Chemistry: 4.8 Photons

When you turn on a light, what comes out? Friends...or photons? Well, you'll know the answer as soon as you watch this video.

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Transcript

00:18

transfer causes electrons to be emitted.

00:21

Of course, not all of these scientific [Hertz stood in a lab]

00:23

greats performed their experiments in the

00:24

same place and at the same time but it's

00:26

a whole lot more fun to pretend that they did, so

00:29

here we go. First, we've got the German

00:31

physicist Heinrich Hertz, who discovered [Hertz waving in a lab]

00:34

while working with a spark-gap

00:36

transmitter, which is an ├╝ber-old radio

00:39

broadcasting device, that substances

00:41

would sometimes give off a spark after

00:43

absorbing light. Rather than chalking the

00:46

phenomenon up to it being late and the

00:48

fact that he hadn't slept much in the

00:49

past week, Hertz continued to study the [Hertz using a flash light on the spark-gap transmitter]

00:52

effect, which has been sometimes

00:53

referred to as the "Hertz effect." So yeah,

00:57

nothing to do with rental cars. The next [Hertz appears at a car rental store]

00:59

guy to throw his hat into the lab was

01:01

Phillipp Lenard, who was a former

01:04

assistant of Hertz's. So it makes sense that [Lenard walks into the lab]

01:07

they'd both be there, right? Well, Lenard

01:09

piggybacked on Hertz's research and

01:11

eventually put together a little [Lenard giving Hertz a piggyback]

01:12

experiment of his own that shed a lot of...

01:14

light... on the subject. He wanted to test

01:17

the theory of thermal emission posed by

01:20

classical mechanics that when light

01:23

shines on a surface, the transfer of

01:25

energy to that surface increases the

01:28

energy of the particles, which in turn

01:30

emit electrons. So, he built this thing:

01:33

cathode here, anode here, and this thing [Finger points to cathode and anode]

01:36

is a vacuum tube, and an ammeter here to

01:39

measure the current. Well, when Lenard let

01:42

lights of varying intensity reach the [Lenard using a flash light on the transmitter]

01:44

cathode, he found that below a certain

01:46

threshold frequency, no electrons at all

01:49

were emitted, while above that threshold

01:51

frequency, the number of electrons that

01:54

made their way to the anode was

01:55

proportional to the intensity of the

01:58

light. So what Lenard observed flew a bit

02:00

in the face of classical physics. There [Electrons flying around Lenard]

02:02

had to be some mysterious explanation

02:05

for what was happening. Enter Albert

02:07

Einstein. Go ahead, squeeze on in

02:09

there Albert; they'll make space for you. [Einstein squeezes in between Hertz and Lenard]

02:11

Albert introduced the idea of "photons,"

02:14

basically packets of light that aren't

02:16

just waves, but also actual particles of

02:19

energy. Einstein's theory of

02:21

wave-particle duality proposed that [Electrons moving around]

02:23

these photons would strike electrons and

02:25

force them to say au revoir to

02:27

their atom. Kind of like a new boyfriend

02:29

moving in in the old one moving out. You [Boyfriend moves out of house and Lenard appears]

02:31

know what we're talking about, there,

02:32

Lenard. Anyway, Einstein found that

02:35

each electron will only be ejected from

02:38

its atom if the frequency of light is [Rainbow colored light with different frequencies]

02:41

high enough. Well, this discovery

02:43

kickstarted something we now call

02:44

"quantum mechanics." Yeah, just those words

02:48

are enough to give you a headache, but it

02:49

was a huge breakthrough in the

02:50

understanding of light, energy, and

02:52

thermodynamics, and... Okay, that was bound [Hertz and Lenard shaking hands and Einstein giving thumbs up]

02:55

to happen.