 write down the summary of the observations, summary of the observations. I mean these are obvious things just write down that the photoelectric current is directly proportional to the intensity of the incident light, directly proportional to the intensity of the incident light, okay. Second saturation current is found to be proportional to intensity, saturation current it is found to be proportional to intensity, does it make sense? This what is the second graph, okay. Saturation current is found to be proportional to intensity but it is independent of what? Frequency, third point for a given metal there exists a threshold frequency, for a given metal there exists a threshold frequency above which, above which sorry below which no photoelectric emission happens. And if frequency is above that threshold frequency then stopping potential varies linearly with that frequency, is it not? We got a linear graph between stopping potential and frequency, the last conclusion this is very important. Photoelectric emission is instantaneous process, photoelectric emission is an instantaneous process without any apparent time lag, it will not wait, it will not wait for it to acquire energy, if it has to happen it will happen instantaneously, fine. Even if the intensity is very very less, even if intensity is very very less, getting it, fine. If you understand this 90 percent chapter is done now, if you have any doubts let me know, no doubt. So this observation actually Helvet and Lenard have made this observation concluded but never gave any equation or any theory behind it, why it is happening, it is instantaneous process but why, it is independent of intensity but why, current is directly proportional to intensity why, so the reasoning was not given, it was just observation, right. So Einstein came up with particle theory and gave few postulate or gave few some structure to based on which we can understand what is happening here, okay. So write down the concept of photoelectric emission, the first point is this, when there is interaction between matter and wave, wave behaves as particle, wave will behave like a particle, fine. And since it is photoelectric emission, we are not talking about any wave, we are talking about EM wave, okay. Interaction between matter and electromagnetic wave or EM radiation, actually this is not proper to say interaction between matter and wave, we can say EM radiation, electromagnetic radiation, the radiation behaves like this, fine. And these particles are called, these particles are photons, fine, named as photons, each photon, momentum of H by lambda, where lambda is a wavelength of the radiation, although you are treating it as a particle but still you are saying wavelength and frequency, ultimately it has a dual nature, it will have a frequency or wavelength but while interacting it behaves like a particle, fine. It is like you having two natures, while interacting with your parents, you behave something else, while interacting with your friends you behave in some other manner, fine. Similarly here also, so momentum is this and energy is H into mu, where mu is the frequency of the radiation, mu is also equal to C by lambda, C is speed of light, okay. Intensity is directly proportional to number of photons, photons are charged less, they do not have any charge, they will not get deflected by electromagnetic, they do not have any charge. In collisions between photons and matter, when an energy gets conserved but what does not get conserved, number of photons may change, fine. So this is the framework based on which Einstein has put forward his photoelectric effect theory, can you doubt on this? So once you understand this and there is one more core which is not clearly within the book is that one electron can absorb only one photon, it can not absorb more than one photon due to some quantum physics phenomena which is beyond our scope right now but what is clear is that one electron cannot absorb more than one photon. If you assume this, now it is very clear, if your frequency is less than threshold frequency, the electron will not wait to absorb more and more photons, it cannot, it can absorb only one photon, no matter what is your radiation, fine. So if you behave, I mean if you treat photons or if you treat the light to be made up of photons or particles, then only you will be able to understand what is going on in photoelectric emission, fine, right. So enough of theory, let us get down to the equations and numerical part of it.