StemUp: OCR A A level Physics 4.5: Quantum physics

What are photons? (2)

Photons are discrete energy quanta (packets) which is how electromagnetic energy presents itself when interacting with matter.

What is the photon model? (1)

This models light as discrete energy quanta (packets) of electromagnetic radiation instead of waves.

How is the energy of the photon found? (2)

- The energy (E) is directly proportional to the frequency (f) of the electromagnetic radiation. The energy is E = hf = hc / λ as f = c / λ

- h is the Planck constant (6.63 x 10⁻³⁴ Js⁻¹)

What is an electronvolt (eV)? (2)

- 1 electronvolt is defined as the energy transferred to an electron as it passes through a potential difference of one volt.

- 1 electronvolt is 1.6 x 10⁻¹⁹ J using the formula W = QV

What is the value of 1eV? (1)

1.6 x 10⁻¹⁹

How can LED's be used to estimate the value of the Planck constant (h)? (2)

- Each photon from an LED is emitted when a single electron loses energy which is converted to the energy of a photon.

- If we know the wavelength emitted by the LED and the potential difference across the LED we can equate the energy of the electron to the energy of the photon: eV = hc \ λ to estimate the Planck constant.

What is the photoelectric effect? (1)

This is when electrons are released from the surface of a metal after electromagnetic radiation is shone onto it. It is evidence for the particle model of light.

How can the photoelectric effect be demonstrated? (3)

- It can be shown using a gold leaf electroscope (a zinc plate on a negatively charged stem with a negatively charged gold leaf attached to the stem).

- Initially the gold leaf will repel the stem as they are both negatively charged.

- If UV light is shone onto the zinc plate, free electrons will be released from the surface so the negative charge will be slowly lost and the gold leaf will fall back to the stem as they do not repel anymore.

How does a photon interact with an electron during the photoelectric effect? (1)

A photon interacts one-on-one with only one electron when it is incident on the metal, transferring all of its energy to the electron.

What is the photoelectric effect equation? (1)

This the energy of the photon equated to the energy of the released electron.

hf = Φ + KE_max where f is the frequency of the photon, Φ is the work function of the metal and KE_max is the maximum possible kinetic energy of the electron.

What is the work function (Φ) of a metal? (1)

This is the minimum energy required to release an electron from the surface of a metal. It is unique for each metal.

What is the threshold frequency? (3)

- As E = hf, the work function will have a specific frequency where the work function is equal to the energy of the photon.

- This is the minimum frequency of the electromagnetic radiation incident on a metal at which an electron will be emitted, it is unique for each metal.

- This means certain parts of the electromagnetic radiation (e.g visible light) cannot emit an electron when incident metal but UV could as it has a higher frequency than visible light.

What is the relation between the intensity of radiation and the photoelectric effect? (2)

- If the incident radiation is below the threshold frequency then no electrons will be released regardless of how high the intensity is.

- However if it is above the threshold frequency, a higher intensity means more electrons will be emitted as more photons are incident on the metal and electrons will be emitted at a greater rate.

What is electron diffraction? (2)

This is when electrons are fired at high speeds towards small gaps in between atoms and diffract through them just like waves. This shows a wave-like behaviour for particles.

What is the experimental evidence for electron diffraction? (2)

- A beam of electrons is fired at high speed towards a thin piece of polycrystalline graphite (which contains many layers of carbon atoms).

- The electrons diffract through these atoms and produce an interference pattern at a screen much like waves do.

What is the de Broglie equation? (2)

- All matter can exhibit wave and particle like behaviour. A particle with momentum (p) has a wavelength (λ) given by:

- λ = h /p where h is the Planck constant and this can be applied to all objects with momentum.