Why is glass transparent?

Sixty Symbols regular Professor Phil Moriarty discusses transparent glass and the so-called energy gap.
PS MESSAGE FROM PROF MORIARTY: "I misspoke for the last sentence of the video. What I meant to say is that the photons don't have enough energy to excite electrons across the energy gap and therefore light is transmitted. What I said was precisely the opposite, contradicting everything else in the video. Apologies for any confusion caused."
More physics at www.sixtysymbols.com/

[@Buckets41369]

I like how he held up a piece of glass to show it's transparency when he's wearing a pair of glasses.

[@MrReneShelle]

Glasses are often not made of glass...

[@tonystroemsnaes554]

+René Shelløe then what are they usually made of?

[@caxco93]

plastic?

[@EebstertheGreat]

Plastic, yeah. Often cellulose derivatives, but there is a variety of available materials for lenses. Glass lenses are noticeably heavier, so if every pair you have worn has been even remotely close to the same weight, probably none were made of glass.

[@dave5194]

EebstertheGreat Yep, cheaper, don't shatter, are lighter, and easier to mass manufacture; although interestingly glasses prices accounting for inflation has been pretty much been level for decades because of the monopoly on the industry. Downside of course is plastic is much less scratch resistant than glass

[@zach9886]

I could listen to that guy all day. It's awesome how excited he gets when he explains things.

[@danielhe3557]

+Zach Reynolds LOL, you should watch the Klein bottle video

[@weberplancknashcurie3747]

+Elia Meyer Also Cliff Stoll

[@gary851]

+Zach Reynolds he got so much passion for this! its contagious

[@mattias2576]

i would love to be a student at nottingham university

[@rickardboberg]

Agreed! Love his videos. : >

[@jpif2961]

"it's 4 am what on earth are you doing?" "learning why glass is transparent"

[@adamlevin438]

"Hey its better than catching Pokémon"

[@katzen3314]

Better than late night clowning too.

[@sonajero25]

Maybe we re doing so because we are not having enough energy as to excite our partners to the next level

[@rikschaaf]

+J Pif Wtf, It IS 4 am exactly! Can you see the future or something?! xD

[@GamerBoyDevin]

As I sit here at 4:11am

[@romanticKSA]

I wish u gave him 10 mins, great explanation

[@ahmedhegazy699]

👍👍

[@Tormasan]

it was more than enough time for him

[@zigravos]

the stronger the light the lower the energy level of the photon?

[@ezekiel0606]

Anis Dabchy in the sprectrum of visible light, blue is the highest energy white red is the lowest

[@Milesco]

Correction: "blue is the highest energy _while_ red is the lowest"

[@olivergarsideconeron]

I'm glad to see all those photons getting true..

[@brokentombot]

The light's going true here, true here, true here, and true to there.

[@mikejones-vd3fg]

thats true, whats your point toe? i mean though

[@robertaderholdt890]

True dat

[@Triantalex]

*through

[@rickfranklin4196]

I'm an electrical engineer with a rough understanding of quantum mechanics. But I have always wondered about this question. Thanks for a "crystal clear" answer.

[@jay5729]

Maybe a little more detail about how important the arrangements of the atoms is. That way you could explain why graphite is opaque, yet diamonds are transparent, yet both are almost pure carbon.

[@harrywilson1660]

Yeah, it's a really misleading explanation. It isn't necessarily individual atoms that are absorbing the light, but the whole crystal lattice.

[@Jon-cw8bb]

@@harrywilson1660 ??? He literally says it's the arrangement of atoms, ie the structure of the network dictated by atom locations, that controls the interactions between electrons and light, hence the opacity. There's really nothing misleading in the video.

[@rociopaoloni5080]

@@Jon-cw8bbnot misleading but it's true that for some of us a more extensive explanation may be needed. For me the arrangement and the energy parts ended up a bit disconnected.

[@admiralpercy]

I really like seeing someone this animated about science.

[@duncanwallace7760]

As a keen photographer, knowing why glass is transparent has promoted me from a ground state to an excited level!

[@Socrates...]

I could watch this guy explaining stuff for days, he is the best

[@abcde_fz]

Well that was an unexpected treat. Thanx for explaining exactly how the color filters on my camera lenses work. Have used them for going on 45 years, only knowing the effects the different color filters will give to different kinds of photo situations. Now I know how they work. PS, if you take pictures in anything more than a casual way, LEARN how color filters can help your artistic efforts. Too much done with digital processing these days. Purists that use digital but grew up with chemical get 5 times the fun out of tweaking their pictorial efforts, by knowing what types of effects can ONLY be applied at the time the picture is taken, and ONLY before the light actually hits the CCD parts of the camera. LEARN what you can DO with the LIGHT before it GETS to the computer, and then on to the printer. You'll gain a new appreciation for your talents.

[@StrixTechnica]

About the only filter I find particularly useful is my CPL, but then I don't shoot B&W. I have an ND grad, but I hardly ever use it. Maybe it says more about my inexperience than anything else, but I can't see the point in filters that affect colour, especially when shooting in raw and any coloured filters will likely get thrown out of intended effect in development.

[@thepussygrabbingfamilyvalu557]

The fact that Si-O and Si-Si bonds don't absorb in the visible is NOT the explanation why glass is transparent. It's true that glass doesn’t *absorb* visible light. But why is sand (which is chemically exactly the same as glass) opaque, then? Why is it that other materials that do not absorb in the visible spectrum are opaque? For instance, syndiotactic polypropylene is opaque, yet it is only composed of C-H and C-C bonds that do not absorb in the visible spectrum. Moreover, if Si-O and Si-Si bonds did absorb in the visible light, it would absorb specific wavelengths, say 450-475nm, and the material would be (in this example) red, but it wouldn’t be opaque. Absorption is only one part of the story. Most of the opaque materials around us is opaque because they are (at least partially) crystalline and the size of the repeating cells in the crystalline structure is such that it *refracts* visible light (think Braggs diffraction). That's the reason why most of the transparent materials are transparent: because the repeating cells in the crystalline structure isn't in the same spacial order of magnitude as the wavelength of visible light or there is no crystalline structure, and therefore doesn’t refract visible light and is transparent. As for sand, the minuscule particles that compose a pile of sand are in the size domain that refract light and that’s why light doesn’t go through sand - most of it is refracted. The same explanation is valid for polypropylene; you would expect PP to be transparent because C-C and C-H bonds don’t absorb in the visible spectrum. But it’s opaque because the crystalline structure refracts visible light. Glass is transparent because Si-O bonds don’t absorb light AND because glass isn’t crystalline and doesn’t refract light. The latter is a lot more relevant. Again, if Si-O bond did absorb visible light, glass would be colored, not opaque. Glass is transparent because it’s amorphous.

[@TCBytom]

+Carl Coppens interesting observations. let me bite your theory somewhat :) If material were not absorbing photons (for transparent case) then speed of light in that medium would be C (as in vacuum). But it is not C. for instance electric permitivity of water is about 80 times that in vacuum and speed of light (or any other electromagnetic wave dependent on permitivity of medium) is slower than in vacuum. It is typicaly explained that photons react with electrons and electrons reemit this wave in specific direction (for transparent materials they emit photons on the other side of atom). there are materials which allow slow down speed of light to meters per second. reality is that no one truly understands interactions of matter and electromagnetic waves.

[@thepussygrabbingfamilyvalu557]

Thorsupremecommander +Thorsupremecommander you're right. the speed of light in glass is slower than c. i remember when I was much younger the explanation was "light is absorbed and re-emitted at the save wavelength and this absorption/reemission slows down and diffracts light". We now know that it's not entirely true because light would then be emitted in all directions and it's not what we see experimentally We do know that the electromagnetic fields of the light/glass do interact somehow, but shine visible light through glass, and *eventually* >99% of visible light is transmitted - hence transparency. You're right when you say that we don't truly understand what's going on, but the issue here is transparency vs opacity. Glass is an an amorphous solid which transmit visible light, while other materials that do not absorb in the visible light are opaque nonetheless because they have a microstructure that does not *absorb* light, but scatters it. i.e. sand vs glass: both have the same chemical structure but one is transparent and one opaque - glass is amorphous solid and sand is a bunch of tiny particles that scatter light.

[@TCBytom]

Carl Coppens I agree. we know very little about inner structure of matter. for instance take coal (carbon). it does not conduct electric current. but when you get graphite - it does. When you take crystal, you have transparent material but non conductive electric current. It's fascinating and weired.

[@thepussygrabbingfamilyvalu557]

Thorsupremecommander well... graphite is a 2D arrangement of sp2 carbon, all conjugated - there's a whole layer of pi orbital with delocalized electrons, so it's not that surprising that it conducts electricity. coal doesn't because it's just a bunch of agglomerated tiny tiny graphite particles, so the pi orbitales don't overlap. it's not surprising that it doesn't conduct. as for crystals, yeah, lots of weird stuff is happening. doped silicon monocrystal are cool: depending on the doping agent, it's either a p- or n-type semiconductor. even though it's crystalline and "transparent", it still conducts electricity - not very well, but still...

[@verneukteaap]

+Carl Coppens Please respond to Moriart2112 I would really like to know the answers to the questions he poses as well.

[@toobphish]

So, for instance, a substance like glass that might be transparent to visible light might be opaque, or partially so, to an energy source like X-rays, if the quanta had enough energy to excite the electron to a higher energy state? TNX for the video!

[@omaraljohani9298]

toobphish so why does high energy rays get through concrete is it observed and then sent again 🤔

[@Schlynn]

Another amazing video, I love how the professors are always so excited about their material.

[@sarans97]

Wow. His name is Moriarty, guys. Moriarty. PROFESSOR MORIARTY.

[@TheAngloSoviet91]

And he is Irish. Coincidence? I think not.

[@thesaturdaytechchannelwith553]

CasualBassist513 Illumanti cofiremeded!!

[@VinylSnapz]

CasualBassist513 Well it is an irish name >.> would be weird if he was called that and wasn't irish. although i'm glad this one tells me the truth instead of making me do odd jobs to find out where my dad is

[@lordeden2732]

CasualBassist513 He is a rarity the one mick in a thousand million who's not thick!

[@gristlevonraben]

Lol

[@christianmcgraw2320]

He gives the example of Diamonds when comparing the properties to glass, which brought up a new question. Why can the photons not excite the electrons in the carbon atoms of Diamonds, but they can excite the electrons of elemental Carbon? If you've seen Carbon before, you'd notice that it is the complete opposite of Diamond in the fact that it is completely opaque and black, whereas diamonds are clear or translucent. Why is this so?

[@skhotzim_bacon]

In diamond, each carbon atom is covalently bonded to four neighboring carbon atoms in a tetrahedral arrangement. This arrangement of strong covalent bonds creates a three-dimensional lattice structure with a high degree of rigidity. As a result, the valence electrons in diamond are tightly held within the structure and are not easily excited by photons of visible light. This property gives diamond its transparent or translucent appearance. On the other hand, elemental carbon in different forms, such as graphite or amorphous carbon, has a different bonding arrangement. In graphite, for example, carbon atoms are arranged in layers and bonded by strong covalent bonds within each layer, but there are weaker bonds between the layers. Due to these weaker bonds, the valence electrons in graphite are more loosely held and can move more freely within the layers. This gives graphite its ability to absorb and scatter a wider range of photons, resulting in its opaque and black appearance. In summary, the distinct bonding arrangements in diamond and elemental carbon cause differences in the properties of the electrons. The strong and rigid covalent bonds in diamond restrict the movement of electrons, making them less easily excited by photons of visible light and allowing light to pass through. Conversely, the weaker bonds in elemental carbon allow for greater electron mobility, resulting in the absorption and scattering of a wider range of photons, leading to its opacity

[@ameyas7726]

So the trick to becoming invisible is not to get exited....hmmm!!...but that would defeat the purpose of becoming invisible...darn!!!

[@Savalanoghlu]

lol

[@kauhanen44]

Your "wavelength" is so short that you can't get excited anyway.

[@Milesco]

I get excited when I see a couple of nice, Fermi bosons. Then I have to clean up the resulting Bose-Einstein condensate.

[@flubadubdubthegreat1272]

excited*

[@hang-sangitch]

Ameya S 😃

[@volodyanarchist]

Why is paper opaque, but when it gets greasy it becomes transparent? Does it change atomic energy levels?

[@sterlingarcher5698]

From this video, Id have to guess the oil molecules soak into the paper and increase the energy gap in the paper molecules.

[@brinastranger]

Great question. I'd have to guess it does not change papers energy gap. It's simply that grease itself is transparent and as it soaks through paper it becomes a medium for light to pass through. Simple as that, but still fascinating.

[@bite027_ketansharma6]

You can watch Steve Moulds video on white colour, there he explains about it. The video is quite recent (today is 15 December 2020)

[@michaeladdis3323]

Link?

[@moukafaslouka4796]

Some videos on KZbin are absolutely fantastic. This is one of those.

[@DrBenson21]

When i was 5 or 6 i used to bug my parents by repeatedly asking them how and why glass is transparent. They ordered me an information pack from pilkington glass but it didn't give any answers. 25 years later this video randomly pops up on my youtube feed and now i know the answer. Your videos are awesome, thank you.

[@Evan_Bell]

Does that mean that glass is opaque to higher energy (frequency) EM radiation, such as xrays?

[@liboud22]

Yupp.

[@adits9633]

Square Root Get rational, bruh.

[@gonyea12]

That bothers me too. His explanation would imply that I could shield gamma radiation with glass!

[@valken666]

gonyea12 not block entirely.

[@HaraceHavoc]

gonyea12 Maybe not gamma radiation. In order to promote an electron to a higher energy level, you not only need light of a high enough frequency, but that frequency range is pretty narrow. Any higher and like the lower frequency light, it would pass through the medium. That's how we can build glass that's transparent, but is opaque to ultraviolet rays because they absorb UV rays at those wavelengths.

[@justicewarrior9187]

Now this is what I call a professor!! In 5 minutes I understood what I've never been able to in class

[@JebJulian]

Great video. First thing I was thinking at the start was changing the photons energy and it's effect then boom. Thanks for giving him more than 10minutes and editing down

[@honestsignalz]

That light's going "stroo."

[@Nillavile]

+Honest Signalz So "true". *best pun 2015™

[@widebandrecords902]

tru = through in this video, he sound like a irish Jamaican, he explained everything very well also regardless

[@KessaWitdaFro]

+Wideband Records (wb-recs) I was just thinking this

[@mastod0n1]

Strayt stroo

[@diabl2master]

he just sounds irish

Very very well explained! Nice job. This is the proof that when well explained, science is for everybody.

[@petroskefallinos8735]

So basically glass doesn't absorb photons with its electrons and thats why it is transparent! You're my favourite professor Phil, so nicely explained and always passionate! :D

[@aranyaaparkarmakar4266]

Petros Kefallinos # Doesn't absorb AND also doesn't reflects photons.

[@SolidSiren]

Afaik, 2 things cause transparency: passage of light through it (complete transmission, no scattering/reflection) and (absorption and) re-emission.

[@SuperHeroMG5000]

I'd like to let you know you helped me at my final thesis. Thank you.

[@craaaaig]

Love the video/explanation, you explain a prism, including input to output of a spectrum?

[@graham65]

Why isn't the photon absorbed anyway, just in the trying to knock the electron up a level. Another way to explain my befuddlement is: if a photon bashes an electron, how does it know the future? The future of whether or not it succeeds in pushing the electron up a level, and therefore whether or not it is absorbed?

[@nikolayrayanov2895]

I'd guess it tries to do it but seeing it cannot it goes through. That's why the speed of light in glass is less than C.

[@Archiekunst]

Question: So what happens after the photon is absorbed ? The electron must come down at some point releasing that energy. In fluorescent substances that bit of energy is regenerated as another photon of a slightly higher wavelength. But not everything is fluorescent. What happens then ? Is it emitted as infrared, a.k.a. thermal radiative energy ?

[@radeklukas]

I guess it just heats up the material, as absorbed light tends to do 😀

[@FineBakedPastry]

followup question 1: most glass is opaque at around 250nm wavelength, suddenly becomes transparent at longer wavelengths, but transmittance diminishes again at around 2700nm. When transparency is due to photon not having enough energy to excite electron, how is the material not transparent to lower energy photons? followup question 2: It's transparent because photons don't have enough energy to excite the electrons, yet, if you powder the glass, it becomes opaque once again even though they're the same atoms as before. What role does the structure of the material play in determining opacity?

[@billpiper795]

The best teacher makes every student the best student. We would all have been served well to have Moriarty as our professor.

[@canister0]

I have a doubt. Gamma rays are electromagnetic waves and they can pass through walls which means that the wall is essentially transparent to gamma rays? Does that mean that they don't have enough energy to excite the electrons to a higher energy state? I don't know if this is a silly question but i would appreciate it if anyone can explain.

[@samtam721]

Yes. That's right. There are so many wavelengths of light that are not visible to the human eye that would pass through "transparent" materials as u said for those particular wavelengths.

[@MrTupii]

I don't understand this explanation with glass. So UV light has enough energy to excite the electrons in glass and it gets absorbed, but why gamma radiation and x-rays do not and they pass through? They have even more energy than UV radiation. Please elaborate.

[@jeratzel]

The photon cannot be absorbed partially, and a photon can't excite more than one particle at a time. The wavelength determines the amount of energy of a photon, shorter wavelength = more energy. At low enough energy (radio wavelength) or high enough (gamma wavelength) the amount of energy that photon provide is either too high or too low for a given energy-level. Sorry for my English.

[@Sand_1995]

When a gamma ray or any high energy ionizing wave hits a wall I believe they blow the electrons away from the atom when they hit them.Hence they are ionizing and cause mutations in our body by damaging cells.This is my conclusion though and it could be wrong.

[@George4943]

A photon has a given quantized energy. In order to deliver this it must find an electron which can absorb exactly that amount.* Some photons have too little, the electron cannot be excited by that exact amount. Some photons have too much, the electron cannot be excited by that exact amount. ____ * It takes a tuned circuit to detect photons of radio frequency. Too high or too low they are ignored. Frequency matters to observation.

[@robmckennie4203]

idea for new channel: prof. Moriarty reviews science articles

[@sleepful1917]

ah man moriarty is just the best, nobody explains concepts with such vigor yet beautiful simplicity like he can

[@MomchilAtanasovHome]

Amazing explanation, but creates a whole lot of new questions for me. How do refraction and reflection play into this? What happens with these energy levels? Does the material ever get saturated where all electrons have achieved maximum energy level and it becomes opaque or does it lose energy levels by emitting photons out (is that refraction)? What if a photon has twice the energy needed to excite an electron, does it raise the energy level of two electrons before it disappears?

[@ShawnRavenfire]

But what if the transparent material was blue-tinted instead of red-tinted? Then the lower-energy levels would be absorbed, but the higher levels wouldn't? And why do x-rays pass through objects? They're energy levels are a lot higher, aren't they? So why aren't the all absorbed?

[@bobbybravio9737]

Why do many glasses stop being transparent when they are, say, scratched or pulverized into a powder? As far as I know, the chemical structure of the compounds in the glass hasn't changed, so why does it suddenly absorb light better?

[@Hirschi74]

Bobby Bravio I think its because of refraction and reflection. The photons get reflected in so many different directions at the scratch that only so few reach the receiver (the eye).

[@apinakapinastorba]

+Bobby Bravio Glass does not absorb light better when ground to pulver, I believe (believing as not being sure). The light just changes direction "chaotically".

[@bobbybravio9737]

Ah that makes sense. Thank you for the responses.

[@PunchAPeach]

+Bobby Bravio Ah these guys beat me to it haha. But they're exactly right, as the crystalline structure is no longer aligned in regular patterns, photons get scattered in all directions. It would be like trying to see your reflection in a heavily cracked mirror.

[@johncgibson4720]

+Bobby Bravio Drinking glass do not have alignment of molecular crystal. "Glass" by definition, is a disoriented , non-crystal structure. It is the arrangement of material , silicon atoms and oxygen atoms together, that determines the transparency of low density material like drinking glasses. The video claims that the atoms electron shell is the culprit. It is not the atoms electron shells that dominates the transparency. It is the arrangement of the atoms, not the atoms individually. Dont listen to the questionable university's professor in the video. Of course it is harder to do calculations with forces between atoms than claiming the electron shells. That harder calculation is actually very very hard. Ever heard of engineering mathematics? That can not be taught in less than 4 years of full time college life.

[@LuisLamadridT]

Great video as always! After several views, I finally had the "I get it!" moment from it, and it was by comparing the visible light to a higher energy light like Ultra-Violet light. I think that viewers will understand better this concept of higher electron states and photons if they're shown a comparison between visible light passing through glass and UV light not passing through it at all. This explanation / experiment will help explain two concepts: first, that glass only "absorbs" higher energy photons, and that UV light is of higher energy (and wavelength) than visible light. Just a thought, maybe for a future vid :)

[@enriquema]

Agreed

[@TsH18]

Then what happens if I emit a photon at even higher energy than UV? For example X-Rays or Gamma-Rays. And it passes through glass.

[@LuisLamadridT]

Cool question. I'm gonna research a little bit because I have no idea hahahahaha.

[@KutuluMike]

tshock2000 it's not just about high energy vs. low energy. It's about the precise energy level of the photons: specific energy levels have "the right" amount of energy to move some electrons from one orbit to a higher one and get absorbed, other energy levels have too littler *or too much* and those pass through. Different substances have different chemical structures, thus different combinations of electrons in different configurations, and thus can absorp different ranges of energy levels. In addition, substances that absorb high energy photons have to re-emit them at some point, so even if a gamma-level photon is absorbed, there's a decent chance a gamma-level photon will be re-emitted and "look like" it passed through; when glass absorbs UV light, for example, it reemits that light at a different wavelength (mostly as heat), again due to the specific electron configuration of the molecules.

[@TsH18]

Michael Edenfield Thanks! Makes sense.

[@glenben92]

I'd love to see an explanation of diffraction, tied in to this. I'm confused as to how the light changes direction if it doesn't interact with the electrons in the material

[@roofusonna1846]

Right! What I am going to do know is pretend to be a photon.

[@George4943]

If I were a photon going c in absolute vacuum, then no time would pass for me. There is no such thing as absolute vacuum, though, thank Heisenberg, so nothing really travels c.

[@nh-a6713]

+George Steele Light travels at c regardless of the medium it is traveling in

[@George4943]

Noah haoN That happens not to be the case.

[@nh-a6713]

George Steele the speed of light is a constant anywhere, the only reason it appears to slow down in some mediums is because it's being absorbed and re-emitted by electrons which gives it a longer path

[@George4943]

Noah haoN ... Close but no cigar. Photons, as I understand, weakly interact and take a longer time. They "see" if they have the right energy, do no get absorbed, just slowed. A single photon changes direction when encountering a different medium. One side of its guiding wave gets slowed down or sped up.

[@Bassotronics]

Can we make things like transparent wood or transparent steel depending on how we arrange the atoms in a certain matter without changing the chemical properties which makes those materials?

[@dgalloway107]

+Bassotronics Yes. Not wood because there are so many various structures and materials in organic matter, but various researchers have been attempting to create various materials in a transparent state. Look up "Aerogel".

[@lcbp2009]

+Bassotronics Yes and no... The arrangement of atoms also define the nature of the material. For example, graphite, graphene, diamond are all compose made of carbon, but the difference between them is how the atoms is arranged. But there are some exception.

[@pedrogrimaldisemeghinimart759]

That is a brilliant explanation. Despite of the wrong justifications for glass, most people just blow up a lot of quantum physics concepts, which are not accessible or elucidative at all. What you did was a marvelous visual representation of energy levels, which made possible understand the general idea of it. Thank you so much for that.

[@goneutt]

Thanks! I had been wondering if the photons that came through glass were the same that went in. I'd had a picture of something more like electron conduction.

[@DavidHager1]

So if an electron can only exist in a ground or exited state and a photon has more than enough energy what happens to the extra energy? Does the electron absorb it and stay in the exited state longer, does it get reflected, or can an electron just go to a higher exited state? Sixty Symbols

[@KuldeepSharma-uf3sh]

I need a 2 hr video on this

[@johncgibson4720]

+Kuldeep Sharma You need a 4 year degree on one of the 2 toughest hard sciences to understand this. These mini videos are just here to please and pacify the public, to make the lay people feel involved in the scientific process. Who knows, maybe they just need the votes for some public elected official for their funding. And votes come from the lay people. There are hard sciences, and there are 2 toughest majors of the hard sciences.

[@roycebracket276]

+John C G Whats the other toughest one? Chemistry?

[@johncgibson4720]

+Royce Bracket The one that says to the quantum entanglement crowd, "hey, your entanglement theory based quantum bits can not solve factoring 15=3x5 in my lab" those kind of things. The one that sets the top scientists panic scrambling to explain the things, and succumb to life long quest and death, he does next to his breakfast on a daily bases.

[@SocksWithSandals]

You could watch this video 32 times :)

[@chrisb3420]

Question regarding the amber semiconductor. I get that the red would go through due to the lower energy level not bumping the electrons up to the next level and how the violet, given its higher energy, is absorbed due having enough energy to bump the electrons to the next level. But if we continue that reasoning, wouldn't x-ray and gamma rays also be blocked given the energy-to-electron bump-up rule?

[@arkadybron6120]

From something that I didn't know to something that I now understand in 5 mins.Wonderful; thank you.

[@gasser5001]

i really like this guy. he gets so excited about science! i love it! can he be my professor?

[@BAKI_H4NMA]

For some strange reason, I love this video.

[@bigrockets]

great explanation on electron energy levels and why glass is transparent, thank you...

[@stevenboelke6661]

Materials are transparent to different wavelengths of light. What may be transparent to visible light may be opaque to UV light. Something may also be transparent to UV light, but opaque to infrared light.

[@kye4840]

Man, I'm glad I made it 'trew' that video, that was amazing!

[@TheXitone]

your accent isn't the base norm ok.

[@NoriMori1992]

Eek a Mouse Don't think he said it was.

[@Triantalex]

*through

[@benhuggyhes3148]

How does absorption work? I already get reflection and transparency. Where does the energy from the absorbed photons go? It doesn't just disappear.

[@benhuggyhes3148]

+Zeus Blue Yes, I understand that clearly. When the electron falls to the lower energy state, it releases a photon. It doesn't seem to do that when photons are absorbed. When photons are absorbed, it seems like the material heats. How does that heating work on the quantum scale? According to what you've said, it seems like absorption is temporary, which we both know is incorrect. Once absorbed, that energy does not release as a photon. So I ask, on the same scale as you just explained it, how can you explain absorption as it lasts forever?

[@benhuggyhes3148]

+Zeus Blue Very clear, thank you. You mentioned how it could go 3-2-1 or 3-1. If it goes 3-1, has it emitted a higher energy photon than what it would with 3-2-1? Otherwise, you'd have leftover energy.

[@bxcinderella]

+Zeus Blue thanks! you explained things very clearly! it was helpful!

[@CHE6yp]

What did that Zeus guy said?

[@saqibmudabbar]

+CHE6yp say* not said* Just correcting your grammar

[@lukasreuter8835]

Not only do the electrons get exited, him and his students do aswell. Such a great professor!

[@Alex_science]

One of the clearest and precise explanations I have seen about this subject.

[@sebastianjones7589]

How can water let higher energy photons through(visible light), but interact with lower energy photons like microwaves?

[@SkunZielonyJakMech]

Microwaves are absorbed due to other characteristics of water - namely O-H bonds. These bonds can vibrate (like oxygen and hydrogen were attached to a spring) and that's the thing that absorbs microwaves.

[@amishbull6544]

Somewhere in physics (or was it chemistry) they teach that the color (wavelength or frequency) of the light (photons) is more important for raising an electron to a higher energy level than is the intensity of the light. Car window glass in treated to block some ultraviolet (high energy light photons) waves. You might want to check out line spectrum too.

[@eliasjosephsson3994]

Electrons as he said cant exist between energy levels, since there are a lot of diffrent energy levels most photones eighter have to much energy, or to litle to be absorbed by a electron in a given transparent material. Supose that they have to much and a microwave might have just the right amount of energy.

[@CanadianBoardCrew]

So this explains why radio waves can go through walls and buildings. They're just very low energy

[@tripjet999]

The radio waves...or the walls and buildings?

[@mohanagarwal7744]

then explain why x-rays go through everything

[@dylan7476]

@@tripjet999 The waves

[@marc_frank]

@@mohanagarwal7744 they don't if they did, they would be useless for imaging bones and stuff

[@rileystewart9165]

Professor Moriarty is probably my favorite!

[@emreparmaksiz3259]

It was an awesome explanation. Thanks for this amazing channel and Professor Moriarty

[@unlokia]

_"Why is glass transparent?"_ Isn't it clear? ;-)

[@hakanarnklint1070]

Briljant!

[@gmangladman]

what a pan lol

[@jskratnyarlathotep8411]

@@hakanarnklint1070 they're transparent too :3

[@Triantalex]

Same thing, no?

[@MelindaGreen]

Just because a material doesn't absorbed photons doesn't make it transparent. Some materials scatter light and are therefore translucent. The question is then why do photons continue on straight lines after bumping into atoms in transparent materials but not in translucent ones?

[@JohnRankinFunGuy]

After bumping into a nucleus the photon bounces off in another direction but again in a straight line until it possibly hits another nucleus or exits the material. In thin glass the photons rarely hit a nucleus so they mostly go straight through. In translucent glass and other translucent materials the photons hit a nucleus more often and so are almost always scattered and so looking through transluctent glass the images appear fuzzy. Shining very bright white light through a thin piece of red glass, it appears white at the centre because not too many photons get absorbed. Shining low intensity white light through thick red glass allows only red light through because the photons of other colours get absorbed and then re-emitted in random directions.

[@MelindaGreen]

@@JohnRankinFunGuy Photons don't interact with nuclei. They interact with the electrical field of the electrons. You also aren't answering the central question of why the photons get reemitted in the same direction from which they came. That happens with glass, but not the same thickness of sand which is the same thing except for its granularity.

[@grk5159]

That was awesome ! Just wondering if there is a similar explanation for how mirrors work.

[@eliadbu]

interesting explanation it raises few thoughts like when the electrons absorb the photons and go to higher energy levels they shortly after go back to lower energy levels while emitting a photon which could partially explain the reflection of light, also it helps to understand why lower frequency radio or micro waves can pass throw walls better the higher frequency waves

[@ChrisKogos]

Why not

[@mohammed-yw5zm]

yes

[@dhvsheabdh]

What happens if you just shine absolutely huge amounts of light onto an opaque surface? If all the electrons are raised in their energies, then surely (and eventually) one photon could pass through. If this is the case, then surely enough photons could make a material transparent?

[@Schnorchmorch]

The electrons aren't staying in that high energy state - they are reemitting light in every direction almost immediatly.

[@MrDivad006]

Retro .Spekta Actually there is a process called Two-photon absorption where it is possible that 2 photons add up and excite one electron, it's pretty rare compared to the usual type of absorption.

[@johnjr5599]

Njörðr Óðinnsson right

[@woodcutwrat]

Isn't that what happens in X-rays?

[@MrDivad006]

no

[@knightwfu]

Brilliant explanation; however, it has triggered a series of questions about macro phenomena related to propagation of light in a transparent medium. Namely: - refraction and Snell's law - reflection of light through glass I am also pretty interested in reconciling the above and the explanation in video with the interpretation of a photon's energy as (proportional to the square root of) the probability density of a photon's or electron's position? Any help with these questions would be greatly appreciated.

[@okayyyy637]

I just finished chemistry and I'm so happy that for once I completely understand everything the video is talking about, at least this one lol

[@Hybridspasser]

Isn't the electron going back to it's ground state, therefor releasing a photon again? What happens to that? Are those the reflected photons, making the color we see from objects?

[@willTryAgainTmrw]

+Hybridspasser same doubt here

[@DaveScottAggie]

Sometimes the electron may be excited to more than just one level above ground state. Then the electron may make a stop on the way down, emitting lower energy photons (which may not be in the visible spectrum)

[@TheLogmp]

+Hybridspasser Yes. The incident photons are absorbed and if the photons have enough energy to excite the electrons to an energy state where the levels are in the visible range, when the photon is released, it is released as visible light, giving things their characteristic color; however, if the photon doesn't have enough energy to have visible light emitted, the emitted photon may fall in the infrared spectrum, hence why light heats things up.

[@Hybridspasser]

TheLogmp But if the photon falls into the infrared spectrum, surely the object is not heated as such? As I understand it, thermal vibrations are carried by phonons?

[@TheLogmp]

+Hybridspasser I'm not exactly sure what your first question is asking so I'll give you a general answer. Phonons - which are lattice vibration quanta - were introduced because the collective vibrational modes only accept discrete quantities of energy and so it could be usefully modeled with a quasiparticle. In other words, phonons are the lattice vibrations that carry heat in solids. A slightly less accurate but general way of looking at it is that phonons are to conductive heat as photons are to radiative heat. Because thermal radiation is released by the motion of atoms and molecules (and some other deducible reasons) they are very much intertwined as you can imagine. Don't forget that there are several modes of heat transfer. :)

[@anshu957]

Thanks for such a nice explanation. However, do we know the answer to the question : "Why do photons have to excite the electrons in the first place" ? Also, does the energy of material increase when it absorbs a photon ?(If yes, then this must be the mechanism behind making a hole in a material through laser. right ? )

[@henryholden3112]

I have a few questions that if answered would greatly clarify things for me. 1. Why do photons, when passing by/through an atom not absorb into an electron if they cannot move the electron to a higher energy level? Why couldn't it move it to an energy level in between it's current energy level and a higher energy level? 2. Why does wavelength affect energy levels?

[@jvcrules]

Is this the same reason that high frequency, high energy signals like 5 GHz WIFI are easily blocked by a wall, while low frequency, low energy radio signals can be transmitted across the entire world and all throughout space?

[@thebootknifer]

I never thought of it that way, I think you could be correct.

[@floorkillacris]

Possibly. Could also be due to the low energy (large wavelength) photon being able to diffract around solid object easier

[@spidalack]

That's exactly correct as far as I know. It's also why, for the longest time, the only way to catch those transmissions was with a bigger and bigger antenna. It's also why you had to have a specific size antenna to receive them until someone figured out how to tune the circuits. (there are tricks you can use to cheat) It's also why old cellphones had extendable antennas and why packing more then one frequency in the device was, for the longest time, considered impossible. Then some student trying to do amateur radio without breaking the no antenna rule in his apartment building figured out you could use fractals to make the thing and a revolution started. Another interesting point. It's also why you can use some parts of the atmosphere (I think the ionosphere, but I'm not sure) to bounce certain wavelength of radio signals all the way over the horizon and over the ocean if the conditions are right, so you can talk on amateur radio with people from all over the world. (or so I am told)

[@jareddiehl3679]

spidalack That is not why old cell phones had big antennas.

[@centurionl]

i think it depends on the different energy levels the atoms of what walls are made of have. but idk what happens if the light has a higher energy level than what whatever the atoms can take. can anyone enlighten us?

[@InEpicinium]

See this is the stuff of science I like, the reasoning behind things. The math part is what absolutely kills me. At least in my AP Physics High school class it did back when I took it. I can watch these videos all day and understand what they are conveying. Just not the math unfortunately.

[@lsping8]

FallenPlague912 i know rite ? im taking engineering and i can understand all the theory or how stuff works but when math kick in i just confuse...

[@TheColourCyan]

FallenPlague912 Unfortunately at a certain point the physics and mathematics are inseparable.

[@Gnurklesquimp]

FallenPlague912 Yeah i'm thinking of amping up my rusty math skills again (it's mostly getting familiar with some terminology and practising a bit really) as it's super frustrating to understand a concept apart from the technical bit that simply requires complex maths

[@gristlevonraben]

+TheColourCyan I don't believe that. I can visualize things easily, so maybe it is easier for some people to only be able to understand these things as mathematical models. I see your point of view if that is the case.

[@pipsantos6278]

Students should be free to use computers to do the computation. Manual calculation impedes the flow of learning. Calculus should be done by computers. Not by hand. Humanity's job is to set up the derivative or integral. After that, computers should be doing the details. In reality, in actual work, nobody do the computation by hand anymore. And engineer without his computer is like a mechanic without his wrenches.

[@bendalton4123]

After watching the video, I have a couple of questions. 1) at what point does the material saturate and electrons cannot be displaced to high energy states? 2) at some point does the intensity of the light become a factor? 3) Why isn't the energy of the photon absorbed even though the electrons are not moved to higher energy states? In a classical sense, if I am pushing on a immovable object, I still expend energy, I don't get to walk through it.

[@golamkashef5255]

That's such a beautifully simple experiment!

[@kidpresentable]

4:22 Xen crystal! Resonance cascade imminent!

[@NoriMori1992]

kidpresentable "GORDON! GET AWAY FROM THE BEAM!" I love seeing disparate fandoms intersect! ❤️

[@AltumNovo]

So why don't gamma rays get blocked by glass?

[@DLockX]

+Altum Novo Gamma rays have huge amounts of energy which are too high for glass to absorb. A particle won't absorb a photon if it has too much or too little energy.

[@rsingh2083]

+Rocketeerian If photons are colliding with the material whether or not they pass through it, then even if they pass through the material (glass) arent they loosing all their energy and hence unable to pass through?

[@jonathanpark4619]

+rahul singh Quantum levels are weird like that. If you don't have the right amount of energy, it's basically as if you never collided with the electron.

[@DLockX]

***** Fair enough, time to sleep. :D

[@cadenasdeltiempo]

omg this video answers all my questions thanks a lot! btw that machine in your lab is amazing what is it called?

[@stiimuli]

So, with opaque materials, photons coming in and changing the energy positions of those electrons....does that change the material in any significant way? Sounds like it should.

[@TheBaconWizard]

Would someone please explain to me why electrons have specific energy levels they can have around a nucleus, rather than simply occupying a fractionally higher level if it receives a fractionally higher amount of energy from a photon?

[@TheOlliemath]

Essentially the answer is quantum mechanics - you can watch other sixty symbols videos on this

[@be_robot2391]

Do you mean "Why are energy levels DISCRETE (quantized)?"

[@TheBaconWizard]

maxpesaIT yes, precisely.

[@be_robot2391]

TheBaconWizard Well, I'm not sure, but I remember this is caused by the relationship between the energy of a particle and his wavelength, and that this has to be an integer number in order to create an energy level. I believe you can imagine it as a sine function all around the orbit, which alternates moments where the electron is further or nearer the core. If the function is "in phase" after a complete "lap around the core", then the energy level may exist. Please don't take my answer as complete

[@TheBaconWizard]

maxpesaIT that is starting to make a kind of sense lol

[@rgpnovo]

Glass is opaque for IR radiation, that is called the greenhouse effect. But according to your video if visible photons can't bridge the energy gap, then IR photons even less so. Help, please!

[@ikaron-csgololmore7577]

+rgpnovo If you picture the energy levels as rings that an electron can be on, then there can be for example two empty rings. The energy of the photon must be the exact energy of the gap in order for it to be absorbed. two empty rings means that there are two gaps to cross. One gap might be really small, so IR photons are absorbed, but because it's so small, visible light has too much energy to lift the electron to the next state. Out of the visible range, UV light might have enough energy to cross the second gap, so it is also absorbed. It's just random luck that there's a material that has such an energy gap that we humans can look through it. It's basically the same idea as with the orange glass, only that the bridging energy level is in the infrared area.

[@Gerbicide3]

oh you should make a follow up video talking about those bathrooms that go from transparent to opaque when you lock the door. how does that work?

[@bernardputersznit64]

Thanks -- Loved the experiment in particular

[@daisyamelia9359]

Vsause lean back anyone??

[@LiloPLAYS]

I have a begginer question... So photons only can be absorved by electrons? Glass is a mass made of atons, with protons and electrons... so I thought that the most of the photons would be "absorved" or be blocked by the protons that is much larger than an electron. If the glass is solid and made of atoms, how can be possible to light gets through!? I still don't get it! Help me!

[@TheMrRow]

a photon is actually a 'pack' of energy, when this 'energy' hits an electron, the electron uses that energy to climb up to an outer ring (don't know if you know anything about atomic models, let's say the electrons move away from the core). But there is no reason for the protons to absorb anything, they ARE the core, they won't budge anyway. The term "absorbing" is actually not really accurate here, as people will confuse it and think the electron is a sort of sponge now. Forget the "absorbing" part and see a photon as a fuel for your electron. And your final question: everyting is made of atoms, but they are very small while the space between them is immense. They are kept in place through electromagnetic fields, but photons can travel through this space.

[@LiloPLAYS]

The Benjamin Thank you! Let me see if I understand well. Electrons if I am right, they don't have a specific place to be, they are always moving making a electromagnetic field, which a photon can collide. Right? The protons can't do much because they are small and don't have any kind(or in that case, no important kind) of "field" to hold photons. Something like that? Thanks again!

[@TheMrRow]

LiloPLAYS Well it doesn't really have anything to do with electromagnetic fields. the photon just adds energy to the electron, moving it further from the nucleus. but the proton has no need for this energy.

[@sjashari112358]

LiloPLAYS Protons are within the nucleus. An atom is mostly just empty space that electrons are moving within, so even if a photon hits a nucleus it will be bounce slightly, but the chances are low due to the ratio of nucleus compared to rest of atom.(not sure exact ratio..but nucleus is extremely small compared to rest of the space atom takes.) Now if a photon does not have the required energy to raise an electron to its excited state it wont be absorbed, meaning it will continue to travel through substance.(the light can still bounce within material, since it can hit nucleus or other particles) Also glass might have other types of atoms (imperfections) that don't belong within the glass, meaning that some light might get absorbed because the electrons within the imperfections can get excited by that light might absorb some photons. above information is simplified, but should be able help with your question. Also Protons have much higher mass than electrons. "The protons can't do much because they are small and don't have any kind" isn't correct. Protons and electron charges are usually balanced within atoms (not counting isotopes and so on).

[@LiloPLAYS]

Shpendi Jashari Now I'm getting it, thanks for the explanation!

[@stealth_pool8780]

I love how he holders up a glass pane or points to one when he mentions when has GLASSES on

[@jeremylourie5270]

How does this apply to much higher energy light, like x-rays & gamma-rays that go straight through human bodies, walls, luggage, etc. Shouldn't these energy levels be high enough to be absorbed by any material, and therefore see it as opaque?

[@31337flamer]

theres too many books on the shelf :O gravity

[@BingusFodder]

Wow I learned something

[@ivofortes823]

What differs the reflected light from the transmitted light in a bulk of atoms? Ok, when a certain photon strikes an electron and doesn't have the right energy to excite it and move a level up, the electron will emit it back. If the material is transparent, some of the energy will pass through (transmitted) the bulk in the same way, the electrons spitting back the photons, until it crosses the material. However, from opaque ones, some (the remainder is absorbed) are reflected (are sent in the opposite direction). What does really stop the transmission through the bulk? We don't see the air because the light, crossing it, is not reflected. Most it is transmitted. So, atoms of H, O Ar, C are very difficult to be excited.

[@adamkatav9752]

That short video answered so many questions about so many subjects for me. But, it raised another question, if the photon has more then the exact energy required to raise an electron to the next energy level will it simply lose energy and increase the wavelength?

[@tintintt650]

so that is why transparent metals are rare !!!!

[@kevinjin6740]

You sound so rude! when you said ur not getting 10 minutes!

[@xMcCarthee]

Kevin Jin Just some banter.

[@shriramvenu]

so what happens to the electrons that have been excited to a higher state by absorbing a photon? Surely they do not remain permanently excited because that will mean any opaque object would quickly become saturated with excited photons, causing it to become transparent. Surely the electrons must fall back down to their ground state, emitting a photon in the process?

[@Micklemoose]

Yes, but, they aren't necessarily going to emit that photon in the same direction it started out going in. It will probably bounce back out, which is how you can see the surface of an opaque object. A mirror is a special type of opaque object where the light bounces back at exactly the angle it struck at and without any real delay. Most objects just spew it out randomly, and even after short delays, well extremely short delays from our perspective.

[@goreyjunk8006]

Micklemoose is that how glow in the dark objects work they take the photons energy store it for a short amount of time but long enough to where it can take minutes to hours for it to emit all the energy back out as photons? and is there a point where the clear glass will begin blocking the ultraviolet light? and if it does block the ultraviolet's light how soon would it release it again?

[@Quintinohthree]

gorey junk You're exactly right. Electrons are excited, lose some energy as heat and fall into a metastable configuration from which they can't get back to the ground state as easily, so they're emitted a whole lot later. It's called phosphorescence. Also glass starts absorbing ultraviolet light at about 250 nm. It doesn't phosphoresce to my knowledge, nor is it fluorescent, so ultraviolet is simply re-emitted with the same wavelength but don't hold me to that.

[@bajaj394]

Professor Moriarty, A nice talk, even if you do bypass the question of how photons can slow down and change direction without ever interacting with anything within the glass. The temptation here, is to immediately switch to the Classical wave explanation of light as it travels through glass, the leading edge of the 'wave' entering the material first and the trailing edge subsequently, accounting for both the delay and the change in direction of light when it enters this new medium. I think the third alternative should be considered , namely that photons are absorbed and re-emitted by valence electrons, without any change in their energy. Valence electrons are chosen because they are only loosely bound to the atom and at the same time can cope with the forces of recoil needed for absorption and emission of photons. The whole process of absorption and emission of photons has to be rethought, it is not a one off static process as your talk seems to imply. Think about a smart phone, it can process data at speeds of several Gigabits per second! Think of the implications of the word process! This being so why can't an electron in an atom oscillate at rates of several hundreds of terahertz ? It is absurd to discount this possibility and continue to regard emission and absorption as a one off process. Remember that in the case of reflection, light is highly anisotropic. Similarly in the case of light travelling through glass, absorption and emission takes place, resulting in a slower speed through the material, the slower speed results in a change in direction. To imagine that the bandwidth is so restrictive in an amorphous substance like glass is not very realistic and to dogmatic by far.

[@rwruger01]

The English accents and speed of babble make these videos irritating and tiresome.

[@TurinTuramber]

Pretty sure that's an Irish accent.

[@gregonasher]

Awesome. I never knew this. Such a simple explanation. bravo!

[@SciHeartJourney]

Wow! I want a Monochromometer for Christmas! Great explanation too!

[@absolutepressur]

So why does the speed of light change when it goes through a medium, and why is that dependent on the wavelength?

[@anon8857]

when the electrons are excited dont they get deexcited very quickly. if they did so, shouldn't there be a deflection in the reading of that photo diode device at 5:19 ?