodes appear at the edges of the box it will collapse therefore only a handful of waves fit in this box our electron orbitals are the same only certain wave functions will fit around the atom and each orbital and subsequent wave function possess a unique energy level so our bounded electron can only change its energy in quantized amounts and these changes in energy stimulate the creation of photons or light but how does he play into this an opera singer singing at the right pitch can cause a wine glass to break molecules naturally vibrate with thermal energy like the breaking glass if a molecule vibrates with the right frequency those vibrations can stimulate the excitation of an electron into a higher orbital when this electron returns back to its ground state it produces light a vibrating molecule doesn't always vibrate with the right frequency to excite one of its electrons but a vibrating molecule can vibrate against their neighbor and transfer some of their internal energy to them this new excess of energy that was transferred can create faster vibrations which in turn can then excite an electron as we increase the temperature more and more vibrations cause more and more excitation of electrons stronger vibrations also cause electrons to jump into even higher orbitals since electrons in higher orbitals possess even greater energy when they return to their ground state they emit even more energy if we can excite an electron into a high enough orbital at some point the light energy it gives off when it returns to its ground state will be in the visible spectrum in matter orbitals of nearby molecules and atoms overlap and create even more orbitals or nooks and cranny's electrons can occupy so even though each orbital requires a specific amount of energy there are so many of them that electrons can release a large variety of light as they transition around so many that the light appears white to us if an object gets hot enough that's the basic mechanism for most blackbody radiation in the visual spectrum but there are a few more sources since the electromagnetic field is just that a field it has an orientation of where it wants things to be pointed exactly like a magnet in most places in the universe the forces of the electromagnetic field are incredibly weak but they exist a lot of molecules are what we call polar that means there exist charges at all times within its geometry will visualize water for this water has very distinct positive and negatively charged poles this means it's kind of like a magnet and has a magnetic field that looks like this if the universe had its way it would prefer for this field to align with its own so there is a force that wants the water molecule to align a certain way in order for the water molecule to rotate it needs to act against that force it needs external energy so if we give it energy in the form of heat it may rotate and this rotation causes a change in the electromagnetic field and thus a photon is created to mediate that change but since these forces are so weak the energy of the photon is very insignificant as well therefore any time a polar molecule even ever so slightly changes its orientation in space it produces a photon but these photons are extremely unimpressive and as far as we're concerned contribute very little to what we observe as blackbody radiation the other main source of light and blackbody radiation is from free electrons when things get incredibly and stupidly hot like in plasma electrons realize there's really no reason for them to be sticking around this hot mess and attach completely from their respective molecules when an electron isn't bound to an atom or molecule they're called free electrons free electrons can carry as much or as little energy as they want when bound to an atom they could only absorb specific quanta of energy as a free electron they can absorb whatever they want free electrons fly about bumping into each other and ions these collisions greatly reduce their kinetic energy or changes their influence on the electromagnetic field this produces bremsstrahlung which is German for breaking radiation this isn't that common on earth because most of our electrons have a home but some of the light from lightning is due to this process lastly we should be asking why is it called blackbody radiation this is simply a name assigned to a theoretical material that could absorb every single wavelength in reality all materials either reflect or let some wavelengths pass directly through them and since we associate the color black with the absorption of visible light we call a material that could theoretically absorb all light black as well so when we measure the radiation from an object some of it is reflected and transmitted but blackbody radiation is the radiation that comes specifically from the object itself therefore when we heat an object up more and more of that observed radiation comes from the object itself and that is blackbody radiation you

Channel: But Why?

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Keywords: black body radiation, black body, radiation, black, body, heat, glow, hot, temperature

Id: Mj2QOpQkSfI

Length: 9min 4sec (544 seconds)

Published: Tue Jun 23 2020