A packet of electromagnetic energy of size E = hf.

What is the photoelectric effect?

The release of electrons from a metal surface when light of high enough frequency is shone on it.

What is the threshold frequency?

The minimum frequency of radiation that releases photoelectrons from a metal surface.

What is the work function?

The minimum energy needed to remove an electron from the surface of a metal.

Why does bright light below the threshold frequency release no electrons?

Each electron absorbs one photon, whose energy is less than the work function, and energies cannot be added.

What does the intensity of light change above the threshold frequency?

The number of photons per second, so the number of photoelectrons per second and the current.

What is the stopping potential?

The potential difference that just stops the fastest photoelectrons, so the current is zero.

Why do photoelectrons have a range of kinetic energies?

Electrons from deeper in the metal lose energy in collisions before leaving.

Why can the wave model not explain the photoelectric effect?

It predicts no threshold frequency and a delay with dim light, but emission is immediate.

The photoelectric effectAQA A-Level Physics: Flashcards

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What is a photon?

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What is a photon?: A packet of electromagnetic energy of size E = hf.
What is the photoelectric effect?: The release of electrons from a metal surface when light of high enough frequency is shone on it.
What is the threshold frequency?: The minimum frequency of radiation that releases photoelectrons from a metal surface.
What is the work function?: The minimum energy needed to remove an electron from the surface of a metal.
State the photoelectric equation.: hf = φ + Ek(max)
How do you find the threshold frequency from the work function?: f₀ = φ/h
Why does bright light below the threshold frequency release no electrons?: Each electron absorbs one photon, whose energy is less than the work function, and energies cannot be added.
What does the intensity of light change above the threshold frequency?: The number of photons per second, so the number of photoelectrons per second and the current.
What is the stopping potential?: The potential difference that just stops the fastest photoelectrons, so the current is zero.
How is stopping potential related to maximum kinetic energy?: eVs = Ek(max)
Why do photoelectrons have a range of kinetic energies?: Electrons from deeper in the metal lose energy in collisions before leaving.
Why can the wave model not explain the photoelectric effect?: It predicts no threshold frequency and a delay with dim light, but emission is immediate.

Exam questions on The photoelectric effect

A physics class directs light of different colours and intensities at a clean metal surface inside an evacuated tube. A sensitive ammeter detects any electrons emitted from the surface. The work function of the metal is 3.4 × 10⁻¹⁹ J.
The intensity of light below the threshold frequency is increased greatly, but no electrons are emitted. Explain why.2 marks
In a vacuum photocell a reverse potential difference is applied across the tube and increased until the photocurrent just falls to zero. This potential difference is the stopping potential. For one frequency of light the fastest photoelectrons leave the metal surface with a kinetic energy of 2.4 × 10⁻¹⁹ J.
Explain why the photoelectrons are emitted with a range of kinetic energies, up to a maximum value.2 marks
A clean metal surface with a work function of 3.6 × 10⁻¹⁹ J is illuminated with ultraviolet light of wavelength 320 nm.
Calculate the maximum kinetic energy of the photoelectrons.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).