What a Photon Is
Electromagnetic radiation transfers energy from a source to an object. Start by learning what one photon represents before using any equation.
A light beam contains many photons
Visible light, infrared, ultraviolet and X-rays are all forms of electromagnetic radiation. The same photon model applies to every part of this spectrum.
The model is needed because electromagnetic energy is emitted and absorbed in separate amounts. Each separate amount is represented by one .
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The circles in the figure are symbols. They show separate photons; they are not a picture of tiny blue balls. A real beam contains far more than five photons.
- One marker represents one photon.
- Many markers represent the many photons in a beam.
- The arrow shows the direction in which the electromagnetic radiation travels.
This is the particulate nature of electromagnetic radiation. The radiation is modelled as individual particles called photons.
Does the figure mean that a real light beam contains only five photons?
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No. The five markers make the model readable. Each marker represents one photon, and a real beam contains a very large number of photons.
A photon is one quantum of electromagnetic energy
Physics uses the word quantum for one separate amount. A photon is one quantum of electromagnetic energy. The word discrete also means separate.
- A photon belongs to electromagnetic radiation.
- One photon carries one discrete amount of energy.
- A photon is emitted or absorbed as a complete quantum, not as part of that quantum.
Definition
2 marksWhat is meant by a photon?
Model answer: A photon is a quantum, or discrete packet, of electromagnetic energy.
For two marks, the definition needs both quantum of energy and electromagnetic radiation (9702/41/M/J/22 Q7(a)) (9702/42/F/M/25 Q8(a)) (9702/41/O/N/25 Q8(a)).
A student writes, “A photon is a packet of energy.” Which required part of the definition is missing?
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The answer must state that the energy belongs to electromagnetic radiation.
A photon does not have one fixed energy for every type of radiation. Its energy depends on the frequency of the radiation.