The Photoelectric Effect. Light is Particles, not Waves. But wait...... | Doc Physics
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- Опубликовано: 4 апр 2013
- Einstein explained the photoelectric effect in 1905, but the full philosophical implications weren't understood until the early 1920's. (Or maybe they still aren't...) Quantum physics is the only way to understand the photoelectric effect. So light comes in photons - little light packets. But they also have a frequency. So, that's weird. They're wiggling packets, I guess.
The Photoelectric Effect? More like The Uncle Bailout Effect!
Neither waves or particles.
They're just metaphors for describing energy, to make it easier for people to understand.
"No don't bother coming over electrons, I'm busy." Great video!
Why don't others teach the way you do??? These concepts are so simple!! If only they're always explained the way you explain them. Instead of using words that people vaguely understand like "Electrode" and "Stopping potential", they should draw and illustrate whats going on!
+Maddix I know!!! I felt so confused in lecture and now I'm like "That's it? SIMPLE!"
Just to share my thought, one of the best youtube channels out there, seriously. You know how to explain complicated stuff, simple! (and with humor "or what? you are watching this on your phone? aren't you cool?") I think in a few years I will owe my engineering degree to you! P.S great sound effects :)))
did you get your engineering degree?
I don't think I'm gonna forget what photoelectroc effect is, ever. Thankyou for making this so cool!
I just had to pause this video and say that you explain like Richard Feynman. I LOVE your videos, It covered all the topics in the minimum amount of time. Thank you so much for making them super interactive.
Mate!!! this is like the best video on youtube ever. You are bloody brilliant...
Christy N and you are Australian
Good question. You could think of it as a "wave packet," wiggling as it travels. Watch on, because I have several more videos that explore the wave/particle duality.
re: "wave/particle duality."
It would be nice if you could "clear up" this constant reference to a light as a 'particle' as it REALLY clouds the understanding of EM (electromagnetic) radiation at _low_ frequencies (compared to light) such as demonstrated here at 9 GHz and a little further on at 140 MHz in this MIT video: ruclips.net/video/wHU-TIZvJTI/видео.html
I give you full credit if I pass my A level exams this year. Your videos are extremely helpful, thank you so much!
Did you pass your Alevels?
Happy to have you! My ancestors on both sides came from Germany in the 1890's! I hope to visit someday.
OMG, I'm desperately looking for a lecture about this concept and happy to find this. You made it sooo easy to understand. And thanks for the sense of humor.
my physics guide! i love physics and finding such teachers like you is a blessing in deed uncle xD
"Or what? Are you watching this from your mobile phone? Arent you cool!" Love this guy ^.^
maffak321 I definitely lol'd too When he said that
This might be the greatest video ever on youtube. How can physics be so fun and simple dude??
thanks, you got me to start solving problems.
you make physics so interesting and understandable compared to lectures at my school
Thanks for shedding some light on this topic
This is a great video. Taught me what I needed to know and more.
Nice find. Slide two has a schematic where you see the PV is doing the EXACT thing that the photoelectric effect apparatus was doing. As you continue this investigation, you may become sidetracked by the ways solid-state components (diodes and transistors) replaced vacuum tube components (rectifiers and triode tubes). This had HUGE repercussions in the evolution of the way music sounds, and it enabled computers to get very small. Imagine an iPhone with billions of tubes in it. That's hot.
Wow. this guy is a genius. Totally understand now, thanks to you! Hope you can make more physics videos for MCAT preparation.
hey i need a bit of help, when you were talking about the second classical prediction and talking about the amplitude you said more wave packets? i though frequency is the amount of waves and amplitude the strength? btw i love your vids, wish we had teachers like you back in the uk :)
Thanks for the explanation...in a this much simple way...
Fantastic video. Thanks!
Great Video, well explained and amusing.
Oooh!! It's very simple explanations Mr doc phys, l like it.
Thanks! This helped A lot!
Hi doc,
I read that the electrons ejected are the free electrons from the photo electric plate
Which are donated by each atom.
But you said they are ejected from the energy levels of atoms.
Which is right??
Hey Doc, I have a question.
Why would the electrons want to go the the metal? It's also negative. And even if you did have enough energy to get it to the metal wouldn't it just be all like "see ya" and go back to the emmittor? I'm so confused...
Lol my uncle is actually in jail right now
Ali Marafi One thousand thumbs up! Well, not for your uncle.
for exploding himself?
Well,I just learned this today at school..Oh sorry,I MEMORIZED it,because thats the way of education in Turkey.This video really helped me to get the logic of photoelectric.Thank you so much =)
I'm so sorry! We have many classes in the US that emphasize memory over understanding, too. I agree that understanding is infinitely better.
It is just so good!!!!!~~ I could learn things easily with fun!!!!~~ its just so so so great have those videos help me out with understanding of physics. please keep going!!!
yijing Yao Yay! Thanks!
The guy is off the charts !!.. " I love graphs"!!!
Great q. PE applies to vacuum tubes and actually liberated electrons. PV occurs in a solid and the electrons are freed within the solid. Study semiconductors (silicon in particular) and band gaps to learn more. That's beyond a first-year physics course, so I won't make a PV video for long time. Fun stuff, though. Hugely important to understand.
wow helped me understand a lot :D
Could you please do a video about Lasers?
Chirag Raju Great idea!
Can't wait :)
Dude! You are awesome! Thanks so much
Thank you so much and we all love you, Doc Schuster!!
Hmmm... Nice! Still relative old or young person can learn cool physics out of your presentation. Thank you Doc Schuster! In Finland we do not use too much sir word, but what a heck... thank you sir Schuster!
Thank you so much for this video!
Great video once again...:) Now If we are talking of the particle nature, what is the significance of using frequecy(in E=hf)? What is the frequecy of a photon when we say it is a particle?
You're so amazing Doc :D it helped me a lot! Thanks for the video :) God Bless
Simply brilliant.
Thanks!
Hello doc! I've come across something fairly confusing: photocurrent is independent of frequency of incident radiation.
But my book also says that the current increases with collector plate potential till it reaches saturation.
But isn't the increase in potential just increasing the kinetic energy of the electrons, just as frequency does? So why does frequency not affect it?
amaaaaaaaaazing doc!!!!!!!!!
Helped me pass my first college Chem exam :) THANK YOU
Doc Schuster yes I've found it. Very useful
Thank u so much Doc ❤
اشكر لك الشرح المبسط للظاهرة باسوب سلس وواضح
The work function is the exact energy you need to free the electron,the reflection of the material emitter or rather the reflection of it decides how many electrons will reach the negative pole.As the pole is negative and the electron is positive there is all so an effect of attraction.the light wave tho gets reflected and spreads out on wich the electron rides and that ripple effect makes it even less probable that the electron hits the negatvie collector.
At 15:48 you mention that if we don't hit the electron hard enough it will go up the hill a little bit but then roll back down and remain trapped.
With this analogy is it correct to say that the electron first absorbs the energy of a photon no matter what the frequency is, then it either
a) uses that energy to get liberated from the metal surface and the excess energy is the electron's kinetic energy, or
b) it bounces around for a bit within its potential well but does not have enough energy to jump out so it lets go of this extra unnecessary energy by throwing out the photon again.
I ask because many explanations and explorations of the photoelectric effect don't really talk about the interaction between the photon and the electron in the second case, and it seems that there is an important difference between saying that an electron will only absorb the energy of a photon if it is enough for the electron to move out of potential energy well, versus saying that the electron is always absorbing and emitting photon energy in such a way that its local energy state is minimised.
The best way I can describe of "lights", it seems everything have 'equilibrium'.
wow!!!!!
perfectly and beautifully explained
It holds more charge at a given voltage because of whatever dielectric material is between its plates, so it is "condensing" charge, i.e., holding on to it with a greater surface charge density than would otherwise be possible.
OK, I just found that phototube on ebay and plan to recreate your experiment--One question: concerning the equation Emax=qV, Why does the "q" not have to be the "TOTAL CHARGE"? It appears that only the charge "q" of one of the many electrons is used.
Thanks,
So, just because they crowd or "condense" the size of the charge distribution? Cool - thanks!
My teacher snuck this question by accident into our test- we haven’t covered it yet but I’ll be going into class an expert on photoelectric mechanics now
Hello, Doc!
I have one doubt. Why does the photoelectrons emitted from the metal do not all have the same energy?
Btw, I love your videos! Good job!
Japjee Kaur, well in any atom, electrons exist in several orbitals and they don't have the same energy, it's the same here
I am a physics student and you sir just earned yourself a subscriber :))
Thank you!
Thank you again. My p.chem book is awful and my prof is even less help. You are awesome.
I'm glad I can help! Keep rocking.
Excellent video man. God damn I wish you were my teacher...
Was that slope Plancks constant? What is the frequency of photon if we are assuming it is a packet of energy or a sort of a wierd particle as 'frequency' is a characteristic of a wave ?
Hey Doc! I have a doubt. If there are electron emitting, are they large in numbers? Can these photoelectric be measured in amperes or it should be measured in milli or micro-amperes?
BTW, cool videos!
Neeraj Khadagade It's microamps. It's actually pretty hard to measure, but you can put it into a little 741 wired up as a current amplifier. It's a worthy experiment to build if you can get your hands on an old photoelectric tube.
Doc Schuster Alright! Thanks!
But how do you know what freq and energy photons you were using in the first place???
I can put them into a spectrometer. A diffraction grating or prism and a photocell is all we really need.
It refers to the ability of it to store a higher density of electrical charge when compared to a simple isolated conductor
two things, first electron as particle was at that time of pee controversial second even if energy of light is frequency dependent this still doesn't mean they are made of blobs or particles, right?
a response video: ruclips.net/video/FW1LGsekb18/видео.html
Thank you very :D greetz from germany just subscribed (y)
You seem to have forgotten ... OLD CP was used to make that prediction, which was BEFORE the nature of the atom was known ... this needs a "revisit" in that 'light' (yes, a pun)
The video is great. It really helps to experimental physics. But in the video, while using color pencil, you saying that the yellow color pencil & Violet color pencil shows same no of electrons & it's interesting. But also confusing. How its possible? In a real experiment also (using this vacuum tube) it showing this interesting problem. Can you give me a reason behind this? I will be grateful. Thanks.
Dunno if you're still watching, but, question: I thought that hitting a molecule with light energy causes electrons to jump orbits and then produce light when the electron orbit decays. What determines whether a electron jumps orbit or jumps ship (gets out of jail?)?
As far as I understand it. The Beam of the Light must be strong enough to split (Jump Ship).
Frequency = Radiation
Molecule can split light or absorb, yes. NOTE: It seems everything have 'equilibrium'.
No, but I've wondered for a long time. Please let me know!
How does the electron flow to negative?
Merci buckets!!!
@Japjee Kaur: who I can't actually reply to for some reason, I think it's because electrons are emitted from different energy levels. The ones with maximum kinetic energy are always the valence electrons.
Sally Milson I have also noticed that I can't reply to about 25% of comments. Have any idea why? Good answer, BTW.
Doc Schuster it's probably a google+ thing if I had to guess
Sally Milson DANG IT, GOOGLE PLUS! MUST YOU RUIN EVERYTHING I'VE BEEN WORKING SO HARD TO CREATE?
Sally Milson Sally, I think you're very close. I was just reading about this. The model whereby we imagine many free electrons / a "sea" of electrons / a conduction layer doesn't fit with the photoelectric effect. When an electron is ejected from the surface, there's a clear gain in momentum. For momentum to be conserved, there must be an equal gain in momentum in the opposite direction. The photon can't provide this momentum gain because (a) its momentum is negligible, and (b) it disappears / is absorbed. The lattice structure, then, must recoil when the electron is ejected.
In order to assert that the lattice / proton structure is what recoils, we must imagine the electron as bound to a nucleus in a specific energy level--as a valence electron, as you correctly say.
(The conduction band/sea of electrons is built from the "overlapping/mixing" of valence energy states in the many neighboring atoms that are bonded together to form the solid metal.)
www.virginia.edu/ep/SurfaceScience/PEE.html
Did you just contradict your quantum rule #2 with the statement at timestamp 19:20???? (all concerning Kmax and # of emitted electrons).
I thought I had it!!!
OK, I think your comment at 8:53 clears things up. Thanks...
Lol, explained awesomely!!
hey Doc,
Thanks so much for your great videos. It helped me alot :)
however, in my Book the emitter and the collector are switched. So that the collector is at the side having the ammeter and the emitter is at the other side :/
which one is correct and does it matter ?
Abdullah Rushdi I guess I could put the ammeter anywhere on this series circuit, and it would read the same current. I use Walker's intro textbook, and this is his circuit sketch. You've still got light hitting the same side, right?
Doc Schuster
no that is the issue...
i have light hitting the straight side and the curved one is the collector.
Abdullah Rushdi ...but is the battery facing the other direction?
Positive side of the battery at the straight electrode which is, in my book, the emitter. The negative side at the curved electrode which is the collector.
This is Alevel physics syllabus. Edexcel book made by Patrick fullick
veru good video
Awesome!
So I want to try and clarify some terms that seemed a bit ambiguous to me upon 1st viewing this video. So AMPLITUDE relates to INTENSITY, relates to the # of PHOTONS going through, relates to # of PHOTOELECTRONS being freed from the EMITTER, going to the COLLECTOR, which will increase the CURRENT. (Like how much water is flowing in a river, relating to the size of the river)
But FREQUENCY relates to the COLOR/ENERGY of each individual Photon, and determines how FAST each PhotoElectron is going through the vacuum tube. Yes?
YES, your understanding of it is how I understood of "lights". It seems everything have 'equilibrium'.
It seems "Light" is Electro + Magnetic + Wave. Here is how I see it.... Electro = Electric - Photons + Magnetic = +/- & + Wave = Radiation. If you put it all together, that is how its able to travel and radiate and travel fast too due to wave.
what would the Kmax speed be?
it just get better and better...
What happens to photons after they have knocked out the electrons?
Apoorva Joshi they go honeymoon
They cease to exist, they are "absorbed" by the ejected electrons, which now contain the photon's energy.
Since a photon is only energy the energy gets transferred to the electrons in form of the kinetic energy
BRUH, i swear I was getting hung up "Okay how does this effect disprove classicial theory" and if they just say the prediction *REGARDLESS OF THE FREQUENCY* I would understand how Classical theory fail. Thank you much
"I don't even want to tell you how much it's worth"
Proceeds to draw on it with a marker.
Me: Confused
Slope - you bet...if y = mx + b. You want to study the wave/particle duality of light next. Fascinating stuff.
Best video ever :)
I'm glad you liked it!
awesome vedio
why can't we explain the results of photoelectric effect considering light as a wave?
Can we not say that a wave of suitable frequency ( threshold f ) can produce photoelectric effect ? I mean I understand that the predictions of classical physics were wrong but I still don't get why do we really have to consider light as a particle to explain photoelectric effect? can we not simply say that the photoelectric current depends upon the intensity of the wave of incident light ?
can someone please help me with this :)
so freaking cool
You're welcome! Stick around for the free donuts and coffee!
If one would replace the the plate with an electron gas cloud from gold particles of < 10 nm, the high density can be harvested at a greater rate.
Hello Doc, I have two questions:
At 9:15 you say, "If you want more light, you don't increase the amplitude, you increase the number of photons." I'm a little confused-isn't increasing amplitude the same thing as increasing the number of photons?
Secondly, I understand the concept of stopping potential, but I do not understand why Kmax = q * V?
Thanks very much for the video! It helped a lot. :)
Kay Wu 1) You're thinking of light as a wave, but it's a stream particles. 2) kinetic energy of the fastest electrons is converted into electric potential energy. Do you know the equation for the EPE for a charge at a voltage?
Doc Schuster
1) I've always imagined light as both - a stream of particles that travel in a wave. Is it incorrect to think of it like that?
2) Ah, no, I do not. I'll look into it.
Thank you so much for taking the time to reply!
Kay Wu Yes - wave packets. But you can't define an amplitude of the "wave" The photoelectric effect is super-duper quantum.
Kay Wu The intensity of the light beam can be thought of as the "density" of the photons. More specifically, the intensity of the light beam (upon reaching the metal surface) indicates the number of photons striking a square meter per second. This can be illustrated through equations:
E_one photon = hf
E_many photons = (number of photons) x (hf)
P_many photons reaching the metal = (number of photons) x (hf) / t
intensity of light at the metal surface = P / A
intensity = (n / t) x (hf / A)
This is a quantum mechanical interpretation of "light intensity." There is also a classical interpretation of wave intensity, which you've pointed to. Here's the classical interpretation: 'the intensity of a wave is proportional to the amplitude of the wave squared.' This classical interpretation of intensity can be useful in cases/experiments where light exhibits wave behaviors (like diffraction and interference). However, the classical interpretation of intensity (as related to amplitude) is out of place in the particular case of photoelectric effect, since in this effect, light exhibits particle behaviors rather than wave behaviors.
Sir you are good
Damn I wish I had some graph paper.
How do you fabricate from scratch (or procure for under $10) a photoelectric effect tube? How do you achieve the vacuum? Or can these be bought for under $10. I'd like to do a little experiment at home and observe the photoelectric effect.
Dan Hillman Oh, that would be awesome. I have gotten mine from salvage inside old 1950's and 1960's spectrophotometers. Good luck!
amazzzing
Now try the electron as a glass and light as sound wave. Classical prediction, the glass will break the louder the sound?
Slope of the max kinetic energy vs. Frequency is Planck's constant.