Threshold effects

Enough energy per photon

Sometimes you don’t need a key to open door: just kick it down. That’s destructive, but so long as you can kick hard enough, it works. For many processes the photon just needs to shift enough energy: to be above a threshold.


One common threshold effect is making ions. Choose a photon of high enough energy to knock changes out of atoms or molecules, and you’ll get an ion. These photons are ionising radiations. For example, ultra-violet light from the sun causes sunburn because each photon shifts enough energy to make an ion.