Define the photoelectric effect .

The emission of electrons from the surface of a metal upon the absorption of electromagnetic radiation

Define photoelectrons .

Electrons emitted from the surface of a metal via the photoelectric effect

What does the photoelectric effect prove about light?

Light is quantised — carried in discrete packets called photons . Each electron can absorb only a single photon

Only frequencies of light above a .......... will emit a photoelectron

Only frequencies of light above a threshold frequency will emit a photoelectron

In the gold leaf electroscope, a negatively charged zinc plate is illuminated with UV light. Why does the gold leaf fall towards the central rod?

UV light causes the emission of photoelectrons from the zinc plate. This removes negative charge, so the leaf and rod become less negatively charged and repel each other less, causing the leaf to fall

True or False? A bright enough light of any frequency will eventually emit photoelectrons

False. If the frequency is below the threshold frequency , no photoelectrons are emitted no matter how intense the light, because each electron can absorb only a single photon

Define the work function .

The minimum energy required to release a photoelectron from the surface of a metal

Define the threshold frequency .

The minimum frequency of incident electromagnetic radiation required to remove a photoelectron from the surface of a metal

The maximum kinetic energy of a photoelectron depends only on the .......... of the incident photon, not on the intensity of the radiation

The maximum kinetic energy of a photoelectron depends only on the frequency of the incident photon, not on the intensity of the radiation

A graph of maximum kinetic energy against frequency is plotted for a metal. What do the gradient and the x-intercept of the line represent?

The gradient equals Planck's constant ( h ). The x-intercept is the threshold frequency ( f 0 ) — below this frequency no electrons are emitted

True or False? Increasing the intensity of light increases the maximum kinetic energy of photoelectrons

False. Maximum kinetic energy depends only on the frequency of the incident light. Increasing intensity increases the number of photoelectrons emitted per second, not their energy

Define the electronvolt .

The energy gained by an electron travelling, from rest, through a potential difference of one volt 1 eV = 1.6 × 10 -19 J

An electronvolt is the energy gained by an electron travelling, from rest, through a potential difference of ..........

An electronvolt is the energy gained by an electron travelling, from rest, through a potential difference of one volt

How do you convert between electronvolts and joules?

eV → J : multiply by 1.6 × 10 -19 J → eV : divide by 1.6 × 10 -19

Why is the electronvolt used to express quantum energies?

Quantum energies tend to be much smaller than 1 joule , so the electronvolt is a more convenient unit

True or False? The electronvolt is a unit of potential difference

False. The electronvolt is a unit of energy , equal to 1.6 × 10 -19 J. It is not a unit of voltage

A discrete packet (or quantum) of energy of electromagnetic radiation

Which two experiments can only be explained if EM radiation is treated as particles (photons)?

The photoelectric effect and atomic line spectra

In the photoelectric effect, how many electrons does a single photon interact with?

One — a single photon interacts with a single electron

In the gold leaf experiment, why does moving the UV source closer to the metal plate make the leaf fall more quickly?

Moving the source closer increases the intensity at the surface. This increases the number of photoelectrons emitted per second , so the leaf loses negative charge more rapidly

If a photon's energy is equal to the work function of the metal, photoelectrons are released ..........

If a photon's energy is equal to the work function of the metal, photoelectrons are released instantaneously

True or False? Using a higher frequency source makes the gold leaf fall more quickly

False. The rate at which the leaf falls depends on the intensity , not the frequency. Higher frequency increases the maximum kinetic energy of the emitted electrons, not how quickly they are emitted

How is an emission line spectrum produced?

An excited electron moves from a higher to a lower energy level and emits a photon with energy equal to the difference between the two levels

Why does each element produce a unique line spectrum?

Each element has a unique set of energy levels , so its transitions emit a unique set of photon wavelengths — acting as a fingerprint of the element

An emission line spectrum consists of a series of .......... against a dark background

An emission line spectrum consists of a series of bright lines against a dark background

Define excitation of an electron.

When an electron gains energy and moves to a higher energy level

True or False? A larger energy transition produces a photon with a longer wavelength

False. Wavelength is inversely proportional to the size of the energy transition — a larger transition emits a photon with a shorter wavelength

Why do emission spectra contain only discrete wavelengths?

Electron energy levels are discrete , so transitions release photons of only specific (quantised) energies — and therefore specific frequencies and wavelengths

A LevelPhysicsEdexcelFlashcards5. Waves & Particle Nature of LightThe Photoelectric Effect & Atomic Spectra

The Photoelectric Effect & Atomic Spectra (Edexcel A Level Physics): Flashcards

Exam code: 9PH0

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FrontThe Photoelectric Effect
Define the photoelectric effect.

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Cards in this collection (33)

Define the photoelectric effect.
The emission of electrons from the surface of a metal upon the absorption of electromagnetic radiation
Define photoelectrons.
Electrons emitted from the surface of a metal via the photoelectric effect
What does the photoelectric effect prove about light?
Light is quantised — carried in discrete packets called photons. Each electron can absorb only a single photon
Only frequencies of light above a .......... will emit a photoelectron
Only frequencies of light above a threshold frequency will emit a photoelectron
In the gold leaf electroscope, a negatively charged zinc plate is illuminated with UV light. Why does the gold leaf fall towards the central rod?
UV light causes the emission of photoelectrons from the zinc plate. This removes negative charge, so the leaf and rod become less negatively charged and repel each other less, causing the leaf to fall
True or False?
A bright enough light of any frequency will eventually emit photoelectrons
False.
If the frequency is below the threshold frequency, no photoelectrons are emitted no matter how intense the light, because each electron can absorb only a single photon
State the photoelectric equation
$h f = Φ + \frac{1}{2} m v_{m a x}^{2}$
Where hf is the photon energy, Φ is the work function and $\frac{1}{2} m v_{m a x}^{2}$ is the maximum kinetic energy of the photoelectron
Define the work function.
The minimum energy required to release a photoelectron from the surface of a metal
Define the threshold frequency.
The minimum frequency of incident electromagnetic radiation required to remove a photoelectron from the surface of a metal
The maximum kinetic energy of a photoelectron depends only on the .......... of the incident photon, not on the intensity of the radiation
The maximum kinetic energy of a photoelectron depends only on the frequency of the incident photon, not on the intensity of the radiation
A graph of maximum kinetic energy against frequency is plotted for a metal. What do the gradient and the x-intercept of the line represent?
The gradient equals Planck's constant (h). The x-intercept is the threshold frequency (f₀) — below this frequency no electrons are emitted
True or False?
Increasing the intensity of light increases the maximum kinetic energy of photoelectrons
False.
Maximum kinetic energy depends only on the frequency of the incident light. Increasing intensity increases the number of photoelectrons emitted per second, not their energy
At the threshold frequency, the photon energy is equal to which quantity?
The work function, so $h f_{0} = Φ$ . At this point the photoelectron is released with zero kinetic energy
Define the electronvolt.
The energy gained by an electron travelling, from rest, through a potential difference of one volt
1 eV = 1.6 × 10^-19 J
An electronvolt is the energy gained by an electron travelling, from rest, through a potential difference of ..........
An electronvolt is the energy gained by an electron travelling, from rest, through a potential difference of one volt
How do you convert between electronvolts and joules?
eV → J: multiply by 1.6 × 10^-19
J → eV: divide by 1.6 × 10^-19
Why is the electronvolt used to express quantum energies?
Quantum energies tend to be much smaller than 1 joule, so the electronvolt is a more convenient unit
Using E = QV, show that 1 eV is equal to 1.6 × 10^-19 J.
An electron has charge 1.6 × 10^-19 C. Travelling through 1 V:
$E = Q V = (1.6 \times 10^{- 19}) \times 1 = 1.6 \times 10^{- 19} J$
True or False?
The electronvolt is a unit of potential difference
False.
The electronvolt is a unit of energy, equal to 1.6 × 10^-19 J. It is not a unit of voltage
Define a photon.
A discrete packet (or quantum) of energy of electromagnetic radiation
Which phenomena show that EM radiation behaves as a wave?
Diffraction and interference
Which two experiments can only be explained if EM radiation is treated as particles (photons)?
The photoelectric effect and atomic line spectra
In the photoelectric effect, how many electrons does a single photon interact with?
One — a single photon interacts with a single electron
In the gold leaf experiment, why does moving the UV source closer to the metal plate make the leaf fall more quickly?
Moving the source closer increases the intensity at the surface. This increases the number of photoelectrons emitted per second, so the leaf loses negative charge more rapidly
If a photon's energy is equal to the work function of the metal, photoelectrons are released ..........
If a photon's energy is equal to the work function of the metal, photoelectrons are released instantaneously
True or False?
Using a higher frequency source makes the gold leaf fall more quickly
False.
The rate at which the leaf falls depends on the intensity, not the frequency. Higher frequency increases the maximum kinetic energy of the emitted electrons, not how quickly they are emitted
How is an emission line spectrum produced?
An excited electron moves from a higher to a lower energy level and emits a photon with energy equal to the difference between the two levels
Why does each element produce a unique line spectrum?
Each element has a unique set of energy levels, so its transitions emit a unique set of photon wavelengths — acting as a fingerprint of the element
An emission line spectrum consists of a series of .......... against a dark background
An emission line spectrum consists of a series of bright lines against a dark background
What is the energy of a photon emitted during an electron transition?
$Δ E = E_{1} - E_{2} = h f = \frac{h c}{λ}$
Where ΔE is the energy difference between the levels, f is frequency and λ is wavelength
Define excitation of an electron.
When an electron gains energy and moves to a higher energy level
True or False?
A larger energy transition produces a photon with a longer wavelength
False.
Wavelength is inversely proportional to the size of the energy transition — a larger transition emits a photon with a shorter wavelength
Why do emission spectra contain only discrete wavelengths?
Electron energy levels are discrete, so transitions release photons of only specific (quantised) energies — and therefore specific frequencies and wavelengths

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