Define photoelectric effect .

The photoelectric effect is the phenomenon in which electrons are emitted from the surface of a metal upon the absorption of electromagnetic radiation.

Define photoelectrons .

Photoelectrons are electrons emitted from the surface of a metal as a result of the photoelectric effect.

Why does the photoelectric effect provide evidence that light is quantised?

Each electron can only absorb a single photon, so only radiation above a threshold frequency carries enough energy per photon to release a photoelectron.

In the gold leaf electroscope experiment, shining UV light onto the negatively charged zinc plate causes photoelectrons to be .......... , so the gold leaf falls.

In the gold leaf electroscope experiment, shining UV light onto the negatively charged zinc plate causes photoelectrons to be emitted , so the gold leaf falls.

True or False? The photoelectric effect can be explained using the classical wave theory of light.

False. The photoelectric effect can only be explained by treating light as particles (photons) ; wave theory alone cannot account for the existence of a threshold frequency.

Why does the gold leaf in the electroscope initially repel from the central rod?

Both the rod and the leaf carry a negative charge , and like charges repel.

Define work function (Φ).

The work function is the minimum energy required to release a photoelectron from the surface of a metal.

Define threshold frequency ( f 0 ).

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

On a graph of maximum kinetic energy KE max against frequency f , what do the gradient, y-intercept and x-intercept represent?

Gradient = Planck's constant ( h ) y-intercept = −Φ (negative of the work function) x-intercept = threshold frequency ( f 0 )

Why are no photoelectrons emitted for radiation below the threshold frequency?

Each photon's energy ( hf ) is less than the work function Φ, so no single photon carries enough energy to release an electron.

The maximum kinetic energy of emitted photoelectrons depends only on the .......... of the incident radiation, not its intensity.

The maximum kinetic energy of emitted photoelectrons depends only on the frequency of the incident radiation, not its intensity.

True or False? Increasing the intensity of incident radiation increases the maximum kinetic energy of emitted photoelectrons.

False. Increasing intensity increases the number of photoelectrons emitted per second, not their maximum kinetic energy, which depends only on the frequency of the radiation.

Define the electronvolt .

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

Why is the electronvolt used instead of the joule for energies in quantum physics?

Quantum-scale energies are typically much smaller than 1 J, so the electronvolt gives more convenient, manageable values.

For an electron accelerated from rest through a potential difference, the energy gained in electronvolts is equal to its .......... energy.

For an electron accelerated from rest through a potential difference, the energy gained in electronvolts is equal to its kinetic energy.

True or False? To convert an energy value from electronvolts to joules, you divide by 1.6 × 10 -19 .

False. To convert eV to J , you multiply by 1.6 × 10 -19 ; to convert J to eV , you divide by 1.6 × 10 -19 .

A photon is a discrete packet of electromagnetic energy.

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

Moving the source closer increases the intensity of radiation incident on the plate, increasing the number of photoelectrons emitted per second, so the leaf loses negative charge faster.

Why does using a filament (low-frequency) light source cause no change in the gold leaf's position?

Its frequency is below the metal's threshold frequency , so no photoelectrons are emitted, regardless of the intensity.

A positively charged plate causes no change in the gold leaf's position because any emitted electrons are .......... back by the positive charge on the metal's surface.

A positively charged plate causes no change in the gold leaf's position because any emitted electrons are attracted back by the positive charge on the metal's surface.

True or False? Increasing the frequency of the incident light increases the rate at which the gold leaf falls.

False. Frequency affects the maximum kinetic energy of the photoelectrons, not their emission rate; the rate of fall depends on intensity , not frequency.

Why is the emission of photoelectrons essentially instantaneous once radiation reaches the metal surface?

A single photon interacts with a single electron, so if that photon's energy is at least equal to the work function, the electron is released immediately, with no time delay.

Define emission line spectrum .

An emission line spectrum is produced when excited electrons move from higher to lower energy levels, emitting photons with energies equal to the difference between the levels, producing a series of bright lines against a dark background.

Why is an element's emission line spectrum described as a fingerprint of that element?

Each element has a unique set of energy levels, so it produces a unique pattern of emitted wavelengths.

In hydrogen, electron transitions ending at n = 1 (the ground state) produce photons in the .......... region of the electromagnetic spectrum.

In hydrogen, electron transitions ending at n = 1 (the ground state) produce photons in the ultraviolet region of the electromagnetic spectrum.

True or False? The longer the wavelength of an emitted photon, the larger the energy level transition it corresponds to.

False. Wavelength is inversely proportional to the size of the energy transition — a larger transition produces a shorter wavelength, not a longer one.

For hydrogen, which region of the electromagnetic spectrum corresponds to transitions ending at n = 3?

Infrared — these are the lowest-energy, longest-wavelength transitions on the hydrogen energy level diagram.

International A Level (IAL)PhysicsEdexcelFlashcards2. Waves & ElectricityThe Photoelectric Effect & Atomic Spectra

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

Exam code: YPH11

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Define photoelectric effect.
The photoelectric effect is the phenomenon in which electrons are emitted from the surface of a metal upon the absorption of electromagnetic radiation.
Define photoelectrons.
Photoelectrons are electrons emitted from the surface of a metal as a result of the photoelectric effect.
Why does the photoelectric effect provide evidence that light is quantised?
Each electron can only absorb a single photon, so only radiation above a threshold frequency carries enough energy per photon to release a photoelectron.
Which metal is typically used as the plate in the gold leaf electroscope experiment?
Zinc.
In the gold leaf electroscope experiment, shining UV light onto the negatively charged zinc plate causes photoelectrons to be .........., so the gold leaf falls.
In the gold leaf electroscope experiment, shining UV light onto the negatively charged zinc plate causes photoelectrons to be emitted, so the gold leaf falls.
True or False?
The photoelectric effect can be explained using the classical wave theory of light.
False.
The photoelectric effect can only be explained by treating light as particles (photons); wave theory alone cannot account for the existence of a threshold frequency.
Why does the gold leaf in the electroscope initially repel from the central rod?
Both the rod and the leaf carry a negative charge, and like charges repel.
Define work function (Φ).
The work function is the minimum energy required to release a photoelectron from the surface of a metal.
Define threshold frequency (f₀).
The threshold frequency is the minimum frequency of incident electromagnetic radiation required to remove a photoelectron from the surface of a metal.
State the photoelectric equation.
$E = h f = Φ + \frac{1}{2} m v_{m a x}^{2}$
On a graph of maximum kinetic energy KE_max against frequency f, what do the gradient, y-intercept and x-intercept represent?
- Gradient = Planck's constant (h)
- y-intercept = −Φ (negative of the work function)
- x-intercept = threshold frequency (f₀)
Why are no photoelectrons emitted for radiation below the threshold frequency?
Each photon's energy (hf) is less than the work function Φ, so no single photon carries enough energy to release an electron.
The maximum kinetic energy of emitted photoelectrons depends only on the .......... of the incident radiation, not its intensity.
The maximum kinetic energy of emitted photoelectrons depends only on the frequency of the incident radiation, not its intensity.
True or False?
Increasing the intensity of incident radiation increases the maximum kinetic energy of emitted photoelectrons.
False.
Increasing intensity increases the number of photoelectrons emitted per second, not their maximum kinetic energy, which depends only on the frequency of the radiation.
Define the electronvolt.
The electronvolt is the energy gained by an electron travelling, from rest, through a potential difference of one volt.
What is the value of 1 eV in joules?
$1 eV = 1.6 \times 10^{- 19} J$
Why is the electronvolt used instead of the joule for energies in quantum physics?
Quantum-scale energies are typically much smaller than 1 J, so the electronvolt gives more convenient, manageable values.
For an electron accelerated from rest through a potential difference, the energy gained in electronvolts is equal to its .......... energy.
For an electron accelerated from rest through a potential difference, the energy gained in electronvolts is equal to its kinetic energy.
True or False?
To convert an energy value from electronvolts to joules, you divide by 1.6 × 10^-19.
False.
To convert eV to J, you multiply by 1.6 × 10^-19; to convert J to eV, you divide by 1.6 × 10^-19.
Give the equation relating the energy gained (in eV) by an electron accelerated from rest to its final speed.
$e V = \frac{1}{2} m v^{2}$
Define photon.
A photon is a discrete packet of electromagnetic energy.
Which two phenomena demonstrate that electromagnetic radiation behaves as a wave?
Diffraction and interference.
In the gold leaf electroscope experiment, why does moving the UV source closer to the metal plate make the gold leaf fall more quickly?
Moving the source closer increases the intensity of radiation incident on the plate, increasing the number of photoelectrons emitted per second, so the leaf loses negative charge faster.
Why does using a filament (low-frequency) light source cause no change in the gold leaf's position?
Its frequency is below the metal's threshold frequency, so no photoelectrons are emitted, regardless of the intensity.
A positively charged plate causes no change in the gold leaf's position because any emitted electrons are .......... back by the positive charge on the metal's surface.
A positively charged plate causes no change in the gold leaf's position because any emitted electrons are attracted back by the positive charge on the metal's surface.
True or False?
Increasing the frequency of the incident light increases the rate at which the gold leaf falls.
False.
Frequency affects the maximum kinetic energy of the photoelectrons, not their emission rate; the rate of fall depends on intensity, not frequency.
Why is the emission of photoelectrons essentially instantaneous once radiation reaches the metal surface?
A single photon interacts with a single electron, so if that photon's energy is at least equal to the work function, the electron is released immediately, with no time delay.
Define emission line spectrum.
An emission line spectrum is produced when excited electrons move from higher to lower energy levels, emitting photons with energies equal to the difference between the levels, producing a series of bright lines against a dark background.
Why is an element's emission line spectrum described as a fingerprint of that element?
Each element has a unique set of energy levels, so it produces a unique pattern of emitted wavelengths.
Give the equation linking the energy of a transition ΔE to the frequency f and wavelength λ of the emitted photon.
$Δ E = E_{1} - E_{2} = h f = \frac{h c}{λ}$
In hydrogen, electron transitions ending at n = 1 (the ground state) produce photons in the .......... region of the electromagnetic spectrum.
In hydrogen, electron transitions ending at n = 1 (the ground state) produce photons in the ultraviolet region of the electromagnetic spectrum.
True or False?
The longer the wavelength of an emitted photon, the larger the energy level transition it corresponds to.
False.
Wavelength is inversely proportional to the size of the energy transition — a larger transition produces a shorter wavelength, not a longer one.
For hydrogen, which region of the electromagnetic spectrum corresponds to transitions ending at n = 3?
Infrared — these are the lowest-energy, longest-wavelength transitions on the hydrogen energy level diagram.

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