Knocking Electrons With Light—The Photoelectric Effect

Summary

In 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

00:00

🔬 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.

05:00

🌌 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.

10:00

📢 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.

Keywords

💡Photoelectric Effect

The photoelectric effect is a phenomenon where electrons are emitted from a material when it is exposed to light of a certain frequency. In the video, the presenter demonstrates this by using ultraviolet light to knock electrons off a charged plate, which is a clear illustration of the effect. The photoelectric effect was pivotal in establishing the concept of light as quanta, or particles of energy, rather than just a wave.

💡Electroscope

An electroscope is a device used to detect the presence and magnitude of electric charge on an object. In the script, the presenter uses an electroscope to measure the electric charge on a plate, demonstrating how it changes when electrons are transferred via a balloon or light. The electroscope plays a crucial role in visually representing the transfer of charge, which is central to the experiment.

💡Electrons

Electrons are subatomic particles that orbit the nucleus of an atom and carry a negative electric charge. The video script describes how electrons are transferred from the presenter's hair to a balloon and then to a metal plate. The movement of electrons is central to the discussion of electric charge and the photoelectric effect.

💡Binding Energy

Binding energy refers to the energy required to remove an electron from an atom. The script explains that electrons on the plate have a specific binding energy, and only light with enough energy can dislodge them. This concept is key to understanding why certain frequencies of light are ineffective at causing the photoelectric effect, while others are.

💡Visible Light

Visible light is the portion of the electromagnetic spectrum that is detectable by the human eye. The video script mentions that visible light, even when bright, does not have enough energy per photon to cause the photoelectric effect on the charged plate, highlighting the difference between the energy of light and its visibility.

💡Ultraviolet Light

Ultraviolet (UV) light is a type of electromagnetic radiation with a wavelength shorter than that of visible light but longer than X-rays. In the script, the presenter uses UV light to successfully dislodge electrons from the plate, demonstrating that UV light has enough energy per photon to overcome the binding energy of the electrons.

💡Frequency

Frequency, in the context of light, refers to the number of wave oscillations that occur per unit of time. The video explains that higher frequency light, such as UV light, has more energy per photon, which is why it can cause the photoelectric effect. This concept is essential for understanding the energy levels required to dislodge electrons.

💡Lumens

Lumens are a measure of the total amount of visible light emitted by a source. The script mentions using a light source with 2000 lumens, indicating the intensity of the light. However, the presenter explains that the brightness, or lumens, does not affect the ability to cause the photoelectric effect; it only affects the rate at which electrons are dislodged.

💡Wave-Particle Duality

Wave-particle duality is the concept that all particles, including light, exhibit both wave-like and particle-like properties. The video script discusses how light can be considered both a wave and a particle, depending on the context. This duality is exemplified by the photoelectric effect, where light behaves as particles (photons) with specific energy levels.

💡Quantum Field

A quantum field is a theoretical framework in which particles are considered as excitations of underlying fields. The script touches on this concept by explaining that particles like photons are the smallest possible vibrations of their respective fields, such as the electromagnetic field. This explanation helps to reconcile the wave-particle duality of light.

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