[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.36,0:00:03.33,Default,,0000,0000,0000,,- If you shine particular kinds\Nof light on certain metals,
Dialogue: 0,0:00:04.65,0:00:05.73,Default,,0000,0000,0000,,electrons will be ejected.
Dialogue: 0,0:00:05.73,0:00:08.22,Default,,0000,0000,0000,,We call this the photoelectric\Neffect because light is photo
Dialogue: 0,0:00:08.22,0:00:10.86,Default,,0000,0000,0000,,and electrons being ejected is electric.
Dialogue: 0,0:00:10.86,0:00:12.45,Default,,0000,0000,0000,,And this was one of the key experiments
Dialogue: 0,0:00:12.45,0:00:15.51,Default,,0000,0000,0000,,that actually helped us\Ndiscover a completely
Dialogue: 0,0:00:15.51,0:00:17.04,Default,,0000,0000,0000,,new model of light.
Dialogue: 0,0:00:17.04,0:00:19.56,Default,,0000,0000,0000,,But how exactly you ask,\Nwell, let's find out.
Dialogue: 0,0:00:19.56,0:00:22.20,Default,,0000,0000,0000,,What's interesting here is\Nthat this effect depends on
Dialogue: 0,0:00:22.20,0:00:23.64,Default,,0000,0000,0000,,the color of light.
Dialogue: 0,0:00:23.64,0:00:27.60,Default,,0000,0000,0000,,For example, if this metal\Nwas, say potassium, okay,
Dialogue: 0,0:00:27.60,0:00:29.49,Default,,0000,0000,0000,,then if you shine blue light,
Dialogue: 0,0:00:29.49,0:00:31.32,Default,,0000,0000,0000,,then we will get electrons being ejected,
Dialogue: 0,0:00:31.32,0:00:33.37,Default,,0000,0000,0000,,photoeletric effect happens.
Dialogue: 0,0:00:33.37,0:00:35.61,Default,,0000,0000,0000,,But if you were to shine\Nred light on potassium,
Dialogue: 0,0:00:35.61,0:00:38.16,Default,,0000,0000,0000,,we will not get\Nphotoeletric effect at all.
Dialogue: 0,0:00:38.16,0:00:40.02,Default,,0000,0000,0000,,Regardless of how bright you make it.
Dialogue: 0,0:00:40.02,0:00:42.75,Default,,0000,0000,0000,,Even if you were to make\Nit blindingly bright,
Dialogue: 0,0:00:42.75,0:00:44.70,Default,,0000,0000,0000,,we will not get photoeletric effect.
Dialogue: 0,0:00:44.70,0:00:47.70,Default,,0000,0000,0000,,This is what puzzled physicists.
Dialogue: 0,0:00:47.70,0:00:50.91,Default,,0000,0000,0000,,I mean, think about the\Nmodel over here we have atoms
Dialogue: 0,0:00:50.91,0:00:53.43,Default,,0000,0000,0000,,with electron clouds\Nover here and the nucleus
Dialogue: 0,0:00:53.43,0:00:54.63,Default,,0000,0000,0000,,at the center, okay?
Dialogue: 0,0:00:54.63,0:00:56.43,Default,,0000,0000,0000,,When you shine light,\Nthe energy of the light
Dialogue: 0,0:00:56.43,0:00:59.40,Default,,0000,0000,0000,,gets transferred to the electrons\Nand they're able to escape
Dialogue: 0,0:00:59.40,0:01:01.95,Default,,0000,0000,0000,,the clutches of the nucleus and go out.
Dialogue: 0,0:01:01.95,0:01:04.29,Default,,0000,0000,0000,,But why can't that happen\Nover here with red light?
Dialogue: 0,0:01:04.29,0:01:06.66,Default,,0000,0000,0000,,Think about it, I'm shining bright light,
Dialogue: 0,0:01:06.66,0:01:09.21,Default,,0000,0000,0000,,very high intensity, incredible\Namount of energy over here,
Dialogue: 0,0:01:09.21,0:01:11.85,Default,,0000,0000,0000,,and yet electrons are not\Nable to absorb it and get out.
Dialogue: 0,0:01:11.85,0:01:13.92,Default,,0000,0000,0000,,Why does the photoeletric\Neffect depend on the color?
Dialogue: 0,0:01:13.92,0:01:16.89,Default,,0000,0000,0000,,That was a big question\Nthat didn't make any sense.
Dialogue: 0,0:01:16.89,0:01:18.57,Default,,0000,0000,0000,,So what do we do over here?
Dialogue: 0,0:01:18.57,0:01:20.67,Default,,0000,0000,0000,,Well, we do more careful experiments.
Dialogue: 0,0:01:20.67,0:01:22.38,Default,,0000,0000,0000,,First, let's only look at the color
Dialogue: 0,0:01:22.38,0:01:24.63,Default,,0000,0000,0000,,and then think about the\Nbrightness later, okay.
Dialogue: 0,0:01:24.63,0:01:26.91,Default,,0000,0000,0000,,So what is color representing\Nelectromagnetic waves?
Dialogue: 0,0:01:26.91,0:01:30.96,Default,,0000,0000,0000,,Remember that color\Nbasically depends on the wave
Dialogue: 0,0:01:30.96,0:01:31.79,Default,,0000,0000,0000,,length of light.
Dialogue: 0,0:01:31.79,0:01:34.92,Default,,0000,0000,0000,,For example, red color is\Nthe wavelength of light
Dialogue: 0,0:01:34.92,0:01:38.13,Default,,0000,0000,0000,,could be somewhere around 650 nanometers.
Dialogue: 0,0:01:38.13,0:01:40.86,Default,,0000,0000,0000,,What we find is that at 650 nanometers
Dialogue: 0,0:01:40.86,0:01:43.68,Default,,0000,0000,0000,,we don't get any photoelectric\Neffect for potassium.
Dialogue: 0,0:01:43.68,0:01:46.59,Default,,0000,0000,0000,,We don't know why, but\Nnow what we can do is
Dialogue: 0,0:01:46.59,0:01:49.23,Default,,0000,0000,0000,,let's reduce the wavelength\Nand see what happens
Dialogue: 0,0:01:49.23,0:01:51.48,Default,,0000,0000,0000,,if I keep reducing the\Nwavelength, and let's say
Dialogue: 0,0:01:51.48,0:01:54.90,Default,,0000,0000,0000,,I come to orange light of 600 nanometers,
Dialogue: 0,0:01:54.90,0:01:56.13,Default,,0000,0000,0000,,see I've reduced it.
Dialogue: 0,0:01:56.13,0:01:58.56,Default,,0000,0000,0000,,I still get no photoeletric\Neffect, I don't know why,
Dialogue: 0,0:01:58.56,0:01:59.61,Default,,0000,0000,0000,,but I'm just doing an experiment.
Dialogue: 0,0:01:59.61,0:02:01.05,Default,,0000,0000,0000,,This is an observation, okay?
Dialogue: 0,0:02:01.05,0:02:01.88,Default,,0000,0000,0000,,I keep reducing.
Dialogue: 0,0:02:01.88,0:02:06.88,Default,,0000,0000,0000,,I keep reducing the wavelength\Nuntil I hit 541 nanometers.
Dialogue: 0,0:02:06.93,0:02:10.62,Default,,0000,0000,0000,,At this point, I now start\Nseeing photoeletric effect,
Dialogue: 0,0:02:10.62,0:02:14.22,Default,,0000,0000,0000,,and in this particular case,\Nelectrons are barely ejected
Dialogue: 0,0:02:14.22,0:02:15.66,Default,,0000,0000,0000,,from the metal.
Dialogue: 0,0:02:15.66,0:02:17.73,Default,,0000,0000,0000,,That's why I've drawn very\Ntiny arrow marks over here,
Dialogue: 0,0:02:17.73,0:02:19.71,Default,,0000,0000,0000,,they have hardly have any kinetic energy.
Dialogue: 0,0:02:19.71,0:02:21.61,Default,,0000,0000,0000,,I just get some photoeletric effect
Dialogue: 0,0:02:22.57,0:02:24.75,Default,,0000,0000,0000,,and then if I reduce it even\Nfurther, that's where I get my
Dialogue: 0,0:02:24.75,0:02:27.21,Default,,0000,0000,0000,,blue light at about say 500 nanometers.
Dialogue: 0,0:02:27.21,0:02:28.62,Default,,0000,0000,0000,,These are rough numbers, okay?
Dialogue: 0,0:02:28.62,0:02:31.50,Default,,0000,0000,0000,,At 500 nanometers, I now\Nget photoelectric effect,
Dialogue: 0,0:02:31.50,0:02:35.10,Default,,0000,0000,0000,,but the electrons coming\Nout with even more energy.
Dialogue: 0,0:02:35.10,0:02:37.59,Default,,0000,0000,0000,,What happens if I reduce it even further?
Dialogue: 0,0:02:37.59,0:02:42.00,Default,,0000,0000,0000,,I find that electrons are coming\Nout with even more energy.
Dialogue: 0,0:02:42.00,0:02:43.62,Default,,0000,0000,0000,,So what's our observation over here?
Dialogue: 0,0:02:43.62,0:02:47.01,Default,,0000,0000,0000,,We see that if we lower the\Nwavelength, we get more energy
Dialogue: 0,0:02:47.01,0:02:48.93,Default,,0000,0000,0000,,for the electrons coming out over here.
Dialogue: 0,0:02:48.93,0:02:50.37,Default,,0000,0000,0000,,We can also talk in terms of frequency.
Dialogue: 0,0:02:50.37,0:02:53.31,Default,,0000,0000,0000,,Remember, bigger the wavelength,\Nsmaller the frequency,
Dialogue: 0,0:02:53.31,0:02:54.51,Default,,0000,0000,0000,,because if you have big wavelength,
Dialogue: 0,0:02:54.51,0:02:57.09,Default,,0000,0000,0000,,there are less waves passing per second.
Dialogue: 0,0:02:57.09,0:03:00.24,Default,,0000,0000,0000,,So this is low frequency\Nand this is high over here.
Dialogue: 0,0:03:00.24,0:03:02.46,Default,,0000,0000,0000,,So we can say when it comes to frequency,
Dialogue: 0,0:03:02.46,0:03:06.15,Default,,0000,0000,0000,,more the frequency, more\Nthe energy of the electrons.
Dialogue: 0,0:03:06.15,0:03:09.33,Default,,0000,0000,0000,,And you also have some kind\Nof a cutoff over here, right?
Dialogue: 0,0:03:09.33,0:03:12.39,Default,,0000,0000,0000,,So for example, if the wave\Nlength is above 541 nanometers
Dialogue: 0,0:03:12.39,0:03:15.45,Default,,0000,0000,0000,,for potassium, for potassium,\Nif it's above 5 41 nanometers,
Dialogue: 0,0:03:15.45,0:03:17.91,Default,,0000,0000,0000,,no photoeletric effect, only below it,
Dialogue: 0,0:03:17.91,0:03:19.20,Default,,0000,0000,0000,,we will get photoelectric effect.
Dialogue: 0,0:03:19.20,0:03:21.75,Default,,0000,0000,0000,,And so every metal will\Nhave its own cutoff.
Dialogue: 0,0:03:21.75,0:03:24.09,Default,,0000,0000,0000,,We call that the threshold wavelength,
Dialogue: 0,0:03:24.09,0:03:25.98,Default,,0000,0000,0000,,or you can also say threshold frequency.
Dialogue: 0,0:03:25.98,0:03:28.92,Default,,0000,0000,0000,,But the whole idea is if\Nthe wavelength is below
Dialogue: 0,0:03:28.92,0:03:31.100,Default,,0000,0000,0000,,that threshold wavelength\Nonly then you get
Dialogue: 0,0:03:31.100,0:03:33.54,Default,,0000,0000,0000,,photoelectric effect, if\Nit's about you won't get it.
Dialogue: 0,0:03:33.54,0:03:35.97,Default,,0000,0000,0000,,Different metals have\Ndifferent threshold wavelengths
Dialogue: 0,0:03:35.97,0:03:38.49,Default,,0000,0000,0000,,and similarly different\Nthreshold frequencies.
Dialogue: 0,0:03:38.49,0:03:40.29,Default,,0000,0000,0000,,So that's the effect of\Nwavelength of frequency.
Dialogue: 0,0:03:40.29,0:03:42.81,Default,,0000,0000,0000,,We see that the wavelength\Nof the frequency controls
Dialogue: 0,0:03:42.81,0:03:44.61,Default,,0000,0000,0000,,the energy with with\Nthe electrons come out
Dialogue: 0,0:03:44.61,0:03:46.68,Default,,0000,0000,0000,,and that cannot be explained as to why.
Dialogue: 0,0:03:46.68,0:03:49.35,Default,,0000,0000,0000,,Why does the wavelength of\Nthe frequency control it?
Dialogue: 0,0:03:49.35,0:03:51.12,Default,,0000,0000,0000,,Why am I not getting photoeletric effect?
Dialogue: 0,0:03:51.12,0:03:52.89,Default,,0000,0000,0000,,If it's about the threshold wavelength,
Dialogue: 0,0:03:52.89,0:03:54.18,Default,,0000,0000,0000,,it doesn't make any sense.
Dialogue: 0,0:03:54.18,0:03:56.25,Default,,0000,0000,0000,,But anyways, the next\Nquestion could be for us,
Dialogue: 0,0:03:56.25,0:03:58.50,Default,,0000,0000,0000,,how does the brightness\Naffect this whole thing?
Dialogue: 0,0:03:58.50,0:04:00.09,Default,,0000,0000,0000,,Does it have any effect?
Dialogue: 0,0:04:00.09,0:04:01.02,Default,,0000,0000,0000,,The answer is yes.
Dialogue: 0,0:04:01.02,0:04:04.66,Default,,0000,0000,0000,,Remember, brightness or the\Nintensity of light is basically
Dialogue: 0,0:04:04.66,0:04:07.98,Default,,0000,0000,0000,,how big the valleys and\Nthe peaks are, right?
Dialogue: 0,0:04:07.98,0:04:11.88,Default,,0000,0000,0000,,So if you were to make the light brighter,
Dialogue: 0,0:04:11.88,0:04:14.40,Default,,0000,0000,0000,,then it will look somewhat like this.
Dialogue: 0,0:04:14.40,0:04:15.66,Default,,0000,0000,0000,,You can imagine it this way.
Dialogue: 0,0:04:15.66,0:04:17.64,Default,,0000,0000,0000,,This is brighter light, okay?
Dialogue: 0,0:04:17.64,0:04:21.95,Default,,0000,0000,0000,,Now what we find is that we\Nget more electrons, okay?
Dialogue: 0,0:04:23.16,0:04:25.17,Default,,0000,0000,0000,,It doesn't change the energy\Nwith these electrons come out.
Dialogue: 0,0:04:25.17,0:04:28.18,Default,,0000,0000,0000,,See they're coming out with\Nmuch the same energy as before,
Dialogue: 0,0:04:28.18,0:04:29.52,Default,,0000,0000,0000,,but we now get more electrons.
Dialogue: 0,0:04:29.52,0:04:31.97,Default,,0000,0000,0000,,Of course, if you're above\Nthe threshold wavelength,
Dialogue: 0,0:04:31.97,0:04:35.58,Default,,0000,0000,0000,,you'll not get photo\Nphotoelectric effect at all,
Dialogue: 0,0:04:35.58,0:04:36.90,Default,,0000,0000,0000,,regardless of the brightness.
Dialogue: 0,0:04:36.90,0:04:39.03,Default,,0000,0000,0000,,It doesn't matter, okay?
Dialogue: 0,0:04:39.03,0:04:41.76,Default,,0000,0000,0000,,So if you decrease the\Nbrightness or intensity,
Dialogue: 0,0:04:41.76,0:04:43.23,Default,,0000,0000,0000,,you get less electrons.
Dialogue: 0,0:04:43.23,0:04:45.99,Default,,0000,0000,0000,,If you increase the intensity,\Nyou get more electrons.
Dialogue: 0,0:04:45.99,0:04:49.14,Default,,0000,0000,0000,,So intensity only controls\Nthe number of electrons,
Dialogue: 0,0:04:49.14,0:04:52.53,Default,,0000,0000,0000,,but it's the wavelength of\Nthe frequency that controls
Dialogue: 0,0:04:52.53,0:04:54.21,Default,,0000,0000,0000,,the energy with visual electrons come out,
Dialogue: 0,0:04:54.21,0:04:57.15,Default,,0000,0000,0000,,it also controls whether we\Nget photoeletric effect or not.
Dialogue: 0,0:04:57.15,0:05:00.60,Default,,0000,0000,0000,,The big question was why the\Nwave model just cannot explain
Dialogue: 0,0:05:00.60,0:05:03.06,Default,,0000,0000,0000,,this because according to wave model,
Dialogue: 0,0:05:03.06,0:05:05.37,Default,,0000,0000,0000,,you should get photoeletric\Neffect for all colors
Dialogue: 0,0:05:05.37,0:05:06.36,Default,,0000,0000,0000,,of light, right?
Dialogue: 0,0:05:06.36,0:05:08.52,Default,,0000,0000,0000,,If you make light bright\Nenough, electrons should be able
Dialogue: 0,0:05:08.52,0:05:11.07,Default,,0000,0000,0000,,to absorb it and just get,\Nyou know just get admitted.
Dialogue: 0,0:05:11.07,0:05:12.27,Default,,0000,0000,0000,,But that doesn't happen.
Dialogue: 0,0:05:12.27,0:05:16.14,Default,,0000,0000,0000,,And this is why physicists\Nback then were puzzled
Dialogue: 0,0:05:16.14,0:05:19.20,Default,,0000,0000,0000,,and we were desperately in\Nneed of an answer for this.
Dialogue: 0,0:05:19.20,0:05:20.91,Default,,0000,0000,0000,,So what did we do?
Dialogue: 0,0:05:20.91,0:05:22.92,Default,,0000,0000,0000,,Well, to explain these\Nobservations, we came up with
Dialogue: 0,0:05:22.92,0:05:26.73,Default,,0000,0000,0000,,a completely brand new model of light.
Dialogue: 0,0:05:26.73,0:05:29.46,Default,,0000,0000,0000,,Instead of thinking of\Nlight as waves that carry
Dialogue: 0,0:05:29.46,0:05:31.98,Default,,0000,0000,0000,,energy continuously and that can transfer
Dialogue: 0,0:05:31.98,0:05:36.00,Default,,0000,0000,0000,,energy continuously,\Nwe thought maybe light
Dialogue: 0,0:05:36.00,0:05:40.20,Default,,0000,0000,0000,,is made of discrete packets\Nof energy, not waves,
Dialogue: 0,0:05:40.20,0:05:43.23,Default,,0000,0000,0000,,but packets of energy,\Nwhich we call photons.
Dialogue: 0,0:05:43.23,0:05:46.41,Default,,0000,0000,0000,,And then light is being\Nabsorbed by say electrons.
Dialogue: 0,0:05:46.41,0:05:47.94,Default,,0000,0000,0000,,You also absorb it as packets.
Dialogue: 0,0:05:47.94,0:05:50.64,Default,,0000,0000,0000,,You'll either absorb no\Nlight or you'll absorb
Dialogue: 0,0:05:50.64,0:05:53.19,Default,,0000,0000,0000,,one packet of light or\Ntwo packets of light
Dialogue: 0,0:05:53.19,0:05:56.00,Default,,0000,0000,0000,,and so on and so forth,\Nnothing in between.
Dialogue: 0,0:05:56.00,0:05:58.47,Default,,0000,0000,0000,,We call this discrete,\Nwhich is exactly opposite
Dialogue: 0,0:05:58.47,0:06:00.03,Default,,0000,0000,0000,,of what happens in wave model,
Dialogue: 0,0:06:00.03,0:06:02.94,Default,,0000,0000,0000,,there you can absorb continuously.
Dialogue: 0,0:06:02.94,0:06:06.18,Default,,0000,0000,0000,,Okay, so how does this explain\Nthe photoelectric effect,
Dialogue: 0,0:06:06.18,0:06:07.62,Default,,0000,0000,0000,,the observations over here?
Dialogue: 0,0:06:07.62,0:06:09.25,Default,,0000,0000,0000,,Well, let's see.
Dialogue: 0,0:06:09.25,0:06:12.10,Default,,0000,0000,0000,,The key thing over here is\Nthat the energy of the photons
Dialogue: 0,0:06:12.10,0:06:14.10,Default,,0000,0000,0000,,or the packets notice\Ndepends on the color.
Dialogue: 0,0:06:14.10,0:06:15.81,Default,,0000,0000,0000,,If you're dealing with long wavelength
Dialogue: 0,0:06:15.81,0:06:18.79,Default,,0000,0000,0000,,or low frequency light,\Nthen we have less energy
Dialogue: 0,0:06:18.79,0:06:22.50,Default,,0000,0000,0000,,of the packet, the\Nphotons have less energy.
Dialogue: 0,0:06:22.50,0:06:24.87,Default,,0000,0000,0000,,And if you're dealing\Nwith short wavelength
Dialogue: 0,0:06:24.87,0:06:28.05,Default,,0000,0000,0000,,or high frequency light, you\Ncan see that the packets have
Dialogue: 0,0:06:28.05,0:06:29.31,Default,,0000,0000,0000,,more energy.
Dialogue: 0,0:06:29.31,0:06:32.91,Default,,0000,0000,0000,,So shorter the wavelength\Nor more the frequency,
Dialogue: 0,0:06:32.91,0:06:35.31,Default,,0000,0000,0000,,there is more energy in the packet.
Dialogue: 0,0:06:35.31,0:06:37.83,Default,,0000,0000,0000,,There is a relationship between\Nenergy and the wavelength,
Dialogue: 0,0:06:37.83,0:06:39.15,Default,,0000,0000,0000,,which we'll not get into.
Dialogue: 0,0:06:39.15,0:06:41.37,Default,,0000,0000,0000,,But lemme just give you\Nsome rough numbers over here
Dialogue: 0,0:06:41.37,0:06:43.20,Default,,0000,0000,0000,,because the numbers is\Ngonna help us over here.
Dialogue: 0,0:06:43.20,0:06:45.63,Default,,0000,0000,0000,,So here are some numbers.
Dialogue: 0,0:06:45.63,0:06:47.93,Default,,0000,0000,0000,,So it turns out that if\Nyou consider red light
Dialogue: 0,0:06:49.25,0:06:51.02,Default,,0000,0000,0000,,of 650 nanometers, the energy\Nof the packet, the energy
Dialogue: 0,0:06:51.02,0:06:53.85,Default,,0000,0000,0000,,of the photon is about 1.9 electron volt.
Dialogue: 0,0:06:53.85,0:06:56.31,Default,,0000,0000,0000,,Yeah, maybe wondering,\Nshouldn't we be measuring
Dialogue: 0,0:06:56.31,0:06:57.33,Default,,0000,0000,0000,,energy in joules?
Dialogue: 0,0:06:57.33,0:07:00.33,Default,,0000,0000,0000,,Well, joule turns out to\Nbe a big unit of energy.
Dialogue: 0,0:07:00.33,0:07:01.98,Default,,0000,0000,0000,,So we use a smaller unit of energy,
Dialogue: 0,0:07:01.98,0:07:03.60,Default,,0000,0000,0000,,which we call electron volts.
Dialogue: 0,0:07:03.60,0:07:06.68,Default,,0000,0000,0000,,Don't worry too much\Nabout the units over here,
Dialogue: 0,0:07:06.68,0:07:07.76,Default,,0000,0000,0000,,it's just the numbers.
Dialogue: 0,0:07:07.76,0:07:10.56,Default,,0000,0000,0000,,You can see these\Npackets have tiny energy,
Dialogue: 0,0:07:10.56,0:07:12.57,Default,,0000,0000,0000,,but this packet has much bigger energy.
Dialogue: 0,0:07:12.57,0:07:14.55,Default,,0000,0000,0000,,2.8 electron volts, you\Ncan see that, right.
Dialogue: 0,0:07:14.55,0:07:17.04,Default,,0000,0000,0000,,Now for potassium it turns out,
Dialogue: 0,0:07:17.04,0:07:18.36,Default,,0000,0000,0000,,if you want to pluck an electron,
Dialogue: 0,0:07:18.36,0:07:21.54,Default,,0000,0000,0000,,if electron needs to be\Nejected, the minimum energy
Dialogue: 0,0:07:21.54,0:07:26.10,Default,,0000,0000,0000,,that you need is about 2.3 electron volts.
Dialogue: 0,0:07:26.10,0:07:27.75,Default,,0000,0000,0000,,This is for potassium.
Dialogue: 0,0:07:27.75,0:07:29.58,Default,,0000,0000,0000,,Now is a great time for\Nyou to pause the video
Dialogue: 0,0:07:29.58,0:07:30.81,Default,,0000,0000,0000,,and see if you can try and come up
Dialogue: 0,0:07:30.81,0:07:32.67,Default,,0000,0000,0000,,with an explanation over here.
Dialogue: 0,0:07:32.67,0:07:33.87,Default,,0000,0000,0000,,Alright, let's see.
Dialogue: 0,0:07:33.87,0:07:37.29,Default,,0000,0000,0000,,The big idea over here is\Nthat if you want to knock off
Dialogue: 0,0:07:37.29,0:07:38.77,Default,,0000,0000,0000,,an electron, I mean like you know,
Dialogue: 0,0:07:38.77,0:07:41.37,Default,,0000,0000,0000,,make that electron escape,\Nthen a single photon
Dialogue: 0,0:07:41.37,0:07:44.28,Default,,0000,0000,0000,,should have at least this much energy.
Dialogue: 0,0:07:44.28,0:07:47.07,Default,,0000,0000,0000,,If the photons do not have\Nat least this much energy,
Dialogue: 0,0:07:47.07,0:07:48.57,Default,,0000,0000,0000,,then the electron will absorb it,
Dialogue: 0,0:07:48.57,0:07:50.85,Default,,0000,0000,0000,,but it's not enough to escape,
Dialogue: 0,0:07:50.85,0:07:52.95,Default,,0000,0000,0000,,and so it'll just reradiate it back.
Dialogue: 0,0:07:52.95,0:07:56.85,Default,,0000,0000,0000,,And therefore, if you\Nhave consider red light,
Dialogue: 0,0:07:56.85,0:07:58.65,Default,,0000,0000,0000,,it does not have a single photon,
Dialogue: 0,0:07:58.65,0:08:00.27,Default,,0000,0000,0000,,does not carry enough energy.
Dialogue: 0,0:08:00.27,0:08:02.19,Default,,0000,0000,0000,,And that's the reason\Nwhy electrons are not
Dialogue: 0,0:08:02.19,0:08:03.69,Default,,0000,0000,0000,,getting injected over here.
Dialogue: 0,0:08:03.69,0:08:06.09,Default,,0000,0000,0000,,And that's why these lights are unable
Dialogue: 0,0:08:06.09,0:08:07.68,Default,,0000,0000,0000,,to give you photoelectric effect.
Dialogue: 0,0:08:07.68,0:08:10.59,Default,,0000,0000,0000,,Over here, we have just enough energy
Dialogue: 0,0:08:10.59,0:08:13.07,Default,,0000,0000,0000,,for photoelectric effect\Nand therefore electrons
Dialogue: 0,0:08:13.07,0:08:16.40,Default,,0000,0000,0000,,barely make it out over here,\Nbecause all of the energy
Dialogue: 0,0:08:16.40,0:08:18.81,Default,,0000,0000,0000,,is used up in just\Nreleasing the electrons.
Dialogue: 0,0:08:18.81,0:08:20.82,Default,,0000,0000,0000,,There's hardly any energy left over here,
Dialogue: 0,0:08:20.82,0:08:22.80,Default,,0000,0000,0000,,so there'll be hardly moving.
Dialogue: 0,0:08:22.80,0:08:26.22,Default,,0000,0000,0000,,But over here, notice you\Nhave more than the necessary
Dialogue: 0,0:08:26.22,0:08:30.03,Default,,0000,0000,0000,,energy over here and therefore\Nsome residual energy is left.
Dialogue: 0,0:08:30.03,0:08:33.69,Default,,0000,0000,0000,,And so electrons after coming\Nout have some extra energy
Dialogue: 0,0:08:33.69,0:08:36.68,Default,,0000,0000,0000,,remaining that goes out as kinetic energy.
Dialogue: 0,0:08:36.68,0:08:38.19,Default,,0000,0000,0000,,And since this has even more energy,
Dialogue: 0,0:08:38.19,0:08:41.25,Default,,0000,0000,0000,,each photon has even more\Nenergy while electrons now
Dialogue: 0,0:08:41.25,0:08:43.20,Default,,0000,0000,0000,,eject with even more kinetic energy,
Dialogue: 0,0:08:43.20,0:08:46.05,Default,,0000,0000,0000,,'cause there's more residual\Nenergy after getting ejected.
Dialogue: 0,0:08:46.05,0:08:47.88,Default,,0000,0000,0000,,But what about the intensity?
Dialogue: 0,0:08:47.88,0:08:51.06,Default,,0000,0000,0000,,Well, if you increase the\Nintensity in this model,
Dialogue: 0,0:08:51.06,0:08:54.45,Default,,0000,0000,0000,,we are increasing the number\Nof photons, that's it.
Dialogue: 0,0:08:54.45,0:08:57.15,Default,,0000,0000,0000,,Over here notice if a\Nsingle photon does not have
Dialogue: 0,0:08:57.15,0:08:59.57,Default,,0000,0000,0000,,enough energy, then I don't\Ncare how many photons you shine,
Dialogue: 0,0:08:59.57,0:09:01.59,Default,,0000,0000,0000,,it's just not going to work.
Dialogue: 0,0:09:01.59,0:09:03.88,Default,,0000,0000,0000,,That's why here I will still not get any,
Dialogue: 0,0:09:03.88,0:09:05.43,Default,,0000,0000,0000,,you know, photoelectric effect.
Dialogue: 0,0:09:05.43,0:09:08.94,Default,,0000,0000,0000,,But over here now I'm shining\Nmore number of photons,
Dialogue: 0,0:09:08.94,0:09:11.29,Default,,0000,0000,0000,,so more electrons can absorb that energy
Dialogue: 0,0:09:12.18,0:09:13.65,Default,,0000,0000,0000,,and therefore more electrons\Ncan escape per second.
Dialogue: 0,0:09:13.65,0:09:16.11,Default,,0000,0000,0000,,And that's why I get\Nmore electrons over here,
Dialogue: 0,0:09:16.11,0:09:17.64,Default,,0000,0000,0000,,putting it all together.
Dialogue: 0,0:09:17.64,0:09:20.31,Default,,0000,0000,0000,,Since the wave of the\Nfrequency decides the energy
Dialogue: 0,0:09:20.31,0:09:23.70,Default,,0000,0000,0000,,of an individual photon that\Ndecides the kinetic energy,
Dialogue: 0,0:09:23.70,0:09:27.06,Default,,0000,0000,0000,,shorter the wavelength,\Nstronger, more is the energy
Dialogue: 0,0:09:27.06,0:09:30.33,Default,,0000,0000,0000,,of the photon and more\Nis the kinetic energy.
Dialogue: 0,0:09:30.33,0:09:32.91,Default,,0000,0000,0000,,If the wavelength is\Nbigger and it's too big,
Dialogue: 0,0:09:32.91,0:09:35.16,Default,,0000,0000,0000,,the energy of the photon is very tiny,
Dialogue: 0,0:09:35.16,0:09:36.93,Default,,0000,0000,0000,,it'll not be able to knock off anything
Dialogue: 0,0:09:36.93,0:09:39.21,Default,,0000,0000,0000,,and you'll not get any\Nphotoelectric effect.
Dialogue: 0,0:09:39.21,0:09:42.81,Default,,0000,0000,0000,,And since intensity is\Nbasically the number of photons,
Dialogue: 0,0:09:42.81,0:09:45.93,Default,,0000,0000,0000,,if you have more number of\Nphotons, you'll get more number
Dialogue: 0,0:09:45.93,0:09:47.28,Default,,0000,0000,0000,,of electrons coming out.
Dialogue: 0,0:09:47.28,0:09:50.43,Default,,0000,0000,0000,,But over here, it doesn't matter\Nhow many photons you shine,
Dialogue: 0,0:09:50.43,0:09:52.74,Default,,0000,0000,0000,,and therefore it doesn't\Nmatter what the brightness is,
Dialogue: 0,0:09:52.74,0:09:54.66,Default,,0000,0000,0000,,you will not get photoelectric effect.
Dialogue: 0,0:09:54.66,0:09:56.25,Default,,0000,0000,0000,,Beautiful, isn't it?
Dialogue: 0,0:09:56.25,0:09:59.04,Default,,0000,0000,0000,,So wait, does this mean\Nthat light is not a wave?
Dialogue: 0,0:09:59.04,0:10:01.08,Default,,0000,0000,0000,,It's actually particles?
Dialogue: 0,0:10:01.08,0:10:02.40,Default,,0000,0000,0000,,Well, not quiet.
Dialogue: 0,0:10:02.40,0:10:05.13,Default,,0000,0000,0000,,You see certain phenomena\Nof light like diffraction
Dialogue: 0,0:10:05.13,0:10:09.60,Default,,0000,0000,0000,,or interference means that\Nlight must have wave properties
Dialogue: 0,0:10:09.60,0:10:13.44,Default,,0000,0000,0000,,and certain other phenomenon\Nlike photoelectric effect,
Dialogue: 0,0:10:13.44,0:10:17.33,Default,,0000,0000,0000,,black body radiation, scattering of light,
Dialogue: 0,0:10:17.33,0:10:19.14,Default,,0000,0000,0000,,and other such effects makes us believe
Dialogue: 0,0:10:19.14,0:10:22.41,Default,,0000,0000,0000,,that light must also\Nhave the particle nature,
Dialogue: 0,0:10:22.41,0:10:26.52,Default,,0000,0000,0000,,the photon nature, which\Nmeans a light must have
Dialogue: 0,0:10:26.52,0:10:29.76,Default,,0000,0000,0000,,a dual nature, both particle and waves.
Dialogue: 0,0:10:29.76,0:10:31.85,Default,,0000,0000,0000,,It's not that light\Nsometimes behaves as waves
Dialogue: 0,0:10:31.85,0:10:33.67,Default,,0000,0000,0000,,and sometimes wears as particle.
Dialogue: 0,0:10:33.67,0:10:34.63,Default,,0000,0000,0000,,No, no, no.
Dialogue: 0,0:10:34.63,0:10:36.66,Default,,0000,0000,0000,,Light has both wave and particle nature.
Dialogue: 0,0:10:36.66,0:10:38.58,Default,,0000,0000,0000,,And if you're wondering, well,\Nhow does that make any sense?
Dialogue: 0,0:10:38.58,0:10:40.11,Default,,0000,0000,0000,,How can something be both waves
Dialogue: 0,0:10:40.11,0:10:42.15,Default,,0000,0000,0000,,and particles at the same time?
Dialogue: 0,0:10:42.15,0:10:45.78,Default,,0000,0000,0000,,Well, unfortunately, there's\Nno way to really visualize it
Dialogue: 0,0:10:45.78,0:10:47.22,Default,,0000,0000,0000,,because in our macroscopic world,
Dialogue: 0,0:10:47.22,0:10:50.22,Default,,0000,0000,0000,,we don't have any experience\Nof things having both wave
Dialogue: 0,0:10:50.22,0:10:52.50,Default,,0000,0000,0000,,and particle nature.
Dialogue: 0,0:10:52.50,0:10:54.18,Default,,0000,0000,0000,,But this is one of the reasons\Nwhy sometimes when we are
Dialogue: 0,0:10:54.18,0:10:56.04,Default,,0000,0000,0000,,showing photons, we show it this way
Dialogue: 0,0:10:56.04,0:10:59.97,Default,,0000,0000,0000,,with a tiny wave packet,\Nbut this doesn't mean
Dialogue: 0,0:10:59.97,0:11:01.92,Default,,0000,0000,0000,,that the photons are wiggling up and down.
Dialogue: 0,0:11:01.92,0:11:03.90,Default,,0000,0000,0000,,Okay, that's a misconception\Nthat I used to have.
Dialogue: 0,0:11:03.90,0:11:05.87,Default,,0000,0000,0000,,It's not like that.
Dialogue: 0,0:11:05.87,0:11:07.71,Default,,0000,0000,0000,,A better way to sort of\Nthink about this is that
Dialogue: 0,0:11:07.71,0:11:11.04,Default,,0000,0000,0000,,light is not a wave in\Nthe traditional sense.
Dialogue: 0,0:11:11.04,0:11:13.83,Default,,0000,0000,0000,,It's not a particle in\Nthe traditional sense,
Dialogue: 0,0:11:13.83,0:11:17.11,Default,,0000,0000,0000,,it's a brand new object,\Nwhich we don't have
Dialogue: 0,0:11:17.11,0:11:19.41,Default,,0000,0000,0000,,experience within our daily life.
Dialogue: 0,0:11:19.41,0:11:21.27,Default,,0000,0000,0000,,This object has both wave properties
Dialogue: 0,0:11:21.27,0:11:24.24,Default,,0000,0000,0000,,and particle properties,\Nand we call such an object,
Dialogue: 0,0:11:24.24,0:11:26.01,Default,,0000,0000,0000,,a quantum object.
Dialogue: 0,0:11:26.01,0:11:28.26,Default,,0000,0000,0000,,Now this sounds very theoretical, right?
Dialogue: 0,0:11:28.26,0:11:30.12,Default,,0000,0000,0000,,But there are so many\Napplications of the fact
Dialogue: 0,0:11:30.12,0:11:31.71,Default,,0000,0000,0000,,that light is a quantum object.
Dialogue: 0,0:11:31.71,0:11:34.23,Default,,0000,0000,0000,,Let me tell you one of them, okay?
Dialogue: 0,0:11:34.23,0:11:36.21,Default,,0000,0000,0000,,Now, in photoelectric effect from light,
Dialogue: 0,0:11:36.21,0:11:38.13,Default,,0000,0000,0000,,we get electrons ejected, right?
Dialogue: 0,0:11:39.22,0:11:40.05,Default,,0000,0000,0000,,Now there's a very similar,
Dialogue: 0,0:11:40.05,0:11:42.72,Default,,0000,0000,0000,,slightly different effect\Ncalled (indistinct)
Dialogue: 0,0:11:42.72,0:11:45.33,Default,,0000,0000,0000,,when you shine light,\Nyou can generate voltage.
Dialogue: 0,0:11:45.33,0:11:49.41,Default,,0000,0000,0000,,We call such an effect,\Na photovoltaic effect.
Dialogue: 0,0:11:49.41,0:11:53.52,Default,,0000,0000,0000,,Now, the way that works is we\Nneed to first create a crystal
Dialogue: 0,0:11:53.52,0:11:57.03,Default,,0000,0000,0000,,in which there's an already\Ninbuilt electric field.
Dialogue: 0,0:11:57.03,0:11:58.35,Default,,0000,0000,0000,,It's possible to do that.
Dialogue: 0,0:11:58.35,0:12:00.27,Default,,0000,0000,0000,,We'll not get too much\Ndetails of how we build
Dialogue: 0,0:12:00.27,0:12:02.28,Default,,0000,0000,0000,,such crystals, but using semiconductors,
Dialogue: 0,0:12:02.28,0:12:04.08,Default,,0000,0000,0000,,we can build crystals like that.
Dialogue: 0,0:12:04.08,0:12:06.60,Default,,0000,0000,0000,,We don't have to hook it up\Nto any battery or anything.
Dialogue: 0,0:12:06.60,0:12:08.91,Default,,0000,0000,0000,,It'll have an inbuilt electric field.
Dialogue: 0,0:12:08.91,0:12:11.28,Default,,0000,0000,0000,,The crystals is built in\Nsuch a way that one side
Dialogue: 0,0:12:11.28,0:12:13.35,Default,,0000,0000,0000,,of the crystal has slightly\Ndifferent properties compared
Dialogue: 0,0:12:13.35,0:12:14.94,Default,,0000,0000,0000,,to another side of the crystal.
Dialogue: 0,0:12:14.94,0:12:16.32,Default,,0000,0000,0000,,And because of the\Ndifference in properties,
Dialogue: 0,0:12:16.32,0:12:18.18,Default,,0000,0000,0000,,an electric field gets built up.
Dialogue: 0,0:12:18.18,0:12:20.64,Default,,0000,0000,0000,,Now the important point is\Nthere are electrons everywhere,
Dialogue: 0,0:12:20.64,0:12:23.46,Default,,0000,0000,0000,,but if you focus on this\Nregion, there are a lot of
Dialogue: 0,0:12:23.46,0:12:26.94,Default,,0000,0000,0000,,electrons, but they're\Nall bonded and they're not
Dialogue: 0,0:12:26.94,0:12:27.77,Default,,0000,0000,0000,,free to move.
Dialogue: 0,0:12:27.77,0:12:29.70,Default,,0000,0000,0000,,So even if there's an electric\Nfield pushing on them,
Dialogue: 0,0:12:29.70,0:12:32.58,Default,,0000,0000,0000,,they cannot move because\Nthey're stuck in bonds.
Dialogue: 0,0:12:32.58,0:12:35.04,Default,,0000,0000,0000,,You can imagine that this is\Nlike the sea of electrons.
Dialogue: 0,0:12:35.04,0:12:37.83,Default,,0000,0000,0000,,They're all kind of\Nfixed inside the crystal,
Dialogue: 0,0:12:37.83,0:12:39.02,Default,,0000,0000,0000,,they cannot move.
Dialogue: 0,0:12:39.02,0:12:43.65,Default,,0000,0000,0000,,But if we shine light in this region,
Dialogue: 0,0:12:43.65,0:12:45.30,Default,,0000,0000,0000,,and if the light has\Nthe suitable frequency
Dialogue: 0,0:12:45.30,0:12:48.93,Default,,0000,0000,0000,,or the suitable wavelength,\Nthen the electrons can absorb
Dialogue: 0,0:12:48.93,0:12:51.30,Default,,0000,0000,0000,,that energy, but it won't get emitted.
Dialogue: 0,0:12:51.30,0:12:53.23,Default,,0000,0000,0000,,Okay, that's the difference over here.
Dialogue: 0,0:12:53.23,0:12:54.49,Default,,0000,0000,0000,,In photo effect, it gets emitted.
Dialogue: 0,0:12:54.49,0:12:56.12,Default,,0000,0000,0000,,But here, instead of getting emitted,
Dialogue: 0,0:12:56.12,0:12:58.02,Default,,0000,0000,0000,,it just gets enough\Nenergy to escape the bond.
Dialogue: 0,0:12:58.02,0:13:01.17,Default,,0000,0000,0000,,And as a result, now it's\Nfree to move and therefore
Dialogue: 0,0:13:01.17,0:13:03.54,Default,,0000,0000,0000,,it'll get accelerated to\Nthe left in this diagram,
Dialogue: 0,0:13:03.54,0:13:05.70,Default,,0000,0000,0000,,because electric field to the\Nright electrons are negatively
Dialogue: 0,0:13:05.70,0:13:08.04,Default,,0000,0000,0000,,charged the experience of force\Nin the opposite direction.
Dialogue: 0,0:13:08.04,0:13:12.21,Default,,0000,0000,0000,,And as a result, it will\Nnow come to the left side
Dialogue: 0,0:13:12.21,0:13:14.88,Default,,0000,0000,0000,,and it'll leave behind a gap.
Dialogue: 0,0:13:14.88,0:13:17.70,Default,,0000,0000,0000,,Now, other electrons, under\Nelectrons, which are bonded,
Dialogue: 0,0:13:17.70,0:13:21.30,Default,,0000,0000,0000,,can swoop into this gap,\Nwhich makes the gap go
Dialogue: 0,0:13:21.30,0:13:23.73,Default,,0000,0000,0000,,to the right, and then the\Nother electrons can swoop into
Dialogue: 0,0:13:23.73,0:13:25.11,Default,,0000,0000,0000,,this gap and so on and so forth.
Dialogue: 0,0:13:25.11,0:13:29.49,Default,,0000,0000,0000,,So it kind of feels that this\Ngap, this latency will move in
Dialogue: 0,0:13:29.49,0:13:30.87,Default,,0000,0000,0000,,to the other side.
Dialogue: 0,0:13:30.87,0:13:34.10,Default,,0000,0000,0000,,This way, a lot of electrons\Nand a lot of vacancies
Dialogue: 0,0:13:34.10,0:13:35.91,Default,,0000,0000,0000,,can be created.
Dialogue: 0,0:13:35.91,0:13:38.31,Default,,0000,0000,0000,,And so look, if you can\Ncomplete this circuit,
Dialogue: 0,0:13:38.31,0:13:40.71,Default,,0000,0000,0000,,electrons would love\Nto go from here to here
Dialogue: 0,0:13:40.71,0:13:42.27,Default,,0000,0000,0000,,through that external circuit.
Dialogue: 0,0:13:42.27,0:13:45.69,Default,,0000,0000,0000,,In other words, there\Nis a voltage created.
Dialogue: 0,0:13:45.69,0:13:49.32,Default,,0000,0000,0000,,And so what we have done is\Nwe have used the energy from
Dialogue: 0,0:13:49.32,0:13:53.49,Default,,0000,0000,0000,,light to create voltage\Nphotovoltaic effect.
Dialogue: 0,0:13:53.49,0:13:58.49,Default,,0000,0000,0000,,If you put a lot of these\Ntogether, we create a solar panel.
Dialogue: 0,0:13:58.80,0:14:01.50,Default,,0000,0000,0000,,That's how solar cells\Nand solar panels work.
Dialogue: 0,0:14:01.50,0:14:04.53,Default,,0000,0000,0000,,They work on the photovoltaic effect.
Dialogue: 0,0:14:04.53,0:14:07.14,Default,,0000,0000,0000,,Whether you consider them\Non the roofs of the houses
Dialogue: 0,0:14:07.14,0:14:09.72,Default,,0000,0000,0000,,or you consider the ones\Nwhich are in the spacecraft,
Dialogue: 0,0:14:09.72,0:14:11.61,Default,,0000,0000,0000,,they all use the same idea.
Dialogue: 0,0:14:11.61,0:14:13.50,Default,,0000,0000,0000,,At the end of the day,\Nwe are using the fact
Dialogue: 0,0:14:13.50,0:14:17.10,Default,,0000,0000,0000,,that light is a quantum\Nobject to harness the power
Dialogue: 0,0:14:17.10,0:14:19.29,Default,,0000,0000,0000,,of light, which we get from the sun.