Knocking Electrons With Light—The Photoelectric Effect
Summary
TLDRIn this educational video, the presenter explores the photoelectric effect, demonstrating how light, composed of particles called photons, can transfer energy to electrons. Using an electroscope, they show that while visible light cannot dislodge electrons, high-energy ultraviolet light can. The experiment illustrates the dual nature of light as both a wave and a particle, with higher frequency light behaving more like particles. The video concludes with a discussion on the wave-particle duality and the concept of particles as vibrations of quantum fields.
Takeaways
- 🔬 The experiment demonstrates that light is composed of particles, specifically photons, which can be observed through the photoelectric effect.
- 🔋 An electroscope is used to measure the electric charge on a metal plate by detecting the movement of a needle due to the charge.
- 🎈 Rubbing a balloon on hair transfers electrons to the balloon, which can then be transferred to the electroscope plate to charge it.
- 🚫 Even high-intensity visible light (red, green, and blue) does not have enough energy per photon to release electrons from the metal plate.
- 🌞 Shortwave ultraviolet light, also known as UVC, has enough energy per photon to knock electrons off the metal plate, causing the electroscope to discharge.
- ⚡ The brightness of the light affects the rate at which electrons are released but not the ability to release them; it's the energy per photon that matters.
- 🔀 When the plate is positively charged with protons, UV light does not easily release these charges, unlike with extra electrons.
- 💡 Neon light bulbs require a certain voltage to ionize the gas inside and emit light, but UV light can help initiate this process even below the usual voltage.
- 🌌 The concept of wave-particle duality is introduced, explaining how light can exhibit properties of both waves and particles depending on its frequency.
- 📚 The video concludes by highlighting Einstein's Nobel Prize-winning explanation of the photoelectric effect and the importance of understanding light as both a wave and a particle.
Q & A
What is the purpose of the electroscope used in the video?
-The electroscope is used to measure the amount of electric charge on a plate by detecting the movement of a needle due to the presence of charged particles.
How does the presenter charge the electroscope plate with electrons?
-The presenter charges the electroscope plate with electrons by rubbing a balloon on their hair, which transfers electrons from the hair to the balloon, and then wiping the balloon on the plate.
Why does touching the charged plate cause the needle to move back to its original position?
-Touching the charged plate allows the electrons to escape from the plate into the hand, neutralizing the charge and causing the needle to return to its original position.
What is the significance of the photoelectric effect demonstrated in the video?
-The photoelectric effect demonstrates that light can behave as particles, called photons, which have enough energy to knock electrons off a surface when the light's frequency is high enough.
Why doesn't visible light (red, green, and blue) discharge the electroscope plate?
-Visible light, including red, green, and blue, doesn't have enough energy per photon to overcome the binding energy of the electrons on the plate, so it cannot discharge it.
What type of light is successful in discharging the electroscope plate in the video?
-Shortwave ultraviolet light, also known as UVC light, is successful in discharging the electroscope plate because it has a higher energy per photon compared to visible light.
How does the brightness of the light affect the photoelectric effect?
-The brightness or intensity of the light affects the rate at which electrons are knocked off but does not affect the ability to knock them off. Higher intensity light increases the number of photons hitting the surface per second, increasing the rate of electron ejection.
What is the difference between charging the plate with extra electrons versus extra protons?
-Charging the plate with extra electrons results in a negative charge, which can be discharged with UV light. Charging with extra protons results in a positive charge, which is harder to discharge with light because protons are more tightly bound within the atomic nucleus.
Why does the neon light bulb require a certain voltage to light up?
-The neon light bulb requires a certain voltage to strip electrons off the gas inside, which then emit light. Below this voltage threshold, not enough electrons are excited to produce light.
How does the presenter demonstrate the photoelectric effect with the neon light bulb?
-The presenter demonstrates the photoelectric effect by shining shortwave ultraviolet light on the neon bulb, which is not enough to light it up by itself due to low voltage. The UV light provides the necessary energy to excite the gas, allowing the bulb to light up even at a voltage below the normal threshold.
What is the wave-particle duality and how does it relate to light?
-Wave-particle duality is the concept that light and other quantum entities can exhibit both wave-like and particle-like properties. The manifestation as a wave or particle depends on the frequency or wavelength; higher frequencies tend to exhibit particle-like properties (photons), while lower frequencies exhibit wave-like properties.
Outlines
🔬 Demonstrating the Photoelectric Effect
In this segment, the presenter introduces the concept of light as particles and demonstrates the photoelectric effect using an electroscope. They charge a metal plate by rubbing a balloon on their hair, transferring electrons to the plate, which is then measured by the electroscope. The presenter explains that while the plate is insulated, it can be discharged by hitting it with particles of light. They test this by shining various colors of light, including red, green, blue, and white light, but find that these do not have enough energy to dislodge electrons. The presenter then successfully uses shortwave ultraviolet light, which has higher energy per photon, to discharge the plate, showing that light behaves as particles with specific energy levels.
🌌 Exploring Particle Physics and Light's Dual Nature
The second paragraph delves deeper into the concept of light as both a particle and a wave. The presenter charges the electroscope positively by rubbing a plastic rod against wool, creating a positive charge due to the absence of electrons. They attempt to discharge the positively charged plate with ultraviolet light but find it ineffective due to the protons' strong binding energy. However, when the presenter touches the plate, allowing electrons to neutralize the positive charge, the plate discharges. This experiment illustrates the photoelectric effect, which was pivotal to Einstein's Nobel Prize. The presenter clarifies the wave-particle duality of light, explaining that light's behavior as a wave or particle depends on its frequency. Higher frequency light, like ultraviolet, behaves as particles, while lower frequency light, like radio waves, behaves as waves. The segment concludes with a demonstration using a neon light bulb, where shortwave ultraviolet light is used to initiate the flow of electrons, turning on the bulb.
📢 Conclusion and Engagement
In the final paragraph, the presenter wraps up the experiment and encourages viewers to subscribe and engage with the content. They mention the Action Lab Shorts channel for shorter videos on similar topics and thank Brandon Fisher for suggesting the photoelectric effect experiment. The presenter also provides a link to an Arbor Scientific video for those interested in replicating the experiment, showcasing the materials and sources used. The segment ends with a reminder to stay tuned for more educational content.
Mindmap
Keywords
💡Photoelectric Effect
💡Electroscope
💡Electrons
💡Binding Energy
💡Visible Light
💡Ultraviolet Light
💡Frequency
💡Lumens
💡Wave-Particle Duality
💡Quantum Field
Highlights
Introduction to the concept that light is made of particles and demonstration of the photoelectric effect.
Explanation of how an electroscope measures electric charge on a plate.
Demonstration of charging a plate with a balloon to show electric charge.
Observation that touching the plate discharges it, illustrating electron transfer.
Isolation of the charged plate to show that it only discharges through direct contact or light particles.
Experiment showing that visible light (red, green, blue) does not have enough energy to discharge the plate.
Use of ultraviolet light to successfully discharge the plate, demonstrating the photoelectric effect.
Discussion on the binding energy of electrons and the energy required to dislodge them.
Experiment with positively charged plate showing that protons are harder to dislodge than electrons.
Explanation of the photoelectric effect and its significance in winning Einstein his Nobel Prize.
Demonstration that light's brightness does not affect its ability to dislodge electrons, only the frequency does.
Alternative experiment using a neon light bulb to show the photoelectric effect with UV light.
Clarification on the wave-particle duality of light and how particles can have vibrations.
Discussion on the relationship between wavelength, frequency, and whether light appears as a wave or particle.
Conclusion and call to action for viewers to subscribe and explore related content.
Transcripts
hey everyone today i'm going to be
showing you that light is actually made
out of small little particles
i'm going to be showing you the
photoelectric effect this device here is
called an electroscope and what it can
do is it can measure the amount of
electric charge on this plate here
so let's try it out here an easy way to
get an electric charge on this is just
to use a balloon
so when i rubbed the balloon on my head
i scraped off some electrons from my
hair and they're stuck on the balloon
now
so i can wipe them onto the plate
you can see the needle move
so once it stops bouncing around you can
see that it settles at about that spot
so we can see kind of a relative measure
of electric charge on this plate
and you can see i can even add more
charge to it
added a bunch more there let it settle
down a little bit
now it'll stay like this for quite a
while because i have all these electrons
trapped on the plate here
but if i touch the plate then all the
electrons are going to escape out of the
plate
into my hand again and it goes back
so this is a really good device in order
to know how many electrons are on this
plate up here
now if i charge up this plate again so
the plate is electrically insulated from
everything around it the only way it can
transfer electrons off
is by the air rubbing against it and the
electrons getting on the air molecules
but that takes some time so for all
intents and purposes this is pretty well
isolated from the environment around it
but there's actually a way that we can
discharge this just by hitting it with a
bunch of particles
and in our case we're going to use light
as particles
the electrons that we're going to try to
knock off of this plate have a specific
binding energy so they're bound to the
nucleus of their atom that they're on
and in order to get them off of that
atom you have to hit them with a certain
amount of energy
so any particle that we throw at these
electrons that has less than a certain
amount of energy won't be able to knock
it out of that conduction band
so to start off let's use some particles
of light that have a pretty good amount
of energy
visible light so i'm going to be shining
some white light on it which is a
combination of red green and blue light
let me charge this up
all right so let's start off using red
green and blue light together or white
light
doesn't look like the needle's moving at
all
let's go higher in our lumens here
so this is this is 2000 lumens shining
on it
so we have plenty of light a ton of
energy going into this
but the problem is each specific photon
doesn't have enough energy
even though there's a lot of energy
coming off the light the specific
photons themselves don't have enough
energy to when they hit a specific
electron
brighter
even if i shine a hundred thousand
lumens on it holy cow
it does not have enough energy to knock
any electrons off of that plate
so this did not move whatsoever even
when i shine a hundred thousand lumens
on a visible light
that's because red green and blue light
simply don't have enough energy when
they hit an electron to knock it out of
that shell
let's try something that has a little
bit more energy ultraviolet light
so this is long wave ultraviolet light
so even this long wave ultraviolet light
doesn't do anything
we're going to have to go with a little
bit higher energy light
shortwave ultraviolet light
this is also called uvc light this is
the type of light that will give you a
sunburn
so now watch when i turn on this uvc
light so
this has a lot more energy per photon
released here we go
you can see that it immediately starts
moving
so we're literally knocking electrons
off of this plate with this light now
so it completely discharged it
watch this again
charge it up a ton
you can see nothing happens when i move
it over without the light on
but as soon as i turn the light on
it just drains it
look at that so this is amazing what's
happening here is literal particles of
light
are hitting electrons off of this plate
and knocking them off
so it's literal particle physics
happening here what's cool about this is
when i use the balloon i have extra
electrons on the plate
but i can actually do it the opposite
way where instead of having extra
electrons on the plate i have extra
protons on the plate
so i need to charge it positively so
let's see what happens if i charge it
positively now
so when i rub this plastic against wool
it's actually going to strip the
electrons off of the plastic so that it
leaves this positively charged
now these positive charges they can't be
knocked loose as easy as these electrons
because the positive charges are really
large they're the protons and the
nucleus of the atoms
let's try to discharge this with the uv
light now
so you can see that nothing happens
but if i just touch the plate and allow
the electrons from my hand go into the
plate
then it can discharge it so this effect
that you're seeing right now is called
the photoelectric effect
and it's actually what won einstein his
nobel prize
so if light were only a wave then that
means that it doesn't matter what the
frequency of light
is that you use if you just use enough
of it then you should be able to knock
electrons loose
for example if you just get a high
enough amplitude and really big waves of
light coming in
you should be able to discharge this
plate but you can see that i used a
hundred thousand lumen flashlight here
and i couldn't do anything so hitting
electrons with a bunch of low frequency
light doesn't do anything to knock them
loose
whereas higher frequency light like
shortwave ultraviolet light
does have enough energy per photon so
even if you have a really dim
ultraviolet light you can still knock
electrons loose the brightness and
dimness of the ultraviolet light only
affects the rate at which the electrons
are knocked off but it doesn't affect
the ability to knock them off
you can actually do this a little bit
easier if you don't have an electroscope
like this but you just have a small neon
light bulb
so my neon bulbs right here and you can
see that if i increase the voltage right
now we're at 67 volts
right when i get around 89 volts
it'll turn on
there it goes so i have to have at least
this voltage to turn it on
but now if i get it to let's just put it
at 88
volts okay so the light bulb is not
lighting
the way these neon bulbs work is there's
two electrodes in here
and in order to get light to occur it
has to strip the electrons off of the
gas that's inside of there
so right now there's not quite enough
voltage to start stripping the electrons
off
but we can actually induce those
electrons to start being stripped off
just by shining that shortwave
ultraviolet light at this light bulb
so you can see it doesn't light but now
let's turn on our light bulb
[Music]
and it turns on and once i start it
flowing it actually keeps flowing i
don't even have to keep the light on
because now it heats up a little bit so
it can keep flowing what's a little bit
confusing about what i've been saying is
i'm talking about light as though it
were discrete particles
but then i'm telling you that it has a
specific frequency
i mean the definition of a frequency is
the number of oscillations
in a given amount of time but i'm
talking about a particle how does a
particle have
oscillations if it's not a wave well to
understand this we have to understand
what a particle even means how do you
define a particle
now this is going to sound weird but the
actual definition of a particle
is the possible smallest vibration of a
quantum field
and the photon like the photons of light
i've been talking about
is the smallest possible vibration of
the electromagnetic field
and then there are even other fields for
example a quark is the smallest possible
vibration of a quark field so that's how
a particle can still have a vibration
because we're still talking about a
vibration of an underlying field but the
vibration of that quantum field
manifests itself as a discrete particle
this is why we have the wave particle
duality why sometimes light can appear
as a wave
and also appear as a particle now
whether we talk about something as a
wave or a particle kind of depends on
its wavelength or its frequency
the higher frequency particles like
ultraviolet light and x-rays
tend to always manifest themselves as
particles so we usually can't see the
peaks and valleys of their wave form
because they're too close together
so we usually call the higher frequency
things particles
whereas particles with lower frequency
we usually just always call them waves
for example radio waves have an
extremely long wavelength and a low
frequency
so we usually only see their wave
properties and never their particle
properties
in fact we've never even been able to
detect a particle of radial weight
because a single photon in the radio
frequency has such low energy that we
can't detect it
and thanks for watching another episode
of the action lab i hope you enjoyed it
if you did don't forget to subscribe if
you haven't subscribed yet
and you can hit the bell so you can be
notified when i release my latest video
and check out the action lab shorts
channel as well it's where i do
similar experiments to this channel
except they're a lot shorter i do them
in less than
a minute and also i'd like to thank
brandon fisher for suggesting that i do
this video
he sent me some good links on this and
suggested that i should do a video on
the photoelectric effect and i'll also
put a link in my description to an arbor
scientific video
where they do the same experiment and
they show you how to use the materials
that i use and where to get them as well
if you want to try it yourself
and thanks for watching i'll see you
next time
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