A2 Unit 22 Quantum Physics

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What is a photon?

A packet/quantum/discrete amount of energy of electromagnetic waves

What is the equation that links photon energy and its frequency?

E = hf

What is the equation that links photon energy and wavelength?

E = hc/λ

What is an electronvolt?

Unit of energy. 1eV = 1.6 × 10^-19J.

Give the equation for the momentum of a photon

ρ = E/cE

Give an example that shows the particle-like nature of light

Photoelectric effect, absorption/emission spectra

Give an example that shows the wave-like nature of matter

Electron diffraction

What is the photoelectric effect?

Emission of photoelectrons from a clean metal surface when illuminated by electromagnetic radiation

What is the threshold wavelength?

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

What is the threshold wavelength?

The longest wavelength of incident electromagnetic radiation that would remove a photoelectron from the surface of a material

What is the work function of a material?

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

Give the equation for the photoelectric effect

E = Φ + E_kmax

Why are photoelectrons emitted with a range of kinetic energyies?

Some electrons will lose energy as they escape the metal

How many photons can each electron absorb?

One. No electron can absorb more than one photon. If the photon does not have an energy greater than the threshold frequency no electrons are emitted.

On a graph of maximum kinetic energy against frequency what is the gradient of the graph?

Planck’s constant

On a graph of maximum kinetic energy against frequency what is the y-intercept?

On a graph of maximum kinetic energy against frequency what is the x-intercept?

Threshold frequency of the metal

What does changing the intensity of light on the metal do for above and below the threshold frequency?

Below the threshold frequency - nothing, no photoelectrons emitted

Above threshold frequency - the number of electrons emitted per second increases, their maximum kinetic energy is the same

What does increasing the frequency of incident light on the metal do?

Increases the maximum kinetic energy of each photoelectron. The number emitted per second is constant.

What is the de Broglie wavelength?

The wavelength associated with a moving particle

What is the equation for the wavelength of a particle?

λ = h/p

What causes an emission spectrum?

When electrons transition from a higher energy level to a lower energy level, a photon is emitted

What does emission spectra show?

Energy levels within an atom are discrete

What is an absorption spectrum?

When white light passes through a cool low-pressure gas, there are some wavelengths missing

Why do some wavelengths of light appear missing?

These wavelengths correspond to an energy level change within the atom. The electron absorbs this photon and remits it in all directions, so a dark band appears at that wavelength.

How do you calculate the energy of the photon emitted?

hf = E₁ - E₂