Photons revisited

Frequency and energy, brightness and number

You can describe what's seen using photons. That’s useful because photons are the simplest quantum object, and physicists think the world is best described by quantum physics.

How bright a thing is depends on the number of photons a second detected and the colour depends on the frequency of the photons that arrive.

The activity (which is the number of photons second-1) at the source and detector set the power absorbed and emitted.

Photons are quantum entities, shifting energy to or from stores in definite chunks. Therefore, counting the number of photons second-1 sets the power in the beam, for a particular frequency of photon.

Changing the number of photons

Change what’s between source and detector to change the number of photons arriving at the detector. (A device switches power between pathways: source, absorber and detector are all devices.) The matter that you put in the way affects what happens to the photons. What happens depends on the energy of the photon.