(Original post by AntiMagicMan)
I would say amount of photons. Because the energy of a wave is proportional to the amplitude squared, and the energy of photons is proportional to the frequency, a bigger amplitude means more energy and hence more photons are required in a given volume, or area.
So basically i'm voting in favour of Willa's explanation.
won't a bigger amplitude also indicate photons with higher frequencies as that also leads to more energy?