Energy Levels & Emission Spectra - A-level Physics

Summary

This educational video script explores the photoelectric effect, explaining how electrons absorb photons and either become excited to higher energy levels or ionize, leaving the atom. It delves into the quantization of energy levels, where electrons can only occupy specific states and release photons when returning to lower levels, creating various colors of light. The script also covers the calculation of photon wavelengths, absorption and emission spectra, and the practical application of these principles in fluorescent tubes, providing a comprehensive understanding of electron energy levels and their interactions with light.

Takeaways

• 🌟 The photoelectric effect demonstrates that electrons can absorb energy from photons, which is quantized and equals to Planck's constant times the frequency of the photon (E=hν).
• 🔬 When a photon is absorbed by an electron, it can either excite the electron to a higher energy level or ionize the electron, causing it to leave the atom entirely if the photon's energy is sufficient.
• 🚀 Electrons can only occupy discrete energy levels, and the energy difference between these levels dictates the energy of the photon that can be absorbed or emitted.
• 🔄 After excitation, electrons tend to return to their ground state, releasing energy in the form of a photon during a process called de-excitation or relaxation.
• 🌈 The energy levels and the transitions between them result in the emission of photons with specific energies, which can correspond to various colors of light.
• 🔢 The energy of a photon is directly related to its wavelength, as shown by the equation E=hC/λ, where h is Planck's constant, C is the speed of light, and λ is the wavelength.
• 📊 Each element has a unique absorption and emission spectrum, which can be used to identify it based on the specific wavelengths of light that are absorbed or emitted.
• 💡 The difference between absorption and emission spectra lies in the fact that electrons can take multiple paths to return to the ground state, leading to a greater variety of emitted wavelengths.
• 💡 Fluorescent tubes operate on the principle of excitation and de-excitation, where electrons collide with mercury atoms to emit UV photons, which are then converted to visible light by the tube's coating.
• 🔬 The video script provides a comprehensive explanation of electron energy levels, the photoelectric effect, and the practical application of these concepts in lighting technology.

Q & A

What is the photoelectric effect?

The photoelectric effect is a phenomenon where electrons in a material absorb energy from photons and are ejected from the material, resulting in the emission of electrons or ionization.

How is the energy of a photon related to its frequency?

The energy of a photon is directly proportional to its frequency, as described by the equation E = hν, where E is the energy, h is Planck's constant, and ν is the frequency of the photon.

What happens when an electron absorbs a photon with more energy than needed for excitation?

If an electron absorbs a photon with energy exceeding the energy gap to the next level, it may not just get excited but could be ionized, leaving the atom and becoming a free electron.

What is the term for when an electron returns to its ground state after being excited?

The process where an electron returns to its ground state after being excited is called de-excitation or relaxation, during which it releases energy in the form of a photon.

Why do electrons only absorb photons of certain frequencies?

Electrons only absorb photons of frequencies that match the energy difference between their current energy level and a higher one, as per the quantization of energy levels in atoms.

What is the significance of electron volts in measuring electron energy?

Electron volts (eV) are used to measure the energy of electrons because they provide a convenient unit for dealing with the very small energy scales involved in electron transitions.

How does the energy of a photon relate to the color of light it represents?

The energy of a photon, and thus the color of light it represents, is inversely related to its wavelength. Higher energy photons correspond to shorter wavelengths, which are typically in the blue/violet end of the visible spectrum, while lower energy photons correspond to longer wavelengths, in the red/infrared end.

What is an absorption spectrum and how is it different from an emission spectrum?

An absorption spectrum shows the wavelengths of light that are absorbed by a substance, typically represented by dark lines on a bright background. An emission spectrum, conversely, shows the wavelengths of light emitted by a substance, represented by bright lines on a dark background. The two spectra are complementary, but the emission spectrum typically has more lines due to the various possible transitions electrons can make as they return to the ground state.

How does the fluorescent tube work in terms of electron energy levels?

In a fluorescent tube, electrons are fired through a low-pressure mercury vapor, causing excitation of mercury atoms. When these excited electrons de-excite, they emit ultraviolet (UV) photons, which are then absorbed by the tube's coating, causing it to emit visible light.

Why do different elements have different absorption and emission spectra?

Different elements have unique atomic structures with specific energy level arrangements, leading to the absorption and emission of light at distinct wavelengths. This results in characteristic spectra that can be used to identify elements.

Outlines

00:00

🌟 Understanding the Photoelectric Effect

The paragraph explains the photoelectric effect, where an electron can absorb a photon of energy. The energy of a photon is denoted by the equation \( E = h\nu \), where \( h \) is Planck's constant (6.63 x 10^-34 Js) and \( \nu \) is the frequency. The paragraph illustrates how electrons in atoms can absorb photons, leading to excitation or ionization. Excitation occurs when an electron absorbs energy and jumps to a higher energy level, while ionization happens when the electron absorbs so much energy that it escapes the atom entirely. The paragraph also discusses the release of energy when an electron returns to its normal energy level, known as de-excitation or relaxation, and the emission of a photon in the process.

05:01

🔬 Energy Levels and Photon Emission

This section delves into the quantized nature of electron energy levels within atoms. It explains that electrons can only exist at specific energy levels, and when they absorb a photon, they may jump to a higher level. The paragraph uses the example of an electron absorbing a photon with 7 electron volts of energy, which allows it to reach a higher energy state. The electron can then return to the ground state by emitting photons at various energy levels. The discussion also includes the calculation of photon wavelength using the equation \( \lambda = \frac{hc}{E} \), where \( c \) is the speed of light, resulting in a wavelength that corresponds to visible light. The concept of absorption and emission spectra is introduced, highlighting how atoms absorb certain wavelengths and emit light at specific frequencies, leading to the characteristic spectral lines of elements.

10:05

💡 Applications in Fluorescent Lighting

The final paragraph discusses the practical application of electron excitation and de-excitation in fluorescent lighting. It describes how low-pressure mercury vapor in a tube is excited by electrons, leading to the emission of ultraviolet (UV) photons. These UV photons are then absorbed by a phosphor coating on the tube's interior, which in turn emits visible light photons. The paragraph explains that the initial excitation is not due to photon absorption but rather from electron collisions. The UV light is transformed into visible light through the phosphor coating, illuminating the room. The paragraph concludes with an invitation for feedback and questions, and a prompt for viewers to engage with the content.

Keywords

💡Photoelectric Effect

The photoelectric effect is a phenomenon where electrons are emitted from a material when it absorbs light, particularly photons. This concept is central to the video's discussion, demonstrating that one photon can transfer its energy to one electron. In this case, the energy of the photon, determined by its frequency, causes either excitation or ionization of the electron.

💡Photon

A photon is a particle representing a quantum of light or electromagnetic radiation. In the video, photons are described as the carriers of energy, which, when absorbed by electrons, can excite them to higher energy levels or completely ionize them. The energy of a photon is proportional to its frequency, as described by the equation E = hf.

💡Excitation

Excitation occurs when an electron absorbs energy and moves to a higher energy level within an atom. The video explains how a photon with enough energy can excite an electron, causing it to jump to a higher shell or energy level. However, the electron prefers to return to its ground state, emitting a photon in the process.

💡Ionization

Ionization refers to the process where an electron gains enough energy to completely leave an atom, resulting in the formation of an ion. The video illustrates this when discussing how a photon with very high energy can cause an electron to escape from the atom entirely, leaving behind an ionized atom and a free electron.

💡Ground State

The ground state is the lowest energy level an electron can occupy in an atom. In the video, it is described as the electron's default position, from which it can be excited to higher energy levels when it absorbs energy. The electron tends to return to this ground state, releasing energy in the form of photons.

💡De-excitation

De-excitation is the process in which an excited electron returns to a lower energy level, releasing energy as a photon. In the video, this process is illustrated when an electron drops back to its original state after being excited, emitting a photon whose energy depends on the difference between the energy levels.

💡Energy Levels

Energy levels refer to the specific quantized states that electrons can occupy within an atom. The video discusses how electrons can only exist in certain energy levels, and when they absorb photons with specific energies, they can move between these levels. The concept of energy levels helps explain phenomena like excitation and ionization.

💡Planck's Constant (h)

Planck's constant (h) is a fundamental physical constant used to describe the energy of a photon, with the equation E = hf, where f is the frequency of the photon. In the video, it is given as 6.63 × 10⁻³⁴ J·s, showing how it plays a key role in determining photon energy and, consequently, how much energy an electron can absorb.

💡Electron Volt (eV)

An electron volt is a unit of energy commonly used to describe the energy of particles on a microscopic scale. In the video, energy levels are measured in electron volts, which helps simplify calculations involving electrons. For instance, the energy required for ionization or excitation is expressed in eV.

💡Emission Spectrum

The emission spectrum refers to the distinct wavelengths of light emitted by electrons as they de-excite and return to lower energy levels. In the video, it explains how an electron emits photons of specific energies when dropping down energy levels, producing an emission spectrum that is unique to each element.

Highlights

The photoelectric effect demonstrates that electrons can absorb energy from photons.

Photon energy is calculated using the equation E=hf, where E is energy, h is Planck's constant, and f is frequency.

Electrons can either become excited to a higher energy level or be ionized, depending on the photon's energy.

Excitation occurs when an electron absorbs a photon and jumps to a higher energy level within the atom.

Ionization happens when a photon provides enough energy for an electron to escape the atom completely.

Electrons prefer their ground state and will release energy when returning to it, a process known as de-excitation.

De-excitation can result in the emission of photons as the electron drops from an excited state to a lower one.

Electrons can only occupy specific energy levels within an atom, which are quantized.

The energy levels are typically measured in electron volts (eV) for convenience in discussing small energy values.

An electron can absorb a photon and move to a higher energy level, but it must match the level's specific energy to be absorbed.

If a photon's energy is insufficient to reach a higher energy level, the photon will pass through unabsorbed.

When an electron returns to its ground state from an excited state, it can release photons of various energies, leading to multiple colors of light.

The wavelength of a photon can be calculated using the equation λ = (hc) / E, where λ is wavelength, h is Planck's constant, c is the speed of light, and E is energy.

Atoms absorb specific wavelengths of light, leading to characteristic absorption spectra with dark lines representing absorbed frequencies.

Emission spectra display the wavelengths of light emitted by atoms, which can include more lines than absorption spectra due to the various possible de-excitation paths.

Fluorescent tubes operate by exciting mercury atoms with electrons, which then emit UV photons that are converted to visible light by the tube's coating.

The process of excitation and de-excitation in fluorescent tubes results in the emission of visible light, illuminating the room.

Transcripts

so the photoelectric effect proves that

one electron can absorb one Photon of

energy the energy of a photon I'm going

to put e gamma gamma is the general

symbol for a photon doesn't necessarily

mean a gamma Photon Photon energy equals

HF F being frequency and H being planked

constant that's

6.63 * 10us 34 dual seconds now in the

photo electric effect we had electrons

in metals absorbing photons but it

doesn't have to be a metal it can be any

electron so I'm going to draw an atom

here I'm going to draw the nucleus which

is

positive then I'm going to

draw just one

electron in a shell around the nucleus

now when a photon comes

in and it's absorbed by an electron one

of two things can happen if it gets more

energy then the electron becomes more

excited and it

jumps to a shell that is further away

from the nucleus we can say that it's

gone up an energy

level and we call this

excitation photon is absorbed electron

go goes up an energy level now what

we're going to find out a little bit

later on is that electrons can only be

in certain energy levels another thing

can happen as well

though if the photon has Lads of

energy so it's got a higher frequency

then the electron has way too much

energy to go to the next energy level or

an energy level above that it actually

just completely

completely moves away from the atom what

are we left with we're left with an ion

this is ionization ionization is what

happens when an electron is given enough

energy by a photon or it can be given

energy by another free electron and it

jumps out of the atom completely and

we're left with a free electron and an

ion left obviously more energy is needed

for ionization than excitation now this

electron here doesn't like being in the

energy level above where it's supposed

to be usually so after it's absorbed the

energy and gone up it will come straight

back down when this electron does go

back to its normal energy level energy

is released and we call this de

exitation sometimes also called

relaxation the electron returns to

energy level and a

photon is

emitted so with the excitation of phot

phon can come in be absorbed by an

electron it can go up an energy level

come straight back down and by doing so

it releases its own Photon so let's draw

our atom again now this is where our

electron usually is but it could also be

if it has enough energy in any of these

shells or energy

levels above but they are very specific

to an

atom what we're going to do is take a

snapshot of this which just going to

take a slice of this and we're going to

blow it up here so instead of drawing

shells we're actually just going to draw

our energy levels like this so here's

our electron in its normal shell we're

going to call this its ground state

that's when it has no energy or rather

no more energy than usual it's all

relative now we're going to measure

these in electron volts now remember

that one electron volt is equals to 1.6

6 * 10 -9

Jew it's just an easier number to deal

with because when we're talking about

individual electrons energies we're

talking very small numbers of Jews so we

deal with electron volts instead now

when a photon comes

in the electron can jump to one of these

energy levels now I can tell you that

these energy levels here are five

electron volts seven electron volts and

eight electron volts this electron volts

here though once it reaches this point

it's out of the atom so that's when it's

actually being

ionized so if this Photon and the energy

of this photon is HF PL constant times

the frequency if the photon had eight

electron volts that's eight lots of

these or more to give the electron then

the electron would jump all the way up

past this eight electron volts and it

would escape the atom the atom is now

being ionized but we're not going to

think about that for now we're just

going to think about what would happen

if it didn't go quite to that point if

the energy of the photon was equals to

three electron volts it's not enough

energy to get the electron to its next

energy level so it's not like the photon

is absorbed and the electron goes

partway to this five electron volts it

can't do that instead the photon would

just pass straight

through so that means that it's not

absorbed but but if it did have exactly

let's say seven electron volts it's

enough to get it to its third energy

level we give these by the way numbers

N1 is the ground state then N2 then N3

so here we go the electron has absorbed

the

photon and it goes up to this energy

level here hrah it's absorbed a photon

of the right amount of energy cuz it has

just the right frequency and it's gone

up to that third energy level we said

that electrons like being at their

ground state they want to come back to

here as soon as possible now on the way

back down from here the electron now has

a choice either it can go straight back

down to

here and it can release a photon because

it has to get rid of

energy and that's going to be a photon

that has seven electron volts of energy

that makes sense but it has another

option as well so it can go from

n 3 to N1 or it can stop

halfway it can go to

here and then down to here so it's gone

from Nal

3 to n =

2 to Nal 1 so it's gone from the third

energy level to the second and then back

to the ground state there both of these

will release photons what's the energy

of this Photon going to be well it's

gone from Seven electron volts to five

electron volts so this photon is going

to have two electron volts of energy so

it's going to be 3.2 * 10- 19 Jew this

one here is going to release a photon

that has five electron volts of energy

so it's going to be 8 * 10 -9 Jew so

when an electron absorbs some energy

from a photon it only absorbs one Photon

but on its way down it has choices it

can go straight to the bottom or it can

go to energy levels in between then fall

to the bottom from there even though

we're absorbing one Photon here one type

of photon how many photons can be

emitted 1 2 3 so we're going to have

various colors of photons and light

being emitted when this electron goes

from the third energy level down to its

ground state what about if I wanted to

find out the wavelength of this Photon

here let's let's have a look we know

that the energy of a

photon is equals to

HF from the wave equation we know the V

or C is f Lambda frequency times

wavelength so that means that

frequency is equals to speed of light

divided by the wavelength pop this in

here we end up with energy equals

HC over Lambda rearrange this and we end

up with wavelength equals PL constant

times the speed of light divided the

energy of the photon give that a go if

you want pause the video if you feel

like

it this is going to be 6.63 * 10-

34 * 3 * 10 8 divided by well it's five

electron volts that's five times this

many Jews whenever we do any

calculations we always have to use

Jewels we never put electron volts

straight into into an equation that's

going to give us a wavelength of

2.5 * 10 - 7 m or about

250 nanm pretty much visible light so

that does mean the atoms only absorb

certain wavelengths of light so you

might see a diagram like

this and you'll

see all the colors in the rainbow going

from

red all the way to

Blue and because we know that electrons

can only absorb certain wavelengths and

certain frequencies of

photons we get these black lines

appearing on

here and it's going to be different for

every single atom and molecule in the

world so this is an absorption Spectrum

so in other words all of these

wavelengths pass through but these black

lines show

the ones that are

absorbed so you can deduce energy levels

from those we also have an emission

spectrum as well other way around this

time the whole thing will be black apart

from a few colored

lines that

show what

frequencies and wavelengths are emitted

from something I've drawn an absorption

spectrum and emission spectrum for two

different elements here but if we did

have an absorption spectrum and an

emission spectrum would they be exactly

the same well no they wouldn't be you

would expect more lines on the emission

spectrum why is that well it's

because all electrons when they absorb

they have to start at the ground

state and go up to a specific energy

level on the way back down though they

can take any route they want so there's

always going to be a greater number of

different wavelengths that can be

emitted that could be absorbed you take

multiple Roots back to the ground

state one place that we see excitation

and de exitation

happening is in a fluorescent tube what

we have in here is low pressure Mercury

gas and what we do is we

fire

electrons from one end to the

other the electrons collide with the

Mercury atoms and they

excite the

electrons like we saw

earlier or we could draw it like

this the electrons de exite the problem

is is that they

emit UV photons

it's a high

frequency what do these photons

do these UV photons are absorbed by the

coating on the outside of the tube so

you have this

coating it's all around the outside if

it wasn't for this coating if you scrape

off the coating off a fluorescent tube

you're just getting UV photons coming

straight out but if we have this coating

the UV photons are absorbed by the

coating and have excitation again what

do they do in turn when they de exite

they emit visible photons therefore

lighting up a room that they're

in so that's how a fluorescent tube

Works notice that the excitation that we

get to begin with isn't due to the

absorption of photons it's an electron

coming in whacking into another electron

that electron goes off CU it's still got

loads of energy but it's given some of

its energy to this electron raising it

an energy level to get rid of that

energy it releases this high frequency

UV Photon that's absorbed by the coating

that then releases visible light photons

to light up a room so that's electron

energy levels if you think I've missed

anything or have any questions please

put it in a comment down below and don't

forget to leave a like if you found this

helpful and I'll see you next

time