0:01 the topic of this video is
0:04 electromagnetic energy and specifically
0:05 the photoelectric effect
0:07 uh we'll be describing the particle
0:09 nature of light in this video
0:11 before i jump into um some observations
0:13 of the photoelectric effect i just want to
0:13 to
0:16 uh briefly describe what the
0:18 photoelectric effect is based off of so
0:20 there's an observation that if we take
0:23 a metal plate okay so this is a metal plate
0:24 plate
0:27 if you shine light onto it
0:30 okay uh what sometimes depending on the
0:31 light source
0:33 we can actually observe so this is light
0:35 coming in
0:38 we can observe an electron or electrons
0:40 being ejected from
0:42 the surface of the metal plate and the
0:44 energy of those electrons
0:46 can be uh determined based off of the
0:48 velocity that they are ejected from the plate
0:49 plate
0:52 so what is the photoelectric effect then
0:55 let's take a look uh we have three
0:56 examples of different wavelengths here
0:59 so we have 700 nanometer
1:02 light 700 nanometer 550 nanometer and
1:04 400 nanometer
1:07 so the the uh
1:09 what i want to do before jumping into
1:10 the observation is to to remind you that
1:12 at 700 nanometer
1:15 light this is going to be the um
1:17 low frequency light in this particular
1:19 case 550 nanometer light is going to be the
1:20 the
1:23 medium frequency and 400 nanometer is
1:24 going to be the
1:26 high frequency so remember frequency and
1:28 wavelength are inversely related so the
1:30 higher the wavelength the lower the
1:32 frequency and vice versa
1:35 so in the case of 700 nanometer light on
1:37 the left what happens is
1:40 um when this light is being uh
1:42 exposed to the metal surface no
1:45 electrons are ejected
1:48 even if you increase the the brightness
1:49 of the light source
1:50 okay you increase the brightness of the
1:53 light source no electrons are ejected
1:54 which is an interesting
1:56 thing because if light is behaving
1:57 exactly as
1:59 a wave and only as a wave then
2:01 increasing the brightness should
2:04 in theory increase the energy of the
2:06 wave okay that's sort of the classical
2:07 view of
2:10 of waves but what we can see here is that
2:11 that
2:13 so so this uh no electrons are ejected
2:15 at 700 nanometers regardless of the
2:17 intensity of the light
2:20 but for 550 nanometer light which is has
2:21 a higher frequency
2:24 we do see electrons being ejected at a
2:26 velocity of 2.96 times 10 to the fifth
2:28 meters per second
2:30 if we increase the frequency yet again or
2:31 or
2:32 decrease the wavelength however you want
2:35 to view it
2:37 what we see is not only are electrons
2:38 still being ejected
2:40 right but they are being ejected now
2:42 with even greater kinetic energy at
2:44 with a with a velocity of six point two
2:45 two times ten to the fifth
2:49 meters per second so so really what this
2:52 exposed was a was a a paradox that
2:55 if light strictly behaved as a wave
2:56 there's no way that that could this these
2:57 these
2:59 um observations could could uh uh be
3:01 explained with a wave
3:04 alone because again increasing the
3:06 intensity of 700 nanometer light
3:09 did not cause electrons to be ejected
3:10 from the surface
3:12 it appears that there is some energy
3:14 quantization happening in the light itself
3:15 itself
3:17 that is not dependent on the amplitude
3:19 of the light but on the frequency
3:20 or the wavelength however you want to
3:22 view it and recall that this is what max
3:23 planck had been working on
3:26 quantized energy of emission from black
3:27 body radiators
3:30 so einstein made the connection there
3:33 before i jump into um
3:35 the uh the equation well i guess it's up
3:36 here i'll give i'll write out the
3:38 equation again in one second but
3:40 in general what i want you to also keep
3:41 in mind is that
3:45 when energy is released
3:48 by us by a substance or a system when
3:50 energy is released by system we call this
3:54 exothermic
3:58 and when energy is absorbed
4:03 by a system we call this endothermic
4:07 okay um so the equation then
4:10 that um einstein used to assign
4:13 to enable us to calculate energy of of light
4:14 light
4:21 particles is energy is equal to planck's
4:22 constant times
4:24 frequency um so he's just pretty much
4:26 applying planck's
4:30 quantization here to now think about
4:33 uh light as particles
4:35 as well as waves okay so if we think
4:37 about um
4:42 light as
4:45 particles this is what we refer to as
4:47 photons okay so when we think about a
4:49 photon we are thinking about a discrete
4:55 particle of electromagnetic energy
4:57 and again this is now referring to
4:59 something called the wave particle
5:00 duality if we take
5:02 electromagnetic radiation and do an
5:03 experiment like the double slit
5:05 experiment we see into an interference pattern
5:05 pattern
5:08 which is the culmination of constructive
5:09 and interference
5:11 of waves so that's clearly as a wave feature
5:12 feature
5:13 but if we do with the photoelectric
5:16 effect we see behaviors that can only be
5:17 explained by particles
5:20 okay so so this is considered the wave
5:23 particle duality so let's go ahead and
5:24 use this equation
5:27 now which we can now think about um
5:30 uh is really useful for energy being
5:31 absorbed or emitted by
5:34 uh substances to solve a practice problem
5:36 problem
5:37 so you can pause the video now if you
5:40 want to write this down in your notes
5:41 i'm going to go ahead and jump into it
5:43 so we have light from a neon sign
5:45 we are observing radiation from excited
5:48 neon atoms okay so the neon atoms are
5:50 emitting energy this is an exothermic
5:51 process you could say
5:53 and if this radiation has a wavelength
5:54 of 640
5:56 nanometers what is the energy of the
5:58 photon being emitted well we have this
6:00 equation the energy of light being
6:02 emitted is equal to
6:03 planck's constant times the speed of
6:05 light over wavelength so let's go ahead
6:06 and plug those
6:10 in 6.626 times 10 to the negative 34
6:12 joules seconds that's multiplied by each
6:14 other those units
6:16 that's a constant multiplied by another
6:19 constant 2.998 times 10 to the 8
6:21 meters per second i'm using the s to the
6:24 negative 1 to indicate per second
6:28 and we're going to divide all of that by
6:31 640 nanometers but right away you should
6:32 be looking for units to be cancelling out
6:33 out
6:34 and what we can see here is that
6:36 reciprocal seconds will cancel out with
6:38 seconds on the top
6:40 but meters on the top cannot cancel out
6:42 with nanometers we have to reconcile
6:44 those units so we're going to convert nanometers
6:45 nanometers
6:47 to meters in the bottom so i'm putting
6:48 nanometers down here
6:52 and meters on top nanometers will cancel
6:55 here and meters will cancel
6:57 between the entire denominator and the
6:59 entire numerator
7:00 now we need the actual unit conversion here
7:02 here
7:05 so we know that in one nanometer that is
7:07 equal to a really small value of meters
7:08 one times ten to the negative ninth
7:11 meters so now all we're left with in
7:12 terms of units
7:15 is uh joules which isn't the unit of
7:18 is a unit of energy so
7:21 we have three this should give you 3.10
7:22 times 10 to the negative
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