Photons: (b) photon as a quantum of energy of electromagnetic radiation

The idea

A photon is a quantum, meaning a single indivisible unit, of electromagnetic radiation. Each photon carries a specific amount of energy determined by the frequency of the radiation: E = hf. This quantisation means that energy can only be transferred in whole multiples of hf. For example, a photon of red light (f ≈ 4.3 × 10¹⁴ Hz) has energy E = 6.63 × 10⁻³⁴ × 4.3 × 10¹⁴ ≈ 2.9 × 10⁻¹⁹ J. A photon of blue light has a higher frequency and therefore more energy. The concept of the photon as a quantum explains why energy exchanges in the photoelectric effect are discrete and why there is a minimum frequency for electron emission.

What earns the marks

• Defines a photon as a quantum (discrete packet) of electromagnetic energy.
• States that photon energy is given by E = hf.
• Explains that energy is quantised in units of hf.
• Calculates photon energy using the Planck constant and frequency.
• Relates photon energy to frequency: higher frequency means higher energy per photon.

Common mistakes and how to put them right

• Thinking that a photon can have any energy.

  Put it right: Its energy is fixed by its frequency.

• Confusing the quantum of energy with the quantum of charge.

  Put it right: The photon is the quantum of electromagnetic energy.

• Forgetting to convert frequency from kHz or MHz to Hz before calculating energy.

• Assuming that all photons have the same energy regardless of frequency.

Exam tips

• Always write the equation E = hf before substituting values.
• Convert all frequencies to Hz and wavelengths to metres before calculation.
• Use standard form carefully, especially with powers of ten in the Planck constant.
• When comparing photons, state that higher frequency corresponds to higher energy.