(a) the particulate nature (photon model) of electromagnetic radiation

The idea

Electromagnetic radiation can be modelled as a stream of discrete packets of energy called photons. This particulate model explains phenomena such as the photoelectric effect, where light below a threshold frequency cannot eject electrons no matter how intense it is. Each photon carries energy E = hf, where h is the Planck constant (6.63 × 10⁻³⁴ J s) and f is the frequency of the radiation. The photon model treats radiation as localised quanta rather than a continuous wave. For example, a beam of ultraviolet light consists of many photons, each with energy determined by its frequency. Increasing intensity increases the number of photons per second, not the energy of each photon.

What earns the marks

• States that electromagnetic radiation can be modelled as discrete packets of energy called photons.
• Recalls that the energy of a photon is given by E = hf.
• Explains that intensity relates to the number of photons per second, not the energy of individual photons.
• Describes how the photon model explains the threshold frequency in the photoelectric effect.
• Recognises that the photon model and wave model are complementary descriptions of electromagnetic radiation.

Common mistakes and how to put them right

• Believing that increasing the intensity of light increases the energy of each photon.

  Put it right: It increases the number of photons.

• Thinking that the photon model replaces the wave model entirely.

  Put it right: Both models are needed for different phenomena.

• Confusing the Planck constant with other constants or using the wrong value.

• Assuming that photons have mass.

  Put it right: Photons are massless and travel at the speed of light in a vacuum.

Exam tips

• Use E = hf and remember that f = c/λ if wavelength is given.
• When explaining the photoelectric effect, state that one photon interacts with one electron.
• Check that your answer distinguishes between intensity (photons per second) and photon energy (hf).
• Quote the Planck constant to at least two significant figures when performing calculations.