Photon Energy
Quantum and Nuclear | Light, Sound and Waves

Coloured filters affect photons

Physics Narrative for 14-16 Supporting Physics Teaching

The power in a beam depends on the frequency of the photons and the activity

The frequency of the light associated with each photon sets the energy shifted by each photon.

Take two equally bright beams of monochromatic light (all photons in a beam have the same frequency and so appear to be the same colour to us) – one that shows up blue and one that shows up green. The energy shifted by the blue beam will arrive in fewer and larger chunks. The power in the pathway associated with the green beam is identical to that of the blue beam: they are the same brightness. However, the energy is shifted by more and lower-energy photons.

The power in the beam is the product of two factors of the stream of photons:

  • The energy shifted by each photon.
  • The number of photons emitted each second (the activity).


The energy shifted by each photon increases as the frequency increases (in fact, the energy per photon is proportional to the frequency). So you can compensate for low-energy photons by supplying many of them: there is a trade-off to obtain the same brightness. The character of the pathway alters, just as in the SPT: Electricity and energy topic, where the trade-off was between potential difference and current.

However, there are some effects that depend on the nature of the photons, such as seeing some effects as coloured: only photons of certain frequency are reflected or transmitted.

Differential absorption of photons explains filter action

Beams of white light consist of photons of a range of frequencies. There could be several combinations that all appear white to us. The photon model can be used to explain how filters work. Photons with certain frequencies are absorbed, while others are transmitted. The balance between the numbers of photons of different frequencies in the stream is altered, so the colour of the beam changes.

Different materials can be engineered to make filters that absorb particular frequencies. Such frequency-specific filters are expensive. However, simple filters that absorb a range of different frequencies to change the colour of beams are quite inexpensive. How might this happen?

Photons could be selected by energy as this varies with frequency. So some processes within the material are enabled if the correct quantity of energy is offered by the photon, and the photon is absorbed. The photon is rejected (for filters, by transmission) if there is no match in energies between what the photon shifts and what the process requires. This is a simple model and it fits in well with the idea that the energy available simply tells you what can and cannot happen (more in the SPT: Energy topic). However, notice something new here: there has to be a match in energy, not just more than enough. This is a difference from the previous model that will be explored.

Photon Energy
is a special case of Energy
can be determined for a Photon
appears in the relation E=hf
is used in analyses relating to Photoelectric Effect
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