A level-readyAll subjects · Physics · WPH12 (Unit 2)
Photons and the electronvolt
Light behaves as a wave in some experiments and as a stream of particles (photons) in others. Each photon carries a fixed packet of energy.
Two models of light
- Wave model: light shows diffraction, interference and polarisation (Huygens, Young).
- Particle model: Newton proposed 'corpuscles'; later Planck and Einstein showed that electromagnetic radiation is emitted and absorbed in packets called photons.
The photon model was accepted after it explained new evidence (the photoelectric effect) that the wave model could not. Today we use whichever model explains the observation.
Photon energy
E = hf = hc / λ, with h = 6.63 x 10-34 J s and c = 3.00 x 108 m/s. Higher frequency (shorter wavelength) means more energetic photons: ultraviolet photons carry more energy than red ones.
The electronvolt
Photon energies in joules are tiny, so we use the electronvolt (eV): the energy transferred when an electron moves through a potential difference of 1 V.
1 eV = 1.60 x 10-19 J. To convert J to eV, divide by 1.60 x 10-19.
Worked example
Find the energy of a photon of wavelength 500 nm, in J and in eV.
- E = hc / λ = 6.63 x 10-34 x 3.00 x 108 / 500 x 10-9 = 3.98 x 10-19 J
- In eV: 3.98 x 10-19 / 1.60 x 10-19
Answer: E = 3.98 x 10-19 J = 2.49 eV
Check yourself
- Find the energy of a photon of frequency 6.0 x 1014 Hz, in eV, to 3 s.f.
Show answer
2.49 eV. E = hf = 6.63 x 10-34 x 6.0 x 1014 = 3.98 x 10-19 J = 2.49 eV.
- Convert 4.0 eV to joules. Give your answer in units of 10-19 J.
Show answer
6.4 x 10-19 J. 4.0 x 1.60 x 10-19 = 6.4 x 10-19 J.
- A photon has an energy of 3.0 eV. Find its wavelength in nm, to 3 s.f.
Show answer
414 nm. E = 3.0 x 1.60 x 10-19 = 4.8 x 10-19 J; λ = hc / E = 1.989 x 10-25 / 4.8 x 10-19 = 4.14 x 10-7 m = 414 nm.
- A 1.0 mW laser emits light of wavelength 633 nm. How many photons does it emit per second? Give your answer in units of 1015, to 3 s.f.
Show answer
3.18 x 1015 per s. Photon energy = hc / λ = 3.14 x 10-19 J; number per second = 1.0 x 10-3 / 3.14 x 10-19 = 3.18 x 1015.
- The frequency of light is increased. What happens to the energy of each photon?
- It increases
- It decreases
- It stays the same
- It becomes zero
Show answer
It increases. E = hf, so E is proportional to f.
- Which observation is evidence for the wave nature of light?
- The photoelectric effect
- Line emission spectra
- Diffraction and interference
- The existence of a threshold frequency
Show answer
Diffraction and interference. Only waves diffract and interfere.
- What is one electronvolt?
- The energy transferred when an electron moves through a p.d. of 1 V
- The charge on one electron
- The p.d. across a 1 Ω resistor
- The energy of any photon
Show answer
The energy transferred when an electron moves through a p.d. of 1 V. 1 eV = e x 1 V = 1.60 x 10-19 J.
Sources
- Pearson Edexcel International Advanced Subsidiary/Advanced Level in Physics specification (XPH11/YPH11), Issue 3
- Pearson qualifications: International Advanced Levels Physics (2018)
- Pearson Edexcel IAL Physics Unit 2 (WPH12) question paper, January 2024: list of data (e, m_e, h, c, eV)
Last updated 7 November 2026. Spotted a mistake? Tell us.