Photo electric effect & failure of wave theory

in the late 1800s a man named heinrich herds the person after whom we have named the unit of frequency hurts right this person accidentally discovered that when you shine sometimes when you shine light on metals they eject electrons and we called this phenomena the photoelectric effect because light is photo and electric means electrons coming out so photoelectric effect and this experiment revolutionized physics it changed the way we thought about this world and when i first learned about this i was like what i mean such an innocent looking experiment like what so what's so special about what's so surprising about this you shine light with energy and and you get electrons so what's the big deal and so the goal of this video is actually to understand what was you know what was the big deal about this experiment and why it shook physics and why we had to come up with a completely new theory of light now the answer to this lies in the details of this experiment and a fun fact is this effect was discovered even before electrons were discovered which means it took about 10 to 15 years for you know an account a lot of experiments for us to realize what was going on but in all these experiments people were trying to figure out how does changing the properties of light affect the electrons to be more precise there are two things that you can change about light you can change its brightness brightness or more technically you can say the intensity so that's one thing you can change about the light you can make it more bright you can make it dim the second thing you can change about light is its color or again more technically we can say it's frequency all right so you can change these two things and people wanted to know how does changing these two things affect the photoelectric effect and the question is now what are the things that can change in these electrons well again there are two things that can change in these electrons so in the photoelectric effect there are two things that can change one is it could affect the speed of the electrons electrons could either come out faster or they could go slower so it could affect the kinetic energy of the ejected electrons the second thing it could affect is the number i'm just going to use hash for number number of electrons so you could have a lot of electrons coming out per second or you can have a little amount of electrons coming out per second and so now the stage is set people wanted to figure out how does changing these two things about the light affected these two things about the electrons that was what people were trying to figure out and of course some of you might be curious and you may be wondering well how do we how did we measure these things i mean electrons are so tiny how do you measure their electro their their speed or their energy how do you measure how many electrons are coming out per second what kind of experiments did people perform there are some clever experiments and we will talk about all the fun stuff in some other videos the the experiments and all of that but in this video let's focus on what the experimental results were and how it shocked everyone so let's start with the brightness of the light my question to you is if you think of light as a wave and if you increase the brightness increase its intensity increase the energy of the wave what would you expect to happen to the kinetic energy of the ejected electrons can you pause the video and think a little bit about that what would you expect to happen well if you have if you if you're providing more energy to the electrons by increasing the intensity i would expect electrons coming out with more energy right so we would expect more brightness more kinetic energy less brightness less kinetic energy that kind of makes sense right but to everyone's surprise what they found experimentally is that the kinetic energy of the ejected electrons was independent this was independent independent okay of the intensity of the intensity and people like what what does that mean this means you shine bright light you shine dim light it doesn't matter the speed at which electrons come out doesn't change and i was like what how does that make any sense to understand better let's think of an example instead of thinking about light waves and electrons let's think about water waves and boats i'm sure over here you would agree that because you have a huge wave when this comes and hits the boat it's going to launch that boat into orbit and over here nothing's going to happen right so you see with the speed at which the boat gets launched should depend on the height of the wave or the intensity of the wave right and that's why it was so weird when we found that the kinetic energy of the ejected electrons was independent of the intensity but like how does that make any sense but we did find that increasing intensity if you increase the intensity we did find that the number of electrons that increases that was our experimental finding so that increases but kinetic energy doesn't increase and that's the point like why okay time for a second shock or something even more weird they change the frequency or the color of the light keeping the brightness or keeping the intensity exactly the same you know what they found they found that increasing the frequency increases the kinetic energy what let me read that they found that if you change the frequency if you increase the frequency they found that now the kinetic energy increases if you decrease the frequency the kinetic energy decreases and again people are baffled like why does the frequency matter why if you increase the number of waves per second for some reason electrons are gaining more energy i've kept the intensity the same but somehow they're getting now more energy this was even more weird and let me tell you what this means this means if you keep decreasing the frequency even though you have enough intensity very bright light you keep decreasing the frequency electrons will come out slower and slower and slower and after one point electrons won't come out at all that means if you go below a particular frequency you get no photoelectric effect and this frequency this minimum frequency we give a name to it we call it the threshold frequency that's just a name threshold frequency and what this means is no photoelectric effect below this frequency which is so weird and let me give you an example if you take copper its threshold frequency is 10 to the power of 15 hertz this means even if you shine blindingly bright light below this frequency you get nothing all of that energy nothing and even if you shine very dim light above this frequency you get photoelectric effect electrons will keep coming out you increase the frequency not the intensity but frequency you get electrons coming out faster it's kind of like saying in this case if you increase the number of waves that will start increasing the energy with which the boat launches which makes no sense at all right it's like kind of like saying hey nothing's gonna happen over here because you only have one wave per second you need minimum five waves per second to launch the boat that makes no sense you would say why would the frequency matter frequency shouldn't have mattered what should have mattered is just the intensity so why is there a threshold frequency why frequency matters there should have been a threshold intensity right that's what people were saying and so that's why this did not make any sense physicists were completely baffled at this point so it's the frequency that decides the kinetic energy and people have no idea why and brightness does not decide does not decide the kinetic energy and people have no idea why all right the last one there's another one there's a third shocker this has something to do with the time delay between the incident light and the electrons coming out see we thought light was a wave back then and when you're shining this wave of energy there are billions and trillions of electrons inside over here right so we thought that all of that energy will be distributed among all these electrons and each electron will get a very tiny tiny fraction of the energy which means for it to gather enough energy it would take time it would take some time to gather all the energy needed to finally eject itself from the metal to gain enough energy to eject itself from the metal and calculations showed that if you have very dim light it could take days before the first electron is able to get you know enough energy to eject out if you shine very bright light maybe it would take few hours and maybe if you shine even brighter light maybe it would take few minutes but you know what we found we found that regardless of the brightness regardless of the frequency photoelectric effect was always instantaneous you shine light you immediately get electrons you stop shining no electrons it doesn't matter whether it's bright light or dim light doesn't matter if it's high frequency light or low frequency light you either get photoelectric effect instantly or you won't get it at all so we found instantaneous effect this was an instantaneous effect and this also confused people because according to wave theory the theory back then we predicted that there should be a time delay right so why didn't that happen and so now you know why this is such an important experiment in the history one of the most important experiments in the history of physics because it was at a time where we thought physics was complete we celebrated saying that you know light is an electromagnetic wave we have understood it and then came this experiment which completely shook us and after this a few incredible folks like max planck niels bohr werner heisenberg and of course our beloved albert einstein came along and came up with completely revolutionary ideas of light which you probably know by now as the quantum nature of light to explain this and of course many other phenomena we discovered later and that's how this is one of the experiments that led to the quantum revolution we'll talk about all the fun stuff in the future videos