Photon
A packet of light with energy E = h f = h c / lambda, where h = 6.63e-34 J s.
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Analogy
Light arrives in small bundles, like single drops of rain.
Example
E = 1240 eV nm / lambda= 1240 / 550= 2.25 eV
In practice
A green photon of wavelength 550 nm carries about 2.25 eV.
Practice
Flashcards
- What is the energy of a photon?E = hf = hc/lambda.
- What does the photoelectric effect show?Light behaves as photons. Energy depends on frequency, not brightness.
Interview questions
- Explain the photoelectric effect and why it could not be explained by wave theory.
- Threshold frequency exists
- Brightness does not change electron energy
- Photons carry energy hf
Questions people ask
Is light a wave or a particle?
It behaves as both, depending on the experiment. Interference and diffraction show wave behaviour. The photoelectric effect and single-photon detectors show particle behaviour. Quantum theory treats light as one thing that has both aspects. Physicists call this wave-particle duality, and it also applies to electrons and atoms.
What is an electron-volt for photons?
The electron-volt is a convenient energy unit for atoms, equal to 1.6e-19 J. A useful formula is E in eV equals 1240 divided by wavelength in nanometres. Visible photons have 1.8 to 3.1 eV, while X-rays carry thousands of eV each.